Vegetables – Page 2 – The Garden Professors™

Viral Vegetables? Growing (and Buying) Produce in the age of COVID-19 (and reducing fear with facts)

Now that much of the world’s attention is focused on limiting the spread of pathogens, well one pathogen, it seems like a good time to talk about some of the questions or concerns we’ve seen regarding vegetable gardens, community gardens, and farmers markets. It’s a good time to talk about some of the practices that we should be doing to prevent other human pathogens from handling produce, like E. coli and Salmonella, and how those might fit into preventing the spread of COVID-19.

First things first

First off, we have to remember that SARS-CoV-2, which is the virus that causes COVID-19, is not a food borne illness. I repeat: COVID-19 IS NOT A FOOD BORNE ILLNESS. This means that it is not spread through the consumption of contaminated food like E. coli and Salmonella. I’ve seen many instances of people spreading fear about food online, with many suggesting using soap or bleach on food to minimize risk. Those steps are both unnecessary an actually pose a poisoning risk. There is currently no evidence to suggest that COVID-19 is transmittable by food or food packaging.

The risk from food (which is considered minimal by experts) is from cross-contamination from food or packaging onto hands or onto surfaces that are then touched by hands. The virus would then have to go from a persons hand to mucous membranes in the respiratory system by something like touching your face or picking your…..well, we won’t go there. The best defense against this isn’t necessarily sanitizing all the food you buy, but washing your hands after you handle it and sanitizing any surfaces that packaging or shopping bags touch.

Factsheet: Is Coronavirus a concern on Fresh Produce?

But while we’re on the topic of pathogens and food safety, it’s a good time to talk about some general guidelines that can not only help stop that potential SARS-CoV-2 cross contamination but also food borne illness in general.

Minimizing the risk from produce even further

Whether you grow it in your own garden, buy it at a local farmers market, or purchase it at the grocery store, produce has a minimal risk when it comes to COVID-19.

Factsheet: COVID-19 and Food Safety – Shopping and Handling Groceries

To minimize the very small risk of cross-contamination even further and (probably more importantly) to also reduce any risk from common food borne illnesses, proper washing of the produce should be practiced. But do you know how to do that? Maybe…and maybe not. Here are some steps to help out.

Wash your hands. The most common pathway of contamination for produce is from human touch.
You should use clean water that you would use for drinking (like out of the tap) and not use any bleach or soap.
Providing gentle friction with your hands or a produce brush or by rubbing the produce together is sufficient.
If you’re washing a lot of produce at once, say from a large harvest, and you’re using a tub full or sink full of water to wash multiple “loads” of produce, keep an eye on how dirty the water gets and refresh it when it gets discolored. Remember that washing produce in a tub or sink of water can also present a cross-contamination issue where contaminated produce contaminates the water.
When in doubt, discard produce you may think is contaminated or wash it separately.
To reduce risk of cross-contamination, consider a “single pass” washing technique where you spray the produce with water and it doesn’t sit in water with other produce.

Food Safety in the Garden

There are a few things we can do in the garden to help stop the spread of human pathogens. Most of them are common sense things that most people don’t even think about. Devout GP readers may remember my little missive around this time last year about the food safety risks of using manure in the vegetable garden (See: The Scoop on Poop). Beyond those musings on manure, though, gardeners can take some additional steps to reduce potential contamination. Those are:

Wash your hands. I know it sounds simple, and maybe even more so now that it has been drilled into our brains, but washing your hands before you garden is one of the best ways to reduce the spread of pathogens. It is especially important to wash your hands before you harvest produce or handle harvested produce.
Use clean containers for collecting and storing produce. Using harvest baskets, tubs, and totes is common, but the ones that are best in terms of food safety are those that can be washed and sanitized. This is one tactic that many farmers are encouraged to use as well. Plastic tubs, totes, trugs, and crates are probably best as they can withstand washing and the use of a sanitizer like bleach. Wooden or woven baskets may be cute, but they’re harder to clean and can hold on to pathogens.
Look for signs of wildlife in the garden. Aside from eating more than their fair share of produce, wild animals can also present a food safety risk especially from their droppings. Look for signs of animals in the garden and especially take note of any droppings. Don’t harvest produce that has signs of droppings on them. Many of the big produce recalls over the last decade have been a result of wild animals like birds or wild hogs.
Keep pets out of the garden. As much as you like to have Fluffy or Fido in the garden, they present a risk just like wild animals do.
Wash produce using proper techniques (previously discussed).

Best practices for minimizing COVID-19 risks at Community Gardens (and Farmers Markets)

You can practice handwashing anywhere. You can buy portable hand washing stations or build this DIY Model. Plans: https://www.youngfarmers.org/fsma_resources/portable-handwashing-stations/

One other aspect of gardening that could provide some risks for the spread of COVID-19 are the more social aspects of gardening, such as community gardens. I’ve had several local gardens reach out for best practices relating to minimizing risks in the garden – from handwashing stations to shared tool use. Thankfully, NCSU Extension was quick on the draw with resources for lots of aspects of the food system in terms of reducing risks from COVID-19 and they graciously allowed other universities to distribute these resources. Below are some links to the resources that would be helpful to gardeners:

COVID-19 FAQ for Community Gardens

And since some gardeners may sell at (or at least visit) farmers markets:

COVID-19 FAQ for Farmers Markets

Click here for all the resources developed by NCSU

Planting Prognostication: Understanding last frost and planting dates

Except for areas of the US that are more tropical like southern Florida or Hawai’i, most gardener’s planting schedules are set around winter weather and the possibility of frost or freeze. And even for gardeners in those more tropical areas, planting sometimes needs to be planned to schedule around the extreme heat of summer. Understanding these planting times can really lead to success or failure, especially for vegetable gardens, tender annuals, tropicals, and non-dormant perennials. There are a few tools that help us understand weather patterns and predict critical temperatures for planting, namely the USDA Hardiness Zone map and the Average Last Frost/Freeze dates. The USDA Hardiness Map shares data on what the average coldest temperature is, which is key for selecting perennial plants that you want to survive the winter. However, to know when to plant we look at the average freeze and frost dates. There seems to be a little bit of mystery, and even confusion, around the dates and how to interpret them, so let’s take a little time to understand them a little better. And since my background is in vegetable production, I’ll share a bit more detail there in terms of plants – but you can translate the information to ornamentals, especially those that are frost tender pretty easily.

Understanding Average Last Frost Date

What is the average last frost date and how is it figured? The average last frost date is exactly what it says it is – the average date at which the probability of frost has diminished. Just how diminished really depends on the source, so we’ll follow up with that in a bit. The data is computed by NOAA (National Oceanographic and Atmospheric Administration) and the National Weather Service to determine the probability of temperatures relating to frost and freezes based on weather data for an area over the last 30 years. They compute the likelihood of a light frost (36 F), frost/heavy frost (32 F), or freeze (28 F) at three different probability levels – 90% (the temperature is very likely to happen), 50% (the possibility is 50/50), or 10% (the temperature is unlikely). This tool from NOAA provides a chart with probabilities for locations throughout each state.

This data is typically collected and analyzed every ten years or so. I’m not exactly sure when the last data was analyzed, but I did find some maps on the NWS referencing the period 1980/81- 2009/20 (below). Therefore it is likely that new data will be released either this year or next year.

Temperature hardiness of common vegetables

Awareness of tolerance is especially important for vegetable crops, as the growing season and expected productivity of the plants. The following chart is a general guideline, and your mileage may vary based on cultivar difference, microclimates, and other factors. Also note that these temperatures are for both planting in spring and fall kill temperatures. Some of the more tender plants, like tomatoes, may withstand colder temperatures when they’re mature so they may be less susceptible to frost at the end of the season vs. the beginning of the season.

Season extension techniques, such as row covers can be used to protect tender plants in the spring and extend harvests in the fall. Row covers can be selected by the degrees of protection they deliver. For example, a row cover may offer 4 degrees of protection. This allows the protected plant to withstand air temperatures 4 degrees colder that what it would unaided. For fall crops, note that plants may stop growing well before the kill temperature but will hang out in “stasis” until they are killed. The above NOAA chart provides probabilities for both spring and fall – allowing you to not only plan for spring planting but also for fall crops. For scheduling fall crop planting dates, find your first frost date, count backwards the days to maturity (from the seed packet or tag), and add a few weeks for a harvest window and for the slowing growth as temperatures drop.

The Problem with Probability

These probabilities are based on past weather data, so keep in mind that these dates are used as a prediction not as a guarantee. It is especially important to remember this as weather uncertainty increases with climate change. Last frost could occur well before or even well after these predictive dates. This also begs the question – which probability should you use? Looking around at different sources, you might find sources that use either the 50% or 10% probability statistic, and there seems to be a bit of disagreement as to which one should be used. Based on the data for my region, I’ve seen sources share both dates. It really comes down to how much of a gamble you want to take or how much you want to push up harvest or maturity. If you plant on the earlier 50% probability date you may end up having to cover the plants a few times to protect them from frost. But each day that passes means that the chance of frost or freeze decreases.

Whenever I give a talk here at home in Omaha, I often ask my audience to guess what the average last frost date is for planting. Invariably, the answer I get is Mother’s Day…which I guess works as a guidepost in general. However, looking at the data (below), we can see that the 10% probability date for a 32 degree (killing) frost is May 4. The light frost date is May 11 – plants may be damaged but not killed unless they’re very tender. And the 50% probability date for a killing frost is actually April 21, which is the point where the probability of frost is 50% each day (and the probability shrinks each day.

Sometimes produce growers may opt to go early to get vegetables to market – which extends the sales season and allows them to charge a premium price if no other growers are selling. Season extension techniques like high tunnels have also pushed back farm production dates. As climate change makes weather more unpredictable, we may all be finding ways to alter the growing season as a norm rather than an exception. Until then, we’ll rely on the data we have to make the best predictions.

Sources:

https://www.canr.msu.edu/news/freeze_damage_in_fall_vegetables_identifying_and_preventing

http://www.gardening.cornell.edu/homegardening/scene0391.html

https://www.weather.gov/iwx/fallfrostinfo

https://www.ncdc.noaa.gov/cgi-bin/climatenormals/climatenormals.pl?directive=prod_select2&prodtype=CLIM2001&subrnum%2520to%2520Freeze/Frost%2520Data%2520from%2520the%2520U.S.%2520Climate%2520Normals

When Good Seeds Go Bad: How long can you store seeds?

Many gardeners, myself included, have that stash of old seed packets or saved seeds from garden seasons past, just waiting for the right time to be planted. They may be shoved in a drawer, a box, or in the fridge/freezer. Maybe you’re pulling some out of storage to start this spring – will they even germinate? Are those seeds good indefinitely? Do they ever expire? The answer to that really depends on what plant it is and how they are stored. While there isn’t a date where all the seeds go bad, they will eventually go bad over time. Why is this? And how can I make sure to use my seeds before they’re gone? Let’s find out!

Why Good Seeds Go Bad
While we think of seeds as perhaps inert, dormant, or in stasis they’re still very much alive and therefore are still undergoing processes like respiration, though at a much lower rate than a growing plant. During respiration, the seed (and plant within) are converting the stored sugars and starches in the endosperm to release energy. Once the germination process starts with the imbibition of water, the respiration rate increases drastically. A large amount of stored energy is needed to get through germination and sustain the seedling until it has its first set of true leaves and can photosynthesize on its own.

Seeds need to retain enough stored energy to sustain seedlings until they develop their first leaves and start photosynthesizing.

The shelf life of seeds is determined by the amount of energy that is stored, the amount used during storage, and the amount needed from germination to leaf development. This means that there’s a limit to how long a seed can stay in storage. After a while the seed loses viability if it doesn’t have enough energy stores to get it far enough along to photosynthesize on its own or to have that first burst of respiration at the initiation of germination. When searching for resources, keep in mind that viability refers to the ability of the seed to produce a robust seedling while germination refers to breaking of dormancy. The terms are inter-related, but the rates are not necessarily the same.

Some seeds have evolved to sit dormant for a long time, while others have a very short lifespan. It usually turns out that the seeds that last longest in storage are weeds that have evolved to wait long periods of time for an opportunity to germinate. Garden seeds tend to be on the shorter end of the storage time scale. A now 140-year old ongoing experiment at Michigan State University has given some interesting insight. In 1880, William Beal (one of the fathers of horticulture) buried 20 vials full of a variety of seeds (garden and weed) in secret locations around campus. The plan was to dig one up every 5 years and see what germinated. However, after the fist few rounds the cycle was bumped to 20 years. A vial was opened in 2000 and only one species, a weed, still germinated. This year is another germination year – we’ll have to wait and see if the mullein will germinate again this year.

How long will my seeds last?

Data from Nebraska Extension publication.

There are a few good sources that pull data from a variety of sources. The figure below lists some life expectancy times for common vegetable crops published by Nebraska Extension, using two common manuals on seeds as sources. You’ll also find some likes to other data, including average storage times for flowers, herbs, etc. in the references section (while we don’t typically promote commercial sites, the guide from Johnny’s Select Seeds has a good list of plants and has a variety of extension and academic sources listed). Like the MSU experiment, most of this research was done a while ago, but the data is still a good generalization. Most sources say that these time estimates are based on storage in optimal conditions. According to Johnny’s Select Seeds, “The actual storage life will depend upon the viability and moisture content of the seed when initially placed in storage, the specific variety, and the conditions of the storage environment”.

What are these “optimal” conditions? Generally the conditions are low humidity and low temperature. Low humidity ensures that the seed stays dry, avoiding potential initiation of germination. Low temperature reduces the respiration rate, slowing down usage of stored energy and increasing longevity. Optimal temperature for storage is below 42°F (15°C). Relative humidity should be between 20 and 40%.

The relationship between temperature and humidity seems to be inverse – meaning that as storage temperature goes lower, humidity can be higher and vice versa. However, storage times increase as both go down. Many sources state that seed longevity doubles for every one percent drop in humidity or five degree (F) drop in temperature. The relative humidity of the air affects the moisture level in the seeds. Germination usually starts at 25% moisture (and above). Ideal moisture levels for storage range between 8 and 12 percent and levels between 12 and 25 can lead to degradation of seeds, growth of fungi, etc. On the flip side, moisture levels below 5% can decrease vigor. Organizations like seed banks and germplasm centers that store seeds long term often will desiccate seeds to around 8% humidity to extend storage, but this isn’t usually needed for home gardeners.

Image result for seed vault — You don’t have to replicate conditions at the Global Seed Vault to have seed saving and starting success

Storage tips
Knowing that we need low temperatures and low relative humidity to extend seed life gives us some clues on how to store seeds to get the longest shelf life. This is key info if we’re trying to start seeds in spring that have been stored, or if we need to store extra or saved seeds. For the needed temperature levels, your standard home refrigerator is acceptable. Storage temps for cold foods are around the 40°F mark. However, humidity in a refrigerator is very variable. Humidity can skyrocket when doors are open, as condensation settles from warm room air settling on surfaces accumulates. Auto defrost cycles can also alter humidity. You’ll want to think about a desiccant like those silica packs to ensure that your seeds don’t get too moist. Store them in a plastic bag with the desiccant, and for added protection I always put mine in a sturdy container like a plastic box (or even a canning jar). Storing seeds in a freezer may help with the humidity issue, as any moisture that enters is frozen. You might also want to think about letting your bag or container warm up to room temperature before opening so that you don’t get condensation on the packets or the seeds themselves.

Sources:

Vegetable Garden Seed Storage and Germination Requirements – Nebraska Extension

Principles and Practices of Seed Storage – USDA

Seed Storage Guide – Johnny’s Select Seeds

Smith, R. D. (1992) Seed storage, temperature, and relative humidity. Seed Science Research 2, 113-116

120 Year Old Experiment Sprouts New Gardening Knowledge – MSU

Fail to Plan or Plan to Fail? Planning for a year of garden success

It seems like we’re always adhering to one schedule or another these days. We have devices and planners to keep track of our appointments, our work schedules, kids schedules, and more. Heck, even the antique seed company clock in my office is telling me to order seeds. It can seem overwhelming, so you might laugh if I tell you that coming up with a schedule, or a plan, for your garden can be beneficial. It is especially helpful for vegetable gardeners or those who like to any kinds of seeds.

Developing a yearly plan for the garden can help you keep ahead of the big tasks, help you stay on top of issues like weather, as well as make sure you get seeds started on time and transplanting done when it makes the most sense. While some of this may be a review for seasoned gardeners, the number of questions and calls we receive at Extension (and the number of oopsies we see) means that the information could be helpful for many.

Since my background is in vegetable production, I’ll focus there with some bits and pieces added for ornamentals when they fit.

Do you have garden goals?

Whenever you are planning your annual vegetable garden, or planning on adding any ornamentals to your gardens or landscape, you should ask yourself a few simple questions. When you’re dreaming of your garden during the winter is a good time to think of these goals.

1. What are my goals for the garden? Do I have long-term goals? What short-term goals can you set for this year to build momentum toward your long-term goals?

2. What resources am I willing to invest in the plants I’m ordering (money, time, water, space)?

3. What are the things I most want to grow?

4. What has worked (and what hasn’t) in your garden in the past?

While it may sound funny to say that you are going to set goals for your garden, it really isn’t all that far-fetched.

If you are planning to add ornamental plants to your landscape, you should think about what you want from those plants — are you looking for color or for structure? how about perennials vs. annuals (or biennials)?

When you are planning a vegetable garden, you should ask yourself not only what you want to grow, but how much. Are you just planting for fresh-from-the-garden eating, or do you want to preserve some through canning, freezing or drying? Are you growing just enough potatoes to eat for a month or two after the garden season, or do you need to select a variety that keeps well so you can store it?

Tips for Planning a Successful Garden

After you set your goals and decide what you want to plant, developing a schedule of when to do what is a good idea to stay on top of everything. I can’t tell you how many years I had been planning on planting this or that, but then forget to buy what I need or start seeds on time. A plan can help with that, as well as helping you space activities out over time rather than trying to get everything done in a hectic sprint. This is especially helpful to new gardeners or busy folks who may forget to start or plant certain things at the right time (I wouldn’t be speaking from experience here. Nope, this gardener has never been guilty of that. I meant not to plant all of that garlic that I bought last fall.) To borrow the method used in a popular self-help book, you’re “scheduling the big rocks” as one of the habits of highly effective gardeners.

Keep in mind that it can be hard to “garden on a schedule” as weather always plays a factor in what we can and can’t do in the garden. Given the wide variability in weather over the last few years in many parts of the country, which many scientists attribute to changing weather patterns due to climate change, it can be even more difficult to pin garden tasks to specific dates. A plan can help you keep track of everything you need to do, but it should be flexible to take weather into account.

Starting Seeds Indoors

Germinating a variety of plants for our 2018 All-America Selections trials

If you’re starting seeds indoors, decide when you’re going to transplant them to the garden. You can usually find this information on a seed packet, but you can find resources or consult your local cooperative extension office for guidance. Keep in mind that warm-season plants typically need to be planted after your average last frost date (unless you’re adventurous and don’t mind gambling with a potential loss). Cool season crops such as Cole crops (broccoli, cauliflower, cabbage, kale, etc.), leafy greens, and bok choi can be planted before the last frost date, but usually after the risk of a hard freeze has diminished. For a map of the average date for last spring freeze/frost, check out https://www.ncdc.noaa.gov/news/when-expect-your-last-spring-freeze. Note that these ranges are determined by analyzing the last frost dates over a 30 year period and the actual dates can vary due to weather variations (made even less predictable by climate change).

Choose the timeframe you wish to plant in the garden and count backward to when you need to start plants indoors. Put both the planting dates and the seed starting times on your calendar. Also keep in mind that this is the earliest that you can plant warm season crops, but you can plant them later if it works better for you. While we don’t typically share commercial links on this site, the best resource I’ve found for planning your seed starting and transplant dates for both vegetable and common annuals is https://www.johnnyseeds.com/growers-library/seed-planting-schedule-calculator.html

Direct Sowing into the Garden

For some crops like root crops, beans, leafy greens, and even some squash and cucumbers, direct sowing sees into the garden is ideal. You can add timeframes to your plan based on previous practice, like knowing that you’ll sow carrots toward the end of March or early April, but keeping an eye on the weather can be even more helpful here. Success here is more about temperature than timing. Most plants have optimal germination temperatures, so you want to sow outside when the soil temperature (not air temperature) is at or near those levels. The following resource has germination temperatures for common crops: http://sacmg.ucanr.edu/files/164220.pdf

If you’re lucky, you can search for local web-connected weather stations that have soil temperature probes. For example, we have one at our office that we share with clients to make gardening decisions (http://mgextensionwx.com/). If you can’t find one, NOAA has a few in each state for official climate data. https://www.ncdc.noaa.gov/crn/current-observations. Putting “check soil temperature” should be on your garden to-do list regularly until the temps get into good gardening range.

Spreading the planting and harvest through the season

If you’re aiming for harvests throughout the growing season, practice relay planting where crops mature in shifts throughout the garden season rather than all at once. If you’re planning on preserving some of your harvest for winter, planning on larger harvests at certain times in the season can get you the amount of produce you need for a big batch at the time that you need it. Some plants are good at producing through the season, but others, like determinate tomatoes and many beans have a one-time flush of production. Of course, we also have the crops that are once and done, like carrots and radishes, that only have one harvest. If we space out planting over weeks rather than planting all at once, harvests (or flowers if you’re growing annuals) can be spread out over a longer period of the season rather than everything maturing at once. There’s generally a several week (to several month) window for planting crops.

For example, tomatoes can be planted as early as the average last frost date, but can be planted for several weeks afterward. To figure out how late you can plant a crop, look for the first frost date and count backwards using the “days to maturity” information for the crop. You’ll want to add on a few weeks to a month to account for having a harvest window and slowing growth as temperatures drop. Keep in mind that many of the cool season crops can last well into the fall and winter, withstanding frosts and even light freezes, so replanting them for a fall harvest is ideal.

Planning out when to plant annuals, perennials, trees, and shrubs can also help make sure you get those plantings off on the right foot and can allow you to prepare in advance. For example, if I want to add a tree to the landscape, taking the time to research trees and planting techniques, scheduling any prep of the planting area, sourcing the tree, and planting at the right time could all go on your calendar – that way you are prepared and ready to plant at the correct time.

Other garden tasks

While much of the work of a garden plan is front-loaded to the spring, there’s lots of tasks that we should be planning on doing regularly. Scouting for and controlling insects and diseases, removing spent plants, mulching, compost turning, and more all come to mind. Putting these on your schedule rather than doing them when you think of them can really improve your likelihood of getting them done. Also think about some of those big things you might have identified in the goals you set for the year. Do you want to build a compost bin or develop new garden beds? Plant some trees? Take a soil test? Putting these on your calendar can not only help you remember them, but plan ahead as well. What do you need to do before you build that compost bin? Do you need to buy supplies and tools (and look for bargains if you’re planning ahead)? By planning when you’re going to accomplish these tasks, you can plan for success throughout the gardening year, improve your successes, and feel a little less hectic when the planting and growing goes full swing.

Why soil tests matter: lessons from my vegetable garden

Regular blog readers will remember that we moved to my childhood home a few years ago. With an acre or so of landscape I finally have enough room to put in a vegetable garden. My husband built a wonderful raised bed system, complete with critter fencing, and we’ve been enjoying the fresh greens and the first few tomatoes of the season.

Jim puts on the finishing touches to our first raised bed garden.

We filled these raised beds with native soil. During a porch addition I asked the contractor to stockpile the topsoil near the raised beds. The house was built almost 100 years ago and at that time there were no “designed topsoils” (thank goodness) – soil was simply moved around during construction. Some of this soil had been covered by pavers and the rest had been covered with turf. [You can read more about designed topsoils in this publication under “choosing soil for raised beds.”] There had been no addition of nutrients for at least 7 years so I was confident that this was about as natural a soil as I could expect.

Our native soil, ready for adding to our raised beds.

I’ve always advised gardeners to have a soil test done whenever they embark on a new garden or landscape project, so before I added anything to my raised beds I took samples and sent them to the soil testing lab at University of Massachusetts at Amherst (my go-to lab as there are no longer any university labs in Washington State for the public to use).

What I already knew about our soil was that it’s a glacial till (in other words it’s full of rocks left behind by a receding glacier). The area is full of native Garry oak (Quercus garryana), some of which are centuries old. The soil is excessively drained, meaning it’s probably a sandy loam. And that’s about all I knew until my results came back.

Some of our massive, centuries old Garry oaks.

Because nothing has been added to this soil for several years, and because I had removed all of the turf grass before filling the beds, I assumed that the organic matter (OM) would be quite low. Most soils that support tree growth have around 3-7% OM. Hah! Ours was over 12%! All I can figure is that centuries of leaf litter has created a rich organic soil.

So here’s lesson number one: Don’t add OM just because you think you need it. Too much OM creates overly rich conditions that can reduce the natural protective chemicals in vegetation. This means pests and diseases are more likely to be problems.

I think these may be the lowest P levels I’ve seen in home garden soils.

I was pleased to see our P level was low. First soil test I’ve ever seen in my area where P was below the desirable range! Does that mean I’m adding P? No – because there is no evidence of a P deficiency anywhere in the landscape. And my garden plants are growing just fine without it.

No sign of any nutrient deficiencies here (though the mesclun mix got out of control).

Lesson number two: Just because a nutrient is reportedly deficient, look for evidence of that deficiency before you add it. It’s a lot easier to add something than it is to remove it.

Likewise, our other nutrient values are just fine, and I was pleased to see that lead levels were low. Given that this is an older house that had lead paint at one time, and given the fact that the soil being tested was adjacent to the house, I was prepared for lead problems.

Surprisingly low lead given the original location of this soil next to an older house.

However – we do have high aluminum in the soil. Exactly why…I don’t know. Perhaps the soil is naturally high in aluminum? There’s no evidence that aluminum sulfate or another amendment was ever used. In any case, that was an unexpected result that does give us some concern for root crops. I’ll be doing some research to see what vegetables accumulate aluminum.

The aluminum levels may bear some watching if I’m growing root crops.

Finally, note our pH – 4.9! This is completely normal for our area, which is naturally acidic. In addition, the tannic acid accumulation from centuries of oak leaves has undoubtedly pushed the pH even lower. Are we going to adjust it? Again, no. There is no evidence of any plant problems, and even our lawn is green. Why would we adjust the pH if there is no visual evidence to support that?

Which leads to lesson number three: Don’t adjust your soil pH just because you think you should. If your plants are growing well, the pH is fine. Plants and their associated root microbes are pretty well adapted to obtaining the necessary nutrients. If you have problems, don’t assume it’s a pH issue. Correlation does not equal causation! You’ll need to eliminate all other possibilities before attempting to change your soil chemistry. And remember it is impossible to permanently change soil pH over the short term. Permanent pH changes require decades, if not centuries of annual inputs (like our oak leaves).

The cat agrees – no pH issue with this lawn.

Will I test my soil again? Probably not. I have the baseline report and since I don’t plan to add anything I don’t expect it to change much. If I had a nutrient toxicity I would retest until the level of that nutrient had decreased to normal levels. But with everything growing well, from lawn to vegetables to shrubs and trees, there really is nothing to be concerned about.

Viburnum plicatum (I think) is one of many established shrubs on the property.

Lesson number four: Unless you have something in your soil to worry about, don’t.

Ripe for the picking: Which fruits keep ripening after harvest?

“Will my peppers continue to ripen? How about my eggplants?” It is common knowledge to most gardeners (and home cooks) that tomatoes will ripen on the kitchen counter, as will bananas and several other fruits. You know that one day your bananas look perfectly ripe and the next they’re a brown mush But does this work for all fruits? We often get questions about whether specific fruits will continue to ripen after picking. And the answer is….. it depends.

One of these fruits is not like the other

The answer as to whether a fruit will continue to ripen after harvest depends on which one of two groups it falls into. These groups are climacteric and non-climacteric fruits. In short, climacteric fruits are the ones that will continue ripening after harvest and non-climacteric fruits are ones that don’t ripen after harvest.

This refers to the “climacteric phase” of fruit ripening where there is an increase in the gaseous plant hormone ethylene and an increase in respiration, which drives the ripening process. It is the climacteric fruits that will keep ripening once they’ve been harvested, thanks to ethylene. The only stage of maturity for non-climacteric fruits after harvest is…..compost.

As long as you’re green, you’re growing. As soon as you’re ripe, you start to rot. -Ray Kroc

Almost all fruits produce ethylene, but non-climacteric fruits produce them at much lower levels and do not rely upon it as the main driver of ripening. I’ll go into a bit more detail in a bit, but first – which fruits are climacteric and which are non-climacteric?

Common Climacteric Fruits	Common Non-Climacteric Fruits
Apple	Brambles (raspberry, blackberry, etc).
Apricot	Citrus (oranges, lemons, limes, etc.)
Avocado	Eggplant
Banana	Grape
Blueberry	Melon (including Watermelon)
Cantaloupe / Muskmelon	Pepper *
Cherry	Pumpkin
Fig	Squash (summer and winter)
Kiwi	Strawberry
Mango
Papaya
Pawpaw
Peach
Pear
Plantain
Plum
Tomato
Cherry
*Some evidence of climacteric ripening in hot peppers

The ripening process

Ripening is genetically programmed – meaning that it is highly dependent on processes that are regulated by genes and it specific to each species. Parts of the process are started and stopped due to the transcription and translation of genes, which are in turn controlled by signals such as chemical compounds, physiological stages of the plant, climate, and so on. These ripening processes have a lot of end results – sugars accumulate in the fruit, pigments develop, some compounds that have pleasant flavors develop while others that are unpleasant are broken down, some of the pectins in the fruit break down to make it softer, and on and on.

Tomatoes – the classic climacteric fruit

Research shows that ethylene, the simple little gaseous hormone plays a crucial role in the ripening of climacteric fruits by altering the transcription and translation of genes responsible for ripening. Ethylene is the dominant trigger for ripening in these plants. Ethylene receptors in the cells are triggered by the presence of the gas which leads to cascade effect. This is why ethylene can be introduced from other fruits to trigger ripening in fruits that aren’t ready to ripen. If you’ve heard of the tip to put an apple in a bag full of some other fruit to get it to ripen, it actually works – as long as it is a climacteric fruit.

The same ripening processes happen in non-climacteric fruit as well, but they are not dependent on the presence of ethylene. In fact, these pathways are also present in climacteric fruits – the ethylene-dependent processes are just the dominant (and faster) way that they ripen.

Controlling ripening

The dependence on ethylene for a vast majority of fruits to ripen has been used by farmers and the food industry for a long time to keep climacteric fruit more stable for shipping. These fruits are harvested “green” before they ripen and shipped unripe since they are much firmer and much less likely to get damaged in transit. These days, bananas, tomatoes, and other climacteric fruits are likely to be given a treatment that temporarily inhibits the ethylene response before harvest or shipping to extend their shelf life further. Once they’re close to their final destinations they’ll either be allowed to ripen on their own or given a treatment of ethylene to speed back up the ripening process.

What we gain in shelf-life and reduced food waste we do lose in a bit of flavor. Since the fruits are no longer attached to the plant when they ripen they don’t have the chance to transport more sugars and flavor compounds from the mother plant. So “vine ripened” fruits do have a bit more sweetness and flavor than those that are picked green. Having just gotten back from Rwanda, a country where bananas are a common staple food I can attest that the ones that ripen on the plant are much sweeter than those we get shipped in to the US – you know, the ones that will ripen next week sometime if you’re lucky. There were even some in our group that don’t care for bananas here that loved the ones we had at breakfast every morning.

One possible direction for biotechnology is the engineering of plants to alter or eliminate the ethylene ripening response to reduce food waste and spoilage. Since many genes that are responsible for ethylene production such as enzymes that catalyze the production of ethylene precursors, or proteins that serve as ethylene receptors have been identified, work is being done to develop delayed ripening by altering or knocking out these genes in a variety of crops.

Sources

Alexander, L., & Grierson, D. (2002). Ethylene biosynthesis and action in tomato: a model for climacteric fruit ripening. Journal of experimental botany, 53(377), 2039-2055.

Pech, J. C., Bouzayen, M., & Latché, A. (2008). Climacteric fruit ripening: ethylene-dependent and independent regulation of ripening pathways in melon fruit. Plant Science, 175(1-2), 114-120.

Lelièvre, J. M., Latchè, A., Jones, B., Bouzayen, M., & Pech, J. C. (1997). Ethylene and fruit ripening. Physiologia plantarum, 101(4), 727-739.

The Scoop on Poop: Manure in the Vegetable Garden (and potential food safety risks)

“Can I use manure to fertilize my garden?” That’s a common question we get in Extension and on the Garden Professors page. The answer is absolutely, but there’s a “but” that should follow that answer that not everyone shares. And that is…but for fruits and vegetable gardens the manure you apply could be a potential source of human pathogens that could make you or your family sick. There are procedures and waiting periods you should follow to reduce the potential risk to human health from pathogens in manure and other animal products.”

Why manure?

First, application of manures to garden and farm production spaces is a good use of nutrients and provides a way to manage those nutrients to the benefit of growers and the environment. Using the concentrated nutrients in the manures to grow crops reduces what washed downstream in the form of pollution. In addition to adding nutrients to the soil, application of manure and other animal byproducts (bone meal and blood meal, for example) add organic matter to the soil, which improves soil texture, nutrient retention and release, and supports beneficial microorganisms.

Typical N-P-K composition for some manures and composts. Source: UC Davis

For organic production, both in home gardens and on farms (certified organic or not), manure and animal products are an important input for fertility. For the most part, manures offer a more concentrated (higher percentage) of nutrients by weight than composts composed only of plant residues, so less is usually needed (by weight) than plant composts to apply the same amount of nutrients.

While the nutrient levels of manures and composts can be highly variable, there are some general ranges that you can use to plan your application based on the needs you find in your soil test. (And you should be doing a soil test, rather than just applying manure or compost willy-nilly. Just because the nutrient concentrations are lower than a bag of 10-10-10, you can still over-apply nutrients with composts and manures).

So what are the hazards?

As you’ve probably realized from bathroom signs and handwashing campaigns, fecal material can carry a number of different human pathogens such as E. coli and Salmonella. The major risk around application of manures to edible crops is the possible cross-contamination of the crop with those pathogens. The number one hazard leading to foodborne illness from fresh produce is the application of organic fertilizers – mainly manure, but also those other byproducts like blood meal and bone meal. Add in the fact that the consumption of raw fruits and vegetables has increased over the last decade or more, and you’ll soon understand why Farmers who grow edible crops must follow certain guidelines outlined in the Food Safety Modernization Act (FSMA, which you’ll hear pronounced to as fizz-mah) to reduce the potential risk that these pathogens pose to people who eat the crops. Right now, only farms with a large volume of sales are required to follow the guidelines, but smaller producers are encouraged to follow them as best practice to reduce risk and liability. And while there isn’t a requirement for home gardeners to follow the guidelines, it is a good idea to understand the risks and incorporate the guidelines as best practice. It is especially a good idea if the produce is being eaten by individuals who are at higher risk of foodborne illness – young children, the elderly, or those who are immunocomprimised.

The recommendations are also suggested when there’s contamination from unexpected or unknown sources like when vegetable gardens are flooded (click here for a recent article I wrote to distribute after the flooding in Nebraska and other midwestern states).

Recommendations to reduce risk

As previously stated, while these recommendations have been developed for produce farmers, research showing the potential hazards of applying manures means that it is a good idea for home gardeners to understand and reduce risks from their own home gardens.

The set of guidelines outlined by FSMA cover what are called Biological Soil Amendments of Animal Origin (BSAAO – since we government types love our acronyms). Here’s the “official definitions” used in the rules for produce farming:

A Biological Soil Amendment is “any soil amendment containing biological materials such as stabilized compost, manure, non-fecal animal byproducts, peat moss, pre-consumer vegetative waste, sewage sludge biosolids, table waste, agricultural tea, or yard trimmings, alone or in combination”.

A Biological Soil Amendment of Animal Origin is “untreated: cattle manure; poultry litter; swine slurry; or horse manure.”

Image result for manure — Now that’s a pile of crap!

For BSAAO (we’ll call it raw manure), manure should only be applied to the soil and care should be taken not to get it on the plants. There’s also a waiting period between applying the manure and when you should harvest the crop. The length of the waiting period depends on whether the edible part of the crop comes in direct contact with the soil. Right now the USDA is still researching the appropriate waiting period between application and harvest, so the general recommendation until then is to follow the standards laid out in the National Organic Program (NOP) standards. Research shows that while pathogens may break down when exposed to the elements like sun and rain, they can persist for a long time especially in the soil.

For now, here are the recommendations:

For crops that contact the soil, like leafy greens (ex: lettuce, spinach, squash, cucumbers, strawberries) the suggested minimum waiting period between manure application and harvest is 120 days.

For crops that do not contact the soil (ex: staked tomatoes, eggplant, corn) the suggested minimum waiting period between manure application and harvest is 90 days.

For farmers following FSMA, the waiting periods could change when the final rule is released – some early thoughts are that it could increase to 9 – 12 months if the research shows a longer period is needed.

What about composted manure? Is it safe? The guidelines indicate that there isn’t a waiting period between application of manure that has been “processed to completion to adequately reduce microorganisms of public health significance.” But what does that mean? The guidelines lay out that for open pile or windrow composting the compost must be maintained between 131°F and 170°F for a minimum of 15 days, must be turned at least 5 times in that period, must be cured for a minimum of 45 days, and must be kept in a location where it can’t be contaminated with pathogens again (animal droppings, etc). Farmers have the added step of monitoring and thoroughly documenting all of the steps and temperatures. Now we know that that’s a bit of overkill for home gardeners, but suffice it to say that the cow manure that’s been piled up to age for a few years that you got from the farm down the road doesn’t meet that standard.

Image result for compost — Failure to maintain proper temperature on composted manure could mean that your goose is cooked, though this thermometer doesn’t have that setting.

“Aged” manure ≠ “processed to completion to adequately reduce microorganisms of public health significance.” So unless you know for sure that you’ve reached and sustained the appropriate temperatures in your compost, you should assume that it would be considered a BSAAO subject to the 90/120 waiting period. Bagged manure you buy at the garden center is likely to be composted “to completion” or may even have other steps to reduce pathogens like pasteurization. Sometimes the label will indicate what steps have been taken to reduce pathogens, or even state that it has been tested for pathogens.

The recommendations also specifically mention compost teas and leachates (a topic we handle with much frequency and derision here at the GPs, since there’s not much science to back up their use and I mention here with much trepidation). For the sake of food safety, any tea or leachate should only be applied to the soil, not the plant. And for home compost that doesn’t even contain animal manure the 90/120 day waiting period should still be observed in most cases since some of what goes into home compost is post-consumer. Since we put pieces of produce in there that we’ve bitten from or chewed on (post-consumer), plus some animal origin items (eggshells) there’s the potential that we could contaminate the compost with our own pathogens – and the environment is perfect for them to multiply.

The Bottom Line

While these guidelines and rules for farmers may just be best practice recommendations that we can pass on to home gardeners, common sense tells us that taking precautions when applying potential pathogens to our edible gardens. An ounce of prevention is worth a pound of cure, especially when were talking about poop.

Sources/Resources:

Produce Safety Alliance producer training materials
National Good Agricultural Practices Program: Soil Amendments (Cornell)
Biological Soil Amendments of Animal Origin (FDA Fact Sheet)
FSMA Final Rule on Produce Safety (FDA)
Manure in Organic Production Systems (ATTRA)

Supplemental Lights for Home Seed Starting and Indoor Growing: Some Considerations

Whether you’ve already got seedlings growing away or getting ready to start your annual indoor seed starting, one of the important factors in seed starting is light. (Last month I covered heat, which you can see here). Questions like “Do I need to use supplemental light or can I use a window?” and “What kind of light do I need to use?” are ones we often get from gardeners – new and seasoned alike. So I thought I’d take a little time to talk about light – the factors that are important for plant growth some ways that you can make sure you’re providing the right kinds and amounts of light to your new seedlings. Keeping these ideas in mind can help you choose lights for your seeds starting (or other plant needs), whether it is a simple shop light ballast from the hardware store, a pre-fab light cart system, or even higher-tech LED system.

Plants require light for several of their functions, most importantly the process of photosynthesis. The green pigments in plants (Chlorophyll A and B) act as receptors, gathering electrons from the light to use as an energy source to manufacture glucose, which is stored in the plant in a number of ways and then ultimately broken down in respiration to release energy to support plant functions. There are three aspects to light that gardeners should keep in mind for supplemental lighting: quality (color), quantity (brightness/intensity), and duration (day/night length).

Duration is a relatively simple concept when it comes to seeds starting and light set-ups. Gardeners will want to try to mimic the natural environment that will be provided by the sun. For the most part, aiming for 16 hours of light and 8 hours of dark is standard. This gives the plant sufficient light, but also provides a rest period which can be important for plant functions. Most gardeners find it handy to invest in timers to turn lights on and off, rather than trying to remember to do it themselves. This can be a simple on-off set up from the hardware store (after-holiday shopping can be a good way to pick them up on sale in the string light section) to something more elaborate from grower suppliers. Duration could be more important if you’re doing longer term growing beyond seeds starting, as day length affects initiating of flowering in some plants.

Intensity refers to how bright the lights are. Some lucky people have big windows with lots of bright light for starting seeds, but even for them intensity (and duration) may not be enough during the shorter, grayer days of winter. Growing in bright windows can sometimes be a challenge because the light is coming from the side rather than above, so plants often grow toward the window and need to be rotated. Supplemental light can increase intensity and lengthen duration, even for plants grown in windows.

Most commonly, light bulbs are sold by wattage as a measure of their energy (light) output. Standard tube florescent lights are generally around the 40 Watt level, but some of the full spectrum plant lights come in 54W options. If you can find it, the higher wattage can make a big difference in the intensity of light and thus the production of your plants. Even at the higher wattage, you’ll want to get a ballast that holds at least two bulbs (and some grow light ballasts hold more). You can further control the intensity of light reaching your plants by increasing or decreasing the distance between the plants and the lights. This is why the pre-made plant carts have a chain or other mechanism for you to raise and lower the lamps. For fluorescents, lights are sometimes lowered to around an inch above the canopy of the plants. For high intensity LEDs, the distance may need to be more. (If you’re using lights for long-term growth of, say houseplants, you’ll have to experiment with the distance to meet the intensity needs of the plants – closer for high light plants and farther away for low light plants).

Light Quality: The Rainbow Connection

Sunlight, or white light, is composed of all of the colors of the spectrum. Think back to art class and our friend ROY G BIV – the colors of the rainbow. There’s also parts of the spectrum that we don’t see like ultraviolet and infrared. For photosynthesis, plants mostly use light in the red and blue spectrum (referred to as Photosynthetic Active Radiation, or PAR), though almost all of the colors have some sort of effect or function on plants. Blue light has a role in promoting vegetative growth in plants, while red has a role in promoting flowering.

For most applications, supplemental light for seed starting or other indoor growing should be full-spectrum. You can achieve this in a variety of ways – buying specific full-spectrum plant light bulbs is the best, but you can buy non-plant specific full spectrum bulbs as well. For small-scale home growers and beginners, it can be as simple as buying a shop light ballast at the hardware/box store with a full spectrum bulb. For more intensive or large-scale growers, there are lots of sources for higher-end, full spectrum grow lights that you can buy from specialty garden retailers, but these are often more than what home gardeners starting seeds indoors need.

Fluorescent vs LED

Image result for fluorescent shop light — Typical shop light ballast

These days you might be presented with a choice of lights – fluorescent vs. LED. There are some positives and negatives to each. While they have a higher up-front cost, LEDs use much less energy than fluorescents and can save money over several seasons of use. The reduced energy usage also means there’s less energy loss in the form of heat, which can be a positive if you are always struggling with creating excess heat that burns your plants, but a negative if you’re relying on that heat to help keep the temperatures up (see my article from last month on heat and seed starting) or have issues with drying out your growing media. Fluorescents on the other hand can be more affordable up-front, but have a higher energy usage that will result in higher electric bills over time.

understanding the basics of grow lights for indoor plants and indoor gardening — LED grow light via Shutterstock by nikkytok

You might have noticed in your searching or in visiting some growers that LED lights for plant growth come in either white (full spectrum) or a red/blue combination which end up giving a purple light. Since LEDs give a larger control over the spectrum of light, growers, especially larger scale intensive operations, use these red/blue combinations as a means to add further energy efficiency since it is the blue and red spectra that are the photosynthetic. By eliminating the spectra that are largely reflected rather than absorbed, less energy is used. This is useful in hydroponic and vertical farming systems where short-term crops are being grown quickly and where profit margins can be slim.

You can read (and listen to) more about light in the Joe Gardener podcast and article on seeds starting I was interviewed for last year with Joe Lamp’l.

However, research has emerged in the last few years that expanding the spectra of light in LED systems increases production. Research has shown that incorporating green LEDs significantly increases production over just red/blue LEDs (some of that research was by Kevin Folta, who is one of the leading science communicators on biotechnology). While green plants largely reflect rather than absorb green light, it does have some effect on plant functions. (Research also shows that adding the green makes the light appear a little more natural to workers in facilities like greenhouses and makes it easier to see issues with the plants – the purple of the red/blue systems washes out the plants and makes it hard to see differences in leaves like diseases).

So if you’re looking at LEDs for seeds starting, and especially if you’re looking at them for longer term indoor plant growing, stick with full spectrum or explore one of the LED systems that incorporates green. Though don’t be afraid to experiment with the colorful LED options – I have a small red/blue system to supplement light to my office potted lime. The key is to experiment and shop around – every gardener’s need for supplemental light is different and the solutions to those needs are different. Don’t be afraid to start small with that shop light from the hardware store before working your way up – especially if you’re just starting a small amount of seeds in the spring.

Feel the Heat: Temperature and Germination

In most parts of the country it is time to dust off the seed starting trays, pick out your favorite seeds, and get a little plant propagation going on. There’s definitely a lot of science (and perhaps a bit of art) to successful seed starting. While the process starts (and relies on) the imbibition of water, one of the biggest factors that affects the success, efficiency, and speed of seed germination and propagation is temperature. Germination relies on a number of chemical and physical reactions within the seed, and the speed and success of those reactions is highly temperature dependent. Respiration, where the seed breaks down stored carbohydrates for energy, is probably the most notable process involved that is temperature dependent (source). Think of it in terms of a chemical reaction you might have done back in your high school or college chemistry class – there’s an optimum temperature for the reaction and any lower and higher the reaction might slow down or not happen at all.

Thinking of it this way, seeds and germination are just like Goldilocks and her porridge – there’s too hot, too cold, and “just” right. Seeds are the same way – there’s a “just right” temperature for germination. The seeds of each species has a different optimal temperature for germination with a range of minimum and maximum temperatures for the process.

Why is important that seeds are started at their optimal temperature?

The optimal temperature is the one at which germination is the fastest. This may seem to only have consequences for impatient gardeners, but slower germination speeds increase the days to emergence for the seeds, which in turns means that the seeds and seedlings have a greater chance of failure. The early stages of germination are when seedlings are most susceptible to damping off, which can be caused by a number of fungal pathogens (Fusarium spp., Phytophthera spp., Pythium spp., etc.) that basically cause the seedling to rot at the soil level. These pathogens (as well as decomposers in some cases) can cause seeds to rot or decompose before emerging as well. That’s why you’ll sometimes see seeds that are slow to germinate (or traditionally direct sown like corn, beans, and peas) treated with those colorful fungicides. The fungicide gives the seed and seedling a little bit of protection (for a week or so, depending on the product), which is handy if you accidentally sow them before soil temperatures are optimal or if the species is slow to germinate.

If emergence is really slow, there’s also the possibility of stunting or failure due to exhaustion of the stored carbohydrates that the seed relies on until it begins photosynthesis. So the closer to the optimal temperature the seed is, the faster the emergence and the highest percentage of germination success.

What does this mean for home gardeners?

Whether you are starting seeds indoors or direct sowing outdoors, knowing the germination temps can help increase your likelihood of success. You can find a variety of resources for the optimal germination temperature for your selected crops. In general, most warm season plants, like tomatoes, peppers, and summer flowers are in the 70-80 °F range. This is why most of the warm season crops are started indoors – so temperatures can be controlled to higher levels.

For vegetable crops, here’s a good resource for basic germination temperatures. And here’s one for a few annual flowers.

Many of the cool season crops germinate at much lower temperatures, which means many of them can be directly sown early in the season rather than started indoors. Crops such as spinach, lettuce, and other leafy greens have these lower germination temps and typically perform better if germinated at lower temps.

It should be noted that this is for the soil temperature, not the air temperature. If you’re starting seeds in your home, most people don’t keep their homes in the 75 – 80 degree range in the winter. Many commercial operations use warmed tables or beds for seed starting, rather than heating the whole facility to the necessary temp – it would be expensive. For home growers, supplemental heat mats can help increase soil temp without having to heat a whole room. In a pinch, you can even clean off the top of your fridge and keep seedlings there. It is higher up in the room (heat rises) and most refrigerators create some amount of external heat as they run.

For any seeds that you’re direct sowing outdoors, whether they require higher or lower germination temperatures, you’ll have more success if you plan your sowing around soil temperatures rather than calendar dates (planting calendars can be good for estimation, though). Investing in a soil thermometer can offer detailed information on the specific temperatures in your garden soil. Or, if you have a good weather station nearby many of them have soil temperature probes that could give you a good idea of what the soil temperatures are in your region.

Direct-sown lettuce germinating for a fall crop

But don’t let the cool/warm season crop designation fool you – the Cole crops like cabbage and broccoli actually have an optimal germination temperature on the warmer side, but grow better in cooler temperatures to keep them from bolting (flowering). This is why they need to be started indoors for spring planting, but you can start them outdoors (even trying direct sowing) for fall crops – they germinate in the heat and then slow growth as the temperatures drop.

Compost in Seed Starting Mix: Recipe for Success….or Failure?

A recent question posted to the Garden Professors blog Facebook group (a place where you can join and join in conversation of garden science) asked about the potential for compost added to seed starting media to cause failure in germination. It is a good question, and one that seems to have several different camps – from garden hero author folks swearing by it in their (non-peer reviewed) books, to fact sheets saying it isn’t a good idea.

I’ve always promoted that the best practice for seeds starting is using a sterile media to avoid such problems as damping off. Many of the problems I’ve heard associated with compost and seed starting are that improperly finished compost can introduce disease microorganisms to the media or cause phytotoxicity, it can make the mix too heavy and thus create anaerobic conditions that starve emerging seedlings of oxygen or cause decomposition, and there is the potential for residues of herbicides in composts using farm waste, manure, or lawn clippings as a feedstock. But does compost really pose a risk to seed starting? I decided to take a very quick spin through the literature to weigh the possibilities. Here are some of the potential issues and what a quick glance at the literature says.

Keeping the Germs out of Germination

Compost, even finished compost, has a high microbial activity. For the most part, the fungi and bacteria in compost are good guys that pose no threats to plants: they are decomposers (that only break down stuff that is already dead) or neutral. But incorrectly managed compost can also harbor fungi such as Pythium and Rhizoctonia that cause damping off or even other diseases such as early and late blight if diseased plants were added to the compost and sufficient heat levels weren’t maintained. Composts that don’t reach 140°F and maintain that temperature for several days to kill off potential pathogens run the risk of introducing diseases into seedlings.

Many promote the use of compost and compost products for potential antagonistic effects on bad bacteria. We’ve discussed compost tea and the lack of conclusive evidence that it has any effect on reducing disease here many times before, and this article found that there is no significant effect of compost tea on damping off. Some other articles, such as this one, did find that commercially prepared composts added to media did suppress damping off. However, it is to be noted that these are commercially prepared composts, which have a strict temperature requirement and often require testing for pathogen and bacterial populations. Many home composters aren’t as proficient at maintaining temperatures suitable for pathogen elimination.

Even if the compost is pathogen free, introduction into a germination media could potentially increase the population of pathogens already present in the media (or that land on it from the air) by providing a source of food for bacterial and fungal growth. The sterile mixes aren’t just sterile from a microorganism perspective, they’re also sterile from a nutrient perspective as well to help inhibit potential pathogen growth. The seeds come with their own food, so it isn’t needed for initial germination – the seedlings should be moved to a more fertile mix once they’ve established their first set of true leaves.

Image result for damping off — Damping off, source hort.uwex.edu

You may be saying- “but we also direct sow seeds outdoors, where there’s lots of pathogens present in the soil.” While this may be the case, damping off is still a definite problem in direct sowing and the loss of investment in materials, lights, and time is generally much lower (and less painful) than in indoor seedling production. This is especially the case for large operations or for home gardeners who grow lots of stuff from seed.

This is the main issue that leads to the best practice recommendation to use a sterile seed-starting mix that doesn’t contain compost. If a mix contains compost, it should be from a commercial enterprise that follows best practices or pasteurized.

Maturity isn’t just for wines, cheeses, and people

Continuing to talk about proper composting, improperly finished compost that hasn’t properly matured (finished composting) can also lead to problems with seed germination. Unfinished compost can still have woody material included, which has a high C/N ratio and also contain/release phytotoxic compounds during the decomposition process. The presence of decomposition microorganisms in a high C/N ratio means that there is still decomposition happening, which requires nitrogen for the process. With absence of nitrogen in the media, the nitrogen from the seed or the seedling can be leeched out, effectively causing mortality after or even before germination. The tender seedling serves as a source of N for the decomposing fungi.

We’ve had this discussion before when it comes mulch. While mulch is perfectly fine on top of the soil, if it gets mixed into the soil there could be potential implications on N availability.

A germination bioassay is one tool commonly used to test for compost maturity. Quickly germinating (and inexpensive) seeds are germinated on the compost (or on filter paper soaked with an extract from the compost in some commercial operations). The rate of germination vs germination failure can give some insight into the maturity of the compost. This paper discusses the use of the technique for commercial sawdust compost used for potting media.

You can use a bioassay of your own to test for compost maturity (or herbicide persistence, discussed later) for applications in your garden. Sow an equal number of inexpensive, fast-germinating seeds like radish or lettuce sown on the compost with a control sown on moist paper towel in a bag. Compare the number of germinated seeds and thriving seedlings after several days to see if there is an issue with the compost.

Keeping Things Light

One other quality required for seed starting media is a good level of porosity (pore spaces) for the media to hold air. Air (oxygen) is important as it is needed by the roots for respiration. If the media is too heavy or holds too much water you run the risk of hypoxia, or lack of oxygen, in the roots. This can result in root die off and subsequent seedling failure. Most seed starting media are composed of very light materials such as peat moss, coir, vermiculite, or perlite for this very reason. Compost, by nature, is a more dense material with less porosity and has a higher water holding capacity. Therefore incorporation of too much compost can create the potential risk of compaction or excessive water holding in the mix.

When Persistence Doesn’t Pay Off

Most herbicides break down during the composting process through a variety of physical and biological interactions. However there have been reports of some herbicides that are persistent after the composting process, resulting in a residue that could damage plants grown using the compost (see this paper for some examples). Many of the reports show the damage manifesting in mostly large applications of compost to gardens. However, the more fragile nature of germinating seeds and young seedlings make them especially susceptible to herbicide residue damage. For further discussion (and examples of bioassays used to detect herbicide residues), check out this paper.

So the potential for pathogens, risk of improperly matured compost, effect on porosity, and potential for herbicide persistence present some significant risks to germination if they are incorporated into seed starting media. These are the risks that cause many sources to promote using sterile seeds starting media, and I think the advice is well founded. While some may not experience these possible issues, the potential is still there.