vegetable gardens – The Garden Professors™

Crop rotation makes the garden go ’round

You might have heard of the concept of crop rotation, or even have had someone tell you that you should be practicing it in your home garden. But does this practice developed for use on large-scale farm fields work for small-scale home gardening or backyard farming? And if so how do you even do it? Let’s take a quick look at the practice and learn how you might implement it in your own garden, no matter the size, if it is practical to do so.

The Benefits of Crop Rotation

The benefits of crop rotation touted to home gardens for annual fruit and vegetable plants focus mainly on Integrated Pest Management. Keeping certain crops in one spot in the garden year after year can lead to a build up of plant disease microorganisms and insect pests in that area. These findings are largely based on large-scale production systems, like where whole fields that are hundreds of acres are grown as the same crop and switched from year to year. But there still is some benefit for the small-scale gardener, especially with disease build up from soil-borne diseases or debris left in the garden.

The benefits are less clear for insect pests. Sure, there might be fewer insects that move from spot-to-spot or bed-to-bed in your home garden, but we must remember that insects can fly and a journey of a few feet from one raised bed to the next might not be much of a barrier. It is sort of like when people ask us if using grub control in the lawn will control Japanese Beetles in their garden. There will likely be a small reduction in the population, but we remind folks that Japanese Beetles can fly pretty good distances, so control will be minimal.

Crops in rows and beds can be easy to rotate.

Benefits of crop rotation lay below the soil. Different plants use different ratios of nutrients and some even add nutrients to the soil. Legumes have nitrifying rhizobia bacteria that colonize their roots in a symbiotic relationship and there is a net nitrogen increase in the soil after legumes such as beans, peas, alfalfa, or clover have been grown. That’s why one of the major crops added to crop rotations in the “corn belt” is soybeans (or just “beans” if you’re from around these parts). The soybeans contribute nitrogen to the soil, which helps in the production system because corn is a heavy nitrogen feeder. This is especially true if the roots are left behind or if the legume is incorporated like a cover crop.

Given that crops uptake nutrients in different ratios and that some input nutrients into the soil, by planting the same crop year-after-year in the same spot you can end up with nutrient imbalances, especially in raised beds or small plot areas. Using cover crops and incorporating them into the soil is another facet of crop rotation that can build soil organic matter and address nutrient issues. Fertility can also be adjusted with fertilizer and compost, but using rotation will help with overall management.

Rotating in root crops like carrots, radishes, and turnips can also allow for some good soil aeration, especially in no or low till garden situations.

There’s also research that shows that different plants exude different compounds, like sugars, etc. that attract and feed different microorganisms and rotations which help increase the diversity of the microbiome in the soil. This will not only improve the soil ecology in your garden, but a healthy microbiome can compete with some disease-causing microorganisms to reduce the likelihood of soil-borne disease. Most of us have been told that eating cultured yogurt increases good microbes in the gut and can reduce the likelihood of some illnesses- its kind of like that, but for plants.

How to rotate crops (especially in small gardens)

While crop rotation might not solve all the world’s problems or your garden insect issues, there are still definite benefits to the practice if you can manage it. For some people garden space is too limited. How do you rotate crops when all you grow is a few tomatoes in the corner of a flower bed? Other than putting them somewhere else every year, there isn’t much you can do.

If you have a large garden and especially if you have one that’s a larger tilled-up spot (don’t get me started on tillage), you should definitely be implementing rotation. While I typically garden in raised beds, that usually makes it easy. Crop rotation usually occurs by plant family, so plant families rotate to a new bed each year (more on plant families in a bit).

In general, a crop rotation plan should mean that plants from the same family aren’t grown in the same place in the garden for at least four years if possible. If I plant tomatoes in “Bed A” in my garden this year, then I shouldn’t plant them in “Bed A” again for at least four years if possible. If you’re not growing in beds, then should rotate by rows or by areas of the garden.

Is this practical for every gardener? No. So I say do what you can. If you have a small space and only grow a few things and want to rotate crops, you can do so by time rotation – growing different crops each year. Or adding some container gardening to the mix and rotating crops into containers in different years. As a side note, using containers can be an effective way to rotate crops because you don’t necessarily need to rotate the plants if you can rotate (replace) the soil from year to year (or at least replace the top layers).

Crop rotation can also aid in succession planting. For example, I don’t follow my beans with corn, I follow them with garlic, which is also a heavy nitrogen feeder and works right into the garden schedule to be planted in October after the beans are done. I’ll often do lettuce or leafy greens in a bed in early spring, then thin them out to plant in tomatoes or peppers when the time comes. Incorporating intercropping where you plant different sized plants around each other, like my lettuce and tomatoes, can also be an effective addition to crop rotation.

For a good crop rotation, you’ll want to plan. This would involve sketching/mapping out your garden or at least labeling different beds or spaces. Then creating a chart for each space where you list what crops will be grown there for the next several years will help you plan out to make sure you are giving ample time for the rotation process.

Keeping it in the family

Family members share lots of things and plant families are no different. Not only do some crops look similar but they can often share the same diseases. Keeping families in mind planning crop rotations is one of the easiest methods to keep track of which crops to include in a rotation. For small gardens it might make sense to keep members of the family together. For example if I only have four beds, I would want all the family members together so that I can have a true four year rotation. In larger gardens, realizing that crops are in the same family will help plan out for rotation of multiple crops.

Also keep in mind that some crops will overwinter (depending on where you are) so a fall crop might also be a spring crop in the same bed the following year. Some crops are also biennial depending on your local climate. For example unless you live in an area with winters cold enough to kill them, swiss chard, onions, and other crops will survive the winter and grow the following spring. I typically leave most perennial vegetables like asparagus and rhubarb, as well as perennial herbs like oregano and chives, out of rotations and put them in their own specific place in the garden (or elsewhere) since they don’t need to be replanted every year.

I’ve put together a list of common annual crops by family, as well as an example crop rotation plan for single garden bed/area. As an example, if I had four raised beds I would use this plan for each bed, starting in a different year for each bed so that I grow all the same crops each year, just in different beds.

Common crops by family. Developed from UF/IFAS Extension

A crop rotation plan I might use in my own garden. If I had four beds, Year 2 crops would be Year 1 crops for bed B and so on.

In conclusion…

Crop rotation isn’t an end-all, be-all garden practice. If you can institute some sort of rotation in your garden you’ll likely see long-term rewards. However, if it isn’t possible or seems like too much work just remember don’t plant things in the same place year after year if you can help it. Any rotation, even if it is hit or miss, will be beneficial.

Sources

Benincasa, P., Tosti, G., Guiducci, M., Farneselli, M., & Tei, F. (2017). Crop rotation as a system approach for soil fertility management in vegetables. Advances in research on fertilization management of vegetable crops, 115-148.

Sasse, J., Martinoia, E., & Northen, T. (2018). Feed your friends: do plant exudates shape the root microbiome?. Trends in plant science, 23(1), 25-41.

Wright, P. J., Falloon, R. E., & Hedderley, D. (2017). A long-term vegetable crop rotation study to determine effects on soil microbial communities and soilborne diseases of potato and onion. New Zealand Journal of Crop and Horticultural Science, 45(1), 29-54.

Yasalonis, A. (2019) A must do in gardening: Vegetable crop rotation (UF/IFAS Extension). UF/IFAS Extension (blog)

The complicated issue of heavy metals in residential soils. Part 3: How can we garden safely in the presence of heavy metals?

This is the last part of our discussion on gardening in soils that contain heavy metals (you can catch up on part 1 and part 2 if you need to). Today we’ll focus on the strategies you can use in your gardens and landscapes to reduce your exposure to soil-borne heavy metals.

Raised beds can be an easy solution for gardeners with contaminated soils

Test your soil!

First – and this should really go without saying – you must test your soil to determine if it contains heavy metals of concern. The COVID19 pandemic provides the perfect comparison: you can’t assume you don’t have the virus just because you don’t have symptoms, and you can’t assume your soil doesn’t have toxic heavy metals just because you don’t think it does. The only way to know for sure, in either case, is through testing.

This eyesore did more than spoil the view.

Most soil tests routinely report aluminum, lead, zinc, and aluminum. Other metals, such as arsenic, cadmium, and chromium, may not be part of a basic soil test and you will need to request additional tests if these metals are likely to be present. Often, county health offices will provide free soil testing if you live in a region where there are known contaminants. For example, I live in the Tacoma area where large amounts of arsenic were deposited for decades downwind of an aluminum smelter. Residents of Pierce County can get free soil testing because of the potential danger.

The aluminum is higher than we would like to see, though everything else looks fine.

Even if you don’t live in an area where industrial or agricultural activity may have added toxic heavy metals to your soils, your soil may naturally contain high levels of some metal of concern. As I’ve mentioned in a previous post, our soils have high levels of aluminum. Because we are not downwind of the smelter site mentioned above, I would not have assumed we had any metals of concern, given the rural location of our land, but knowing this informs my choice of vegetables to plant.

The demolition of the Tacoma smelter. Finally.

Avoid adding more heavy metals

Fortunately, many of the consumer products that contained heavy metals are now gone and no longer will add to existing levels of soil metals. But there are still sources out there that gardeners are well-advised to avoid.

Older treated timbers. As mentioned in my first post, landscape timbers were once treated with a chemical preservative containing arsenic and chromium. Even though gardeners love reusing materials (we are a thrifty bunch!), these older timbers should be removed if they are still on your property. New timbers are treated with a copper-based solution, which is a more environmentally friendly preservative.
Kelp-based fertilizers and amendments. While these products are wildly popular with gardeners, they aren’t very effective fertilizers. Moreover, some kelp species accumulate heavy metals, like arsenic, in seawater and these metals will become a permanent part of your soils. Take a look at this fact sheet for more information.
Recycled rubber mulch. This product should be avoided for many reasons (you can read more about the problems in this fact sheet). As it disintegrates it releases high levels of zinc into the soil. And while zinc is an essential micronutrient in plants (and people!), high levels are toxic.
Unregulated composts and organic products. Certified composts and other organic products have been tested for pesticide residues and heavy metals: unregulated products have not. Unless you are making your own compost from materials you know to be free from contamination, your safest bet is to purchase certified products.

If you have materials like old timbers, you should never burn them or throw them away. They need to be disposed of as a hazardous waste, much like old cans of paint, mercury-containing thermometers, etc. Eventually, we may be able to use these hazardous discards for biofuel production through pyrolysis, or extract the heavy metals from them for reuse. For now, just dispose of them in a legal and environmentally responsible way.

Cedar is naturally decay-resistant and can be a good choice for rasied beds

Suggestions for safe gardening

If soil testing reveals high levels of metals of concern, there are work-arounds to allow you to still enjoy growing vegetables safely. If your soil tests reveal that your soil is safe for growing edibles, congratulations! You may still benefit from some of the suggestions below.

Cover exposed soil with ground covers and mulches (coarse organic or inorganic materials) to eliminate metal-laden dust.
Create raised beds for edibles using untreated wood or other metal-free materials. Line the bottom of the bed with an impermeable membrane to prevent movement of soil-borne metals into the beds.
If raised beds are not possible, use large containers to grow edibles.
Avoid using galvanized tubs, as they will leach zinc (and sometimes chromium) into the soil.
Fill beds and containers with clean (i.e., tested) soils or potting media.
Don’t plant vegetables near roadways, which are a source of airborne lead.

Have I mentioned how great arborist wood chips can be in gardens?

The complicated issue of heavy metals in residential soils, part 1: What are toxic heavy metals, and where do they come from?

The popularity of home gardens is exploding as we wait out the COVID pandemic

So many of us are growing our own vegetables – either as experienced home gardeners or as COVID19-isolated novices. There is a lot of effort in figuring out garden beds, vegetable choices, and growing medium – but one of the issues rarely considered is whether there are heavy metals present in the local soil and/or growing medium. We can’t see heavy metals, or smell them, so we need to have a way of assessing their presence before we plant edibles.

In the next few months, I’ll tackle the complicated science behind this invisible threat. Today, let’s look at the heavy metals that are commonly found in garden soils and where they might come from.

What heavy metals do gardeners need to monitor in their soils?

Heavy metals are exactly that – they are dense elements that have certain chemical properties that define them as metals. In fact, most known elements are considered to be heavy metals. Fortunately, there are only a handful of heavy metals that are commonly found in residential soils. Some of these heavy metals are necessary for life – iron, manganese, and zinc, for example – but others have no known biological function. Arsenic and lead, for instance, can interfere with enzymatic activity and effectively poison biochemical pathways. There is no “safe” level of heavy metals that are not essential nutrients.

Here’s a table of the most common toxic heavy metals that might be found in your soil, and possible anthropogenic sources:

Heavy metal	Sources of contamination
Aluminum*	Smelting
Arsenic	Pesticides, smelting, treated timbers (old)
Cadmium	Paint
Chromium	Fly ash, metals industry, paint, leather tanning, treated timbers (old)
Lead	Gasoline (leaded), paint, pesticides, plumbing, smelting, solder
Nickel	Plumbing, smelting

*Aluminum is a light metal, not a heavy metal, but has similar biochemical poisoning activity as toxic heavy metals

Some of these sources of contamination are not relevant to where I live – why do I need to test my soil?

Gardeners may be tempted to look at the chart above and feel relieved, because pesticides and paint no longer contain heavy metals, they don’t use old treated timbers, and they know that leaded gasoline is a thing of the past. What many don’t consider, however, is that heavy metals are elemental – they don’t break down, though they may change their chemical form. They are a permanent part of soil chemistry unless they are removed by physical or biological means.

Old garden pesticides were very effective – this one contained both lead and arsenic

The underlying soil in housing developments built on old agricultural land often contains high levels of arsenic – because that was the active ingredient in pesticides many decades ago. If the topsoil was removed during construction, it may have been taken to a commercial soil facility where it would have been used to create landscape fill mixes for new landscapes elsewhere. The same is true for land near older roadways where lead from gasoline was released from vehicles over many decades. Not only are lead, arsenic, and other heavy metals in the soil, they also end up in the air when soil is disturbed by erosion or tilling.

Nearly all soils contain some level of some heavy metals. They are naturally occurring, after all, so their presence is not necessarily from anthropogenic activities. Regardless of the source, it’s important to know whether any of these harmful elements are in your garden soils, especially if you are growing edibles. A soil test is the only way to find out.

Here is a soil test of my own raised bed system. While my nutrient levels are optimum, and lead is very low, the aluminum level is quite high. What should I do?

Why aren’t there guidelines on heavy metal uptake in vegetable gardens?

It would be ideal if there was a list of “safe” and “dangerous” vegetables to plant when heavy metals are present. Unfortunately, real life rarely fits into lists and there are numerous sources of variability. Next month I’ll discuss the complications that arise when we consider plant species, heavy metals, and environmental variables.

Tools, tips, and terrible traditions for raised beds – Part 1

Raised beds a month after planting. Adult beverage not harvested here.

Many of us are sheltering at home during the COVID19 outbreak, and that might mean you’re spending more time in the garden. It certainly seems to be true based on my Facebook feed. And given that even more people are showing interest in growing their own food, I thought some practical posts on raised beds dos and don’ts might be fun. John Porter did a nice review of some of the misperceptions about raised beds last year, and that’s worth reading as well. This week’s post will be on siting and materials needed for building a raised bed. At the end of the post is a list of online resources with more information.

Trees to the south will shade vegetable gardens throughout the growing season.

Location

To grow most vegetables, you need direct sunlight at least six hours per day, and more is better in terms of productivity. That means full, unfiltered sunlight, so that your seeds and plants get the entire light spectrum. You’ll need to take into account seasonal changes, like the sun’s angle and the appearance of deciduous canopies, before choosing your site. If part of your bed will unavoidably be in the shade, simply choose plants that will tolerate part- or full-shade conditions for that location.

Building materials

Construction of raised beds. Carpenter contracting not available.

We use pressure-treated hemlock and Douglas fir for our beds, which measure 8′ by 22′ (at the outside dimensions.). Modern pressure treatment uses alkaline copper quaternary, which is nothing like the toxic chromium-arsenic cocktail from earlier times. You don’t have to use wood, of course – other materials will work but do educate yourself on any potential leaching issue into the soil.

Underneath the beds is….nothing. If our underlying soil was contaminated with heavy metals or some other material, we would put down a membrane first to keep our raised bed soil separate from the contaminated soil. But we have no issues, so it’s soil next to soil, meaning we have great drainage.

Planting media

Native topsoil stockpiled from construction project.

The best material for your raised bed is actual native topsoil (if you can find it). If you don’t have enough of your own, see if anyone locally is giving away “free dirt.” People who put in decks, ponds, and other hardscape structures often don’t realize their discarded dirt is real topsoil. Do be cautious with this potential windfall. Ask about pesticides or other chemicals that may have been used in the original landscape. And you should do an initial soil test to see your baseline nutrient values. It’s easier to incorporate amendments BEFORE you fill your beds.

There are exceptions to the native topsoil recommendation – for instance, if your soil is contaminated with heavy metals from industry or agriculture, you shouldn’t use it for growing edibles. In this case, you need to use a commercial topsoil, and isolate it from the underlying soil as described earlier. Commercial topsoils can be heavily amended with compost and other organic material, meaning you have much less actual soil and will constantly need to refill your beds as the organic matter decomposes. Try to find a mix with the greatest possible percentage of topsoil.

Read the label! Is there actually soil in potting “soil”?

The worst choice of all are soilless media. This includes nearly all bagged potting “soils” at garden centers. Read the contents panel carefully – does it say the word “soil” anywhere? If it’s all organic material, you are going to have to fill your beds every year. This is both expensive and time consuming. Plus you could very well have excessive levels of some nutrients that will build every year as you add more.

As you make your decision about what to fill your raised beds with, consider what you will be growing, If you are only growing summer crops, it will be easier to amend the bed every year. If you have a winter crop, or perennial herbs, you can’t incorporate more material without destroying the existing rhizosphere and your plants. Perhaps that means you need two raised beds, or at least have a divided system.

Design

This part is really up to you! Raised beds should be high enough to work comfortably, big enough to hold what you want to grow, and narrow enough to be able to reach all the way across (for one-sided access) or halfway across (two-sided access).

We wanted a design where we could include a critter fence. Once in a while a deer might wander through our property, and rabbits certainly do. The hardware cloth fence keeps larger animals out and also provides a great trellis for beans and other climbers.

We opted for a U-shaped system, with a gate on one end. The inside edges of the beds are topped with 2×6 boards that can be used as a bench. We did run stabilizing boards between interior and exterior posts. They are buried and don’t really interfere with the plants. (Note to self – next time put those stabilizing boards in BEFORE filling with soil.)

What’s next?

Next time I’ll discuss some of my favorite tools for using in raised beds and possibly other places. And we’ll touch on the importance of soil testing before you add organic matter or other fertilizer to your beds. In the meantime, be sure to check out these resources:

Are raised beds for you? This comprehensive fact sheet goes into more detail. https://pubs.extension.wsu.edu/raised-beds-deciding-if-they-benefit-your-vegetable-garden-home-garden-series

Home vegetable gardens – an overview. https://pubs.extension.wsu.edu/home-vegetable-gardening-in-washington-home-garden-series

How much organic material is too much? Don’t overdo – read this first! https://pubs.extension.wsu.edu/organic-soil-amendments-in-yards-and-gardens-how-much-is-enough-home-garden-series

Eggplants getting their buzz on

I was checking my eggplants today, and watching the bumble bees getting busy with the large purple flowers. As they flew in, buzzing away, they landed on the flower and kept buzzing — but the note changed, dropping in pitch. The bumble bee hummed away for a while, then flew off to the next flower.

I was watching buzz pollination at work. Egg plants, and a lot of other flowers, don’t leave their pollen hanging out in the open where any ant or fly that happens by could eat it. Rather they wrap them up in little packages that, when vibrated at just the right rate by a buzzing bumble bee, sends the pollen shooting out, so that bumble bees, which pollinate effectively, can access the pollen, but other insects, that would just eat it all, can’t.

In the garden, it isn’t easy to catch a glimpse of the pollen spewing forth, but luckily there are videos. Thank goodness for youtube. Watch it, and next time you are in your garden and hear a bee land in the flower and suddenly change the tone of its buzz, know you are seeing — and hearing — buzz pollination at work.

Are Soaker Hoses Safe?

By Cynthia Lee Riskin

With drought predicted for the west, southwest, and south through June 2015 (National Weather Service March 2015), many conscientious vegetable gardeners will try to conserve water by using soaker-hoses, those bumpy black hoses that weep water onto the soil through tiny pores.

Soaker hoses are made from fine-crumb rubber, usually recycled from vehicle tires. Research strongly establishes that tire particles leach heavy metals, carcinogens, and mutagenics, among other toxins. Yet soaker hoses have not been studied for potentially increasing the toxicity of edible plants. Are they really safe to use safe on our edible plants?

Soil in the City
Urban soils already contain high levels of heavy metals (Murray et al. 2011) from years of household runoffs—chemicals from pesticides, cars, painting, cleaning, and more. Adding soaker hoses made of crumb tires might exacerbate the problem.

Whether plants take up enough heavy metals to be toxic, however, is a complex equation, depending on a slew of interrelated factors, including:
• Soil pH (Costello 2003) and texture (Singh and Kumar 2006; Murray et al. 2011)
• Temperature (Murray et al. 2011; Lim and Walker 2009)
• The size of the rubber particles (Gaultieri et al. 2004)
• Chemical composition of irrigation water (Singh and Kumar 2006)
Furthermore, the plant species and even the cultivar can affect a plant’s uptake of zinc and other heavy metals (Murray et al. 2009 and 2011).

Growing Healthy Food
If you’re looking for the key to ensuring that your vegetable patch grows healthy food, however, I’m sorry to disappoint you. Too many factors are involved to predict the toxicity of what we grow in our gardens.

A good way to get more information is to contact your local extension agent for a list of laboratories that test soils not only for nutrient composition but for heavy metals. Although this information won’t guarantee you’ll be able to grow heavy-metal-free produce, it’s a step in the right direction while we wait for more research to be done.

Cindy Riskin is a Master of Environmental Horticulture and freelance journalist raising edible plants, an unkempt ornamental garden, and elderly mutts in Seattle, Washington.

NOTE: This article is excerpted from a longer one soon to appear in Cindy Riskin’s upcoming blog, tentatively named Muddy Fingers Northwest. Please contact Cindy Riskin at cindyri@q.com for an advance copy or the blog’s web address.

REFERENCES
1. Costello, Laurence Raleigh. 2003. Abiotic disorders of landscape plants: A diagnostic guide. Oakland, Calif.: University of California, Agriculture and Natural Resources. P. 117.
2. Gualtieri M., M. Andrioletti, C. Vismara, M. Milani, and M. Camatini. 2005. Toxicity of tire debris leachates. Environment International 31 (5): 723–30.
3. Lim, Ly, and Randi Walker. 2009. An assessment of chemical leaching releases to air and temperature at crumb-rubber infilled synthetic turf fields. Albany, N.Y.: New York State Department of Environmental Conservation. http://www.dec.ny.gov/docs/materials_minerals_pdf/crumbrubfr.pdf.
4. Murray, H., T.A. Pinchin, and S.M. Macfie. 2011. Compost application affects metal uptake in plants grown in urban garden soils and potential human health risk. Journal of Soils and Sediments 11 (5):815–829.
5. Murray, Hollydawn, Karen Thompson, and Sheila M. Macfie. 2009. Site- and species-specific patterns of metal bioavailability in edible plants. Botany 87:702–711.
6. National Weather Service Climate Prediction Center. March 19, 2015. U.S. Seasonal Drought Outlook. NOAA/National Weather Service National Centers for Environmental Prediction. http://www.cpc.ncep.noaa.gov/products/expert_assessment/sdo_summary.html.
7. Singh, S., and M. Kumar. 2006. Heavy metal load of soil, water and vegetables in peri-urban Delhi. Environmental Monitoring and Assessment 120 (1-3):1–3.