Holy Hydroponic Houseplants, Batman!: Can you grow houseplants without soil? Yes!

Just when you thought you got the hang of growing houseplants in potting soil (or if you’re a doting plant parent, a special homemade mix someone on the internet told you to use) comes a new trend – hydroponic houseplants!  Or, “semi-hydroponic” to use the more technical term that is used when describing the trend.  How do you grow houseplants semi-hydroponically?  Do they grow this way?  But first, maybe we should ask the question – why? 

Why grow semi-hydroponically? 

I think for most casual houseplant growers, this method is attractive because it is a challenge.  Something new to try after you’ve mastered growing houseplants the old-fashioned way. And quite possibly a pandemic project to provide a distraction after being cooped up in the house for months on end.   But are there benefits to growing houseplants this way?  Turns out, there are some.    

Many articles you find on the subject state that semi-hydroponic houseplant growth can be beneficial for those who struggle with chronic over- or under- watering.  The media used for semi-hydroponics is a big, porous puffed clay stone called hydroton or LECA (Lightweight Expanded Clay Aggregate, of course we have to have an acronym!).  It is used in some hydroponic vegetable (and other plant) production systems.  The large pore spaces it creates and the wicking action it does in the container helps keep a balance of air and water for the roots. The #1 leading cause of death among houseplants is overwatering – it creates a lack of air in the potting media, the roots lack oxygen (called hypoxia), and are either damaged or die.  This can also make it easier for fungal infections to cause root rots.  But how can you stop from overwatering plants if you’re growing them in water?  We’ll talk about that in a bit when we talk about the “how to”. 

There are some houseplants, like epiphytes, that might also benefit from having a media that isn’t like soil.  Plants that are used to growing on tree bark, or in rocky environments in their native habitat that might actually perform better in a media that is a large, rough pebble that does kind of resemble the texture or tree bark or stones.  There are lots of tropical houseplants that also grow in areas with high levels of large particulate organic matter like chunks of wood and bark.  Plants from boggy environments that have high water requirements or grow in a more “mossy” type soil might also benefit. 

One other application of this method is for propagation of cuttings.  Many houseplant growers like to propagate cuttings in water, but this often isn’t the best practice because the water can be depleted of oxygen (causing hypoxia and rot) or become spoiled or soured (and cause infections).  Most horticulturalists will recommend propagation in a light media like seed starting mix, perlite, or sand.  But keeping water consistent without overwatering is difficult in this situation, and media can also be a vector for disease.  The air space and wicking action of the LECA media used for semi-hydroponics can help keep cuttings hydrated without the issues of water propagation.  This method is commonly done in clear class containers, so there’s the added benefit of being able to see root growth to monitor progress. 

How do you grow semi-hydroponically?

Of course, in this short article we won’t be able to cover every detail, so if this is something you’re interested in trying, I’d suggest some self-study.  I’ll be covering some of the basics, but there’s a lot more to learn. 

The Kratky Method - Grow Food The Passive Hydroponic Way (Step by Step  Guide) | Trees.com
Kratky hydroponic method Photo: UpstartFarmers

First, this method somewhat resembles one of the simplest forms of hydroponic production that lots of home hydroponic gardeners use, called the Kratky method.  In this passive hydroponic method, a plant is suspended above a water-based nutrient solution.  At first planting, the nutrient solution is right below the plant, close enough for a few inches of the roots to touch the solution.  As the plant grows, the roots elongate and the nutrient solution level is reduced to keep just a few inches of the roots submerged.  This allows the roots to take up solution, but the space between the plant and the solution allows a majority of the roots to be surrounded by air to avoid the issues of hypoxia. 

In semi-hydroponic houseplant growing, a container (usually clear glass, at least for beginners) is filled with the LECA media and the plant’s roots are distributed through the media.  (The media should be washed and soaked in water first, to remove dust and allow it to hydrate.)  It is easier if it is a young plant or recent propagation so you don’t have too many roots to deal with (you may need to root prune larger plants).  Smaller plants will also withstand the shock of going to this system, especially if they’re moving from a potting soil media.  (Note: Clear glass container + nutrient solution + light = algae. Be prepared to clean up the algae from time to time.)

A (dilute) nutrient solution is added to the container.  The roots should not be submerged in the solution, but rather it should be added to a level where it will wick up through the media to surround the roots.  The basic rule of thumb is to fill the container about 1/3 of solution, but if the container is exceptionally large or the roots are very small, you may need to fill it higher to make sure the media around the roots stays hydrated. 

This nutrient solution is one of the trickier parts.  You can use a general all-purpose hydroponic nutrient mix, available in lots of garden centers now or online.  You can also try some of the general houseplant fertilizers or ones specific to whatever houseplant you’re trying to grow.  You’ll want one with micronutrients as well as the macronutrients like N-P-K – since we’re growing without soil or an organic matter based media you’re going to have to supply all of the plant’s nutritional needs.  You’ll want to mix the solution between ¼ – ½ the recommended strength – you’ll need to see what works for you and your plant.  And then you’ll want to pH balance the water to create the right environment for the plant and make sure that nutrients are available for uptake.  The pH range for most plants is between 6.0 and 7.0 (aim for 6.5), unless you have one with specific needs.  For this you’ll either need pH test strips or a meter (which you can now get for less than $20 online) and some acidic and basic solutions to adjust pH (you can use some household items like vinegar to do this, but your best bet would be solutions specifically prepared for adjusting hydroponic or aquarium pH levels commonly referred to as “pH up” and “pH down”).  This pH adjustment is a lot easier (and maybe unnecessary) if you start with distilled or reverse osmosis water (or if you have a really good water filter that removes dissolved solids).  The pH levels and dissolved solids in some tap water makes it hard to adjust (my water here in Nebraska is very basic because it is very heavy due to high calcium levels, which also throws off the nutrient balance). Rainwater or melted snow can also work (though may not be pH balanced). 

You want to keep the solution topped off so that the media stays sufficiently moist. As with hydroponic production, plants pull nutrients out of the solution at different rates, so you can get build-up of some nutrient salts over time that could result in poor growth and even toxicity.  To avoid this, every few weeks (or more often if your plant is a heavy drinker) you might need to perform a flush, where you drain off the nutrient solution, give a quick rinse with tap water, and start over with fresh nutrient solution. 

More experienced growers might graduate to using this method in containers other than clear glass.  This adds a level of challenge, since you can’t automatically assess the level of nutrient solution by visual inspection.  The use of self-watering pots that have net pot or hole-y insert pots are commonly used for this.  Or you can buy net pot or orchid pot plastic inserts to use in any non-porous container you desire.  Growing in net pots can make the flushing process easier, since you can just pop it out of the container and run tap water through it.  Otherwise, you’ll have to find a way to pour the tap water out of the container or

completely remove the plant and wash the media. 

What can I grow semi-hydroponically?

Well, you can try with a lot of different plants.  I don’t know that there’s a list of plants out there for do’s and don’ts, but there are a few good candidates to try. Most tropical houseplants are good candidates. I’ve seen lots of articles on orchids, and I just recently put a rescue phalaenopsis in semi-hydroponics.  Other epiphytes like holiday cacti and bromeliads are also good candidates – think of things that like to grow on trees/treebark. Hoya, which are all the rage in houseplant circles, are also candidates due to their mostly epiphytic habits.  Lots of tropicals like Monstera, Philodendron, and Pothos also do well in this system.

Things that probably won’t do the best in this system are ones that don’t like to have “wet feet” – I’m thinking mostly desert cacti and succulents. But some of the LECA lovers that I talked to said that some succulents, like “string of pearls” and other strings of things (hearts, dolphins, turtles, etc) do grow well. But if we take a look at their natural habitat, where they grow over rocky outcroppings, it makes sense.

There isn’t really an exhaustive list, so you might want to experiment if you’re wanting to try it out. As long as it isn’t an expensive plant (and there are lots of expensive houseplants out there), a little experimentation can help you find the plants that would work best for you and your situation. 

In conclusion…..

Growing houseplants semi-hydroponically isn’t for everyone.  Getting everything just right can have a learning curve, especially if you weren’t great in chemistry class.  But, it can be a fun way to challenge yourself and may also benefit your plants in the right situations.  It is becoming so common that the materials are getting easier to find – many garden centers now carry the LECA and hydroponic supplies, you can always order them online, and you can even find small bags of the LECA/hydroton in the ever expanding houseplant section at IKEA (of all places, if you’re lucky to have one).  So if you’re up for a challenge, give it a try!  You might find a fun new way to grow houseplants….or a new way to kill houseplants!  But the fun will be in the trying. 

Special thanks to:

  • Anni Moira
  • Sydney Tillotson Sehi
  • Suzi Sellner
  • Tiffany Caldwell
  • Shelbi Sorrell
  • Maggie Pope

Sources:

Houseplant Hubub: The rage about variegation

It is no secret that houseplants are hot right now.  Interest was growing before the pandemic, especially with millennials and younger folks.  Then the pandemic hit.  Houseplant interest skyrocketed since people were stuck at home and wanted to bring a little bit of nature indoors to make their spaces a little more cozy for 24/7 habitation. 

This has caused the demand, and price, of many houseplants to increase, especially if they are on the rarer side.  One thing that increases the price of many plants is when a variegated version of a standard plant has been developed. 

My reading nook/houseplant oasis

Just as an example, after posting a photo of my “reading nook/houseplant oasis” in my home office I was informed that variegated form of a Monstera deliciosa vine that I had was the highly sought M. deliciosa “Albo-Variegata” cultivar, usually referred to as a Monstera albo, or just Albo.  Folks were reaching out to buy cuttings right and left.  I ended up selling 5 single leaf/node cuttings over one weekend and made $675 in the process.  That’s right, $675!  The most variegated of the leaves sold for $200, and that was actually a bargain price.  The garden writer for the local paper, the Omaha World Herald, even picked up the story and shared it as a focus on the four new houseplant shops that have popped up in the city over the last few months.

Had my plant not had the variegation that made it an albo, each of those cutting would have been worth a few dollars apiece.  So what makes some plants variegated and others not?  Sometimes the variegation is the standard form found “in nature” and sometimes it is a cultivar or variety that has been bred or discovered by chance.  Let’s take a look at all the ways that a plant can get that variegation, whether it is standard or rare. 

Chimeric variegation

My Monstera albo that caused the hubub

This is a common form of variegation and the one responsible for the variegation of my Monstera.  In this form, a genetic mutation in some cells changes that cell’s ability to produce chlorophyll.  It may reduce chlorophyll production, resulting in yellowish or silver coloration, or eliminate chlorophyll altogether, resulting in white coloration. 

The name chimeric or chimeral is based on the fact that the plant displays two (or more) chromosomal patterns on one plant.  In Greek mythology, a Chimera is a frightening fire-breathing female monster with the head of a lion, body of a goat, and the tail of a serpent. 

Image result for chimera
An ancient chimera statue

This variegation can be stable, where the pattern persists throughout the plant.  Or it may be unstable, where it is random on certain leaves and parts of the plant can revert back to the standard green form.  These plants can also produce leaves that are almost totally white, which usually results in a leaf that will die since it can’t photosynthesize. 

This type of variegation also means that cutting or propagations may or may not be “true” to the pattern.  It can be random.  For my Monstera, the presence of white striping in or around the node that will become the new plant is the important marker for whether the new plant will be variegated or not. 

One common chimeric houseplant is the plant formerly known as Sanseveria, now a Dracena (Snake plant or mother-in-law’s tongue). Many of the different color patterns on some of the cultivars are due to cuttings taken from different parts of the “original” natural type that display different colors on them.

Viral Variegation

Image result for tulip mania
Viral variegation that was all the rage in Tulip Mania

While beautiful, this variegation will often reduce the productivity of plants if not kill them outright.  There aren’t a lot of houseplants that have this variegation, but some Hosta cultivars do.  Probably the most famous case of viral variegation is the Tulip Mania during the Dutch Golden Age (in the 1600s).  Prices of tulips skyrocketed and people were buying them as investments (maybe like the current houseplant craze, or GameStop stocks, or bitcoins).  Unfortunately, as the virus reproduced plants kept getting weaker and weaker.  Eventually the tulip market collapsed and lots of people went broke.  Let’s hope that doesn’t happen with the houseplant market….at least with my fancy Monstera. 

Natural Variegation

Natural variegation on Tradescantia

This type of variation occurs when the patterns or colors of the variegation are written into the DNA of the whole plant.  It will occur regularly throughout the entire plant, not randomly on some parts as in chimeric or viral variegation.  This variegation is passed through cuttings and usually through sexual reproduction from seeds as well, though different variations may pop up that cause a more desirable or rare cultivar. 

Common houseplants such as Tradescantia, Maranta (prayer plant), and many more common plants have this type of variegation. 

Blister, bubble, or Reflective Variegation

Reflective variegation on Phildodendron ‘Birkin’ following veins in the leaf

This type of variegation occurs when there is an air pocket or bubble between the lower layer of tissue and epidermis, or skin, of the leaf.  The lower level typically has green pigmentation from chlorophyll and the epidermis does not, resulting in a pattern that is usually white, silver, or yellowish though other colors could appear.  This pattern can be blotchy or splotchy like in some types of Pothos and Pepperomia.  It can also occur along the veins of some plants, resulting in white or silver veins on green leaves, as in some Alocacia, Anthurium, and Philodendron varieties. 

In conclusion…..

Even if you don’t have an expensive plant hiding in the corner, houseplants can add lots of fun and color to your living spaces.  And sometimes, your houseplant obsession can even pay for itself.  Online swap and sale groups have houseplant afficionados swapping and selling cuttings and plants all over the place.  So enjoy your plants….and maybe you’ll find a cash cow hiding in the corner.  Don’t mind me….I’m just over here propagating more Albos to fill up my “mad money” jar. 

Sources

Variegation mutants and mechanisms of chloroplast biogenesis

Variegated Indoor Plants: The Science Behind The Latest Houseplant Trend

Chimeras and Variegation: Patterns of Deceit

Saving for the Future: Seed Saving Tricks and Tips

As summer winds down and the summer crops and flowers start to slow down many gardeners start thinking about saving seeds. Who doesn’t love saving seeds from that favorite tomato or beautiful coneflower?  Not only do you have some for next year, but you can also share with your friends! There are definitely some things to consider and some myths out there when it comes to seed saving, so let’s talk about how to do it right. 

You’ll get the most consistent results from open pollinated or heirloom varieties that are self-pollinating.  These plants have genetics stable enough that the seeds you save will come out looking and acting like a close approximation to the plants from the previous season (with some variation based on your selection of the “best” plants you save seeds from. Self-pollinating species are: tomatoes, peppers, eggplant, beans, peas, peanuts (note, peppers and eggplants have more open floral structures that can be cross pollinated).  Most tree fruits like apples and pears are cross pollinated and they are notorious for not “breeding true” – even if you hand pollinate to ensure that the mother and father are both the same cultivar you’re likely to get surprises.  Stone fruits (peaches, plums, etc) are less variable but still not true-breeding.  Bee pollinated plants are also notoriously hard to save seed from, since they can cross pollinate with different varieties and cultivars from miles away.  It is especially interesting for plants that look totally different but are the same species (like pumpkin and zucchini).

A puccini – or a zumpkin? Either way, it was nasty.

Myth: You can’t save seeds from those new modified hybrid plants. They’ve been made to be sterile

First off, hybrids aren’t genetically engineered and there are no GE plants available to home gardeners (most home garden crops don’t even have GE versions).  Hybrid plants do in fact usually produce viable seeds.  However, you won’t get the consistent results you will with open pollinated/hybrid varieties.  Hybrids are the F1 generation of a specific cross between a mother and father plant.  The offspring from that F1 generation (the plants from the seeds you save) is called the F2 generation will be a mix of traits – some will look like the F1 generation, some will look like the mother, some the father, and some the milkman.  So you’ll be in for a mixed bag of surprises.  According to our former GP colleague Joseph Tychonievich’s book “Plant Breeding for Home Gardeners” you can even develop a stable open pollinated variety from hybrids by saving seeds over a few seasons, selecting seeds from the plants that most resemble the cultivar you’re trying to save. 

You’ll want to make sure that the fruit/flower head that you’re saving seed from is mature.  This can be tricky for some vegetables, because we eat them in their immature states.  Peppers need to change from green to whatever their color is (red, yellow, orange, purple, etc),  cucumbers and zucchini (and other squash) need to turn into those massive, bloated fruits that often change to yellow or orange.  Beans often need to change to yellow or tan (and may have stripes).  For flowers, the seed heads or fruiting structures often need to turn brown and dry or start to open. 

If the weather cooperates, you’ll want to collect seeds from dry fruits/structured (beans, some flowers, etc) before significant rainfall so that seeds don’t become wet and potentially mold or break dormancy.  Collect seeds and place in a warm, dry location to let them continue drying out (if they’re small you want to put them somewhere they won’t blow away).  After drying, store seeds in envelopes or containers and put them in a cool dry place.  I often tell people to store seeds in the freezer – the cold temperature slows down respiration in the seeds and can extend their lifespan (the fridge is too moist/humid).  If you do that, drop your envelopes or containers down into a sealable container or bag to help keep condensation minimal when you pull them out of the freezer next year.

For home gardeners, it may not matter that you get plants next year that exactly copy the ones you saved seeds from – the fun can be in the surprise.  Who knows, you may discover a new variety – at least one that is exciting to you.  It can be fun seeing the variation in your new plants and finding something that you love. 

Epilogue: A special case – tomatoes

Most of the vegetable crops we grow don’t need any special treatment to break their dormancy (you’ll have to research flowers on a case-by-case basis) – save the seed and plant it next year and it will pop up.  Tomatoes are a bit of a special case.  If you scrape the seeds out of the fruit you’ll notice they’re still covered with the “goo” from inside the tomato which is called interlocular fluid (interlocular = between seeds).  The coating persists on the seed even if you wash them.  It has long been held that this coating retains some of the hormones of the fruit (like abscisic acid) that inhibits germination (though not all experts agree). So many sources will tell you to go through some process to break down the coating left on the seed, most commonly by placing the seeds and associated goo in a container, adding a bit of water, and letting them ferment for a few days.  You can dump them out and wash off all the gunk. Whether or not this is required to break dormancy is up for debate, but it does provide you with clean seeds that you can store easily.  There is also some evidence to suggest that this fermentation process helps remove pathogens on the exterior of the seed (heat treatment can help remove interior pathogens as well).

Some people just scoop out the seeds and smear the goo on a paper towel and try to scrape them off next year.  Some people add the step of washing, but this will still not remove all of the goo coating the seeds. This works if you’re not trying to share (or sell seeds) since they will stick to the paper towel. My guess is that the in the day or so that it takes for the goo to dry there is enough fermentation or decomposition going on to break dormancy.  If you don’t want the seeds stuck to a paper towel, you can use wax paper or some other non-binding surface, but you’ll still have dried goo on your seeds.

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

Late summer pruning: what happens, what won’t, and why

In the fall a gardener’s fancy lightly turns to thoughts of pruning (with apologies to Alfred, Lord Tennyson).  In particular, people worry that pruning too late in the summer or early fall will stimulate plants to send out new growth, which is then damaged by freezing temperatures. Let’s dissect what actually happens when woody plants are pruned during this time.

Sumac leaves in full autumn glory.

First, we need to separate temperate trees and shrubs from tropical and subtropical species. For the most part, the latter don’t become winter dormant: pruning them at any time means you will have regrowth as long as there are sufficient resources. If planted in more temperate zones, they will continue to grow until they are killed by freeze damage. Instead, we’ll look at temperate species and how they are adapted to surviving winter conditions.

Tropical woody plants like this jade are not winter dormant species. Don’t leave them outside even if you think they are protected (lesson learned).

I wrote a couple of posts last year on cold hardiness (here and here), so I won’t repeat those discussions on how plants survive freezing. Instead, we’ll focus on the process of HOW plants enter winter dormancy and become cold hardy. It’s a two-step process that depends on two different environmental factors: one that never changes from year to year, and one that certainly can.

The first step to dormancy is initiated right after the summer solstice. Plants are exquisitely adapted to changes in the light-to-dark ratio, and days begin shortening after the summer solstice. The changes that occur are largely biochemical, but you can also see some changes in plants themselves. Many trees and shrubs slow their growth during this time so that fewer young leaves and shoots are produced. Instead, resources are put into the existing foliage, or flowers for summer bloomers. Excess resources are routed to woody parts of the plant for storage.

Light and dark ratios vary with latitude, but the seasonal changes are always the same time of year.

From a practical standpoint, this means that when you prune trees and shrubs where growth has stopped, you will NOT get regrowth. The vegetative buds below the pruning cut are dormant. The tricky thing is that the exact time when the switch is thrown varies by species and is affected by environmental conditions. Careful observation will allow you to estimate when the plants will no longer produce new growth.

Some temperate species naturally put on a spurt of late summer growth. The leaves on these new Japanese maple shoots generally die from cold damage, but the branches themselves survive.

The second step begins when night temperatures cool to near freezing, which is not a predictable date. Because many of the biochemical and physiological processes have already begun or are finished, the response to cold night temperatures is rapid and visible. Leaf colors change as the plant begins breaking down leaf materials for mobilization and storage elsewhere in preparation for winter dormancy.

This katsura has started the process of autumn leaf senescence.

This process, honed over millions of years, is unfortunately not infallible especially under abnormal environmental conditions. Two examples spring to mind:

  • High intensity street lights. If the normal light-to-dark ratio change is interrupted by significant levels of night light, the first step of dormancy is hijacked. You can see what happens in these previous blog posts here and here.

That street light in the middle has kept the nearby leaves green while those farther away are senescing.

  • Unseasonably cold weather. With climate change, we are seeing wild shifts in all sorts of weather patterns, including the date of the first hard freeze. Hard, early freezes are not the same as a light evening frost. You can see what happens here:

A hard freeze in early November fried the leaves on this hydrangea.

Given normal conditions, however, temperate trees and shrubs are well on their way to full winter dormancy by late summer and early fall. Pruning them is not going to induce new growth.

A Gardener’s Primer to Cold Hardiness, Part 2

Snowpocalypse!

Last week I discussed the mechanics of how cold hardy plants can survive temperatures far below freezing. Today we’ll consider the practical implications of this phenomenon and what, if anything, you can do to help your plants through cold snaps.

What happens when temperatures change at unusually high rates?

Remember, supercooling occurs when temperatures drop slowly, allowing water to leave living cells and freeze in the dead spaces between cells. When rates drop quickly, which can happen on sunny winter days once the sun goes down, water can freeze inside the cells before it has time to migrate into the extracellular space. When that happens, those cells die when ice crystals pierce the cell membrane. Sometimes this damage will be visible right away – you’ll see water-soaked areas in leaves, for instance, where the contents of the cells have leaked into the extracellular spaces.

Watersoaked leaf on left was frozen, while the one on the right was not.

In other cases you may not see damage until spring, especially in buds that have frozen. The scales prevent you from seeing what’s happened to the tissues in the bud, but once warmer temperatures arrive you will see brown or black leaf and flower buds. These are NOT diseased buds, though they are often colonized by opportunistic pathogens.

Partially damaged Rhododendron flower bud

What about wind chill?

The wind chill question is an interesting one. Despite the way it feels to you, wind chill does NOT lower the temperature below the ambient air temperature. It just cools things off faster than they would without the wind. For cold hardy plants, this has two important effects:

  • The rate of temperature decrease around the plant speeds up – so ice can form faster than normal. This can result in freeze damage to the plant as described above.
  • The wind itself is dehydrating, pulling away water from plant tissues and causing freeze-induced dehydration (as discussed last week). This also causes damage to susceptible tissues and is often called winter burn.

So even though the temperature itself is not lowered by wind, the rate at which it decreases and the additional dehydration stress means that plants can be damaged at temperatures they would normally survive in the absence of wind.

What can we do to help plants survive?

Before cold temperatures are expected, it is critical to mulch the soil well with a thick layer of coarse, woody mulch. This insulates the soil and roots, which are the least cold tolerant of all plant tissues. Roots never go dormant, so they are generally unable to supercool much more than a few degrees below freezing. Oh, and be sure your soil is moist (but not waterlogged). Moist soil is a better heat sink than dry soil.

Arborist wood chip mulch protects soil and roots throughout the year.

Next, be sure insulate freeze-susceptible plants. This can be done by constructing a cage of chicken wire around small trees and shrubs, filling it with leaves, and then wrapping it in burlap. Containers should be moved to the leeward side of the house or other building and grouped together. The containers need to be protected from freezing at all costs.

Heavy wet snow should be removed to avoid structural damage to woody plants.

Speaking of insulation, snow is a great insulator. But it’s not always best to leave it in place. If temperatures are cold and snow is dry and light, leave it in place to insulated tissues. But if temperatures are near freezing and the snow is wet and heavy, remove it as much as possible. Its insulative value is marginal and the damage that heavy snow can do to trees and shrubs is permanent.

 

 

A Gardener’s Primer to Cold Hardiness, Part 1

Ice crystallizing on the outside of plant tissues is often not damaging (Ralf Dolgner)

With record low temperatures in some parts of the country, gardeners are understandably worried about the ability of their perennial and woody plants to survive the cold. What today’s post will do is give you some context for understanding how plants can survive temperatures far below freezing.

Why ice floats and how this damages cells

Ice weighs less than water, but takes up more space (Wikipedia).

Everyone knows that ice floats, whether it’s an iceberg in the ocean or cubes in your favorite chilled beverage. Ice is lighter than water because its molecular structure is different: there is more space between water molecules in ice. When water freezes naturally, the molecules organize into hexagons, forming a crystalline lattice (which helps explain why snowflakes look the way they do). This hexagonal shape forces water molecules farther away from each other, resulting in a porous material that’s lighter than liquid water.

Hexagonal shapes of of ice crystals (Picryl)

As ice crystals grow, they take up more space than the water did in liquid form. You know this if you have ever left a filled can or bottle in a freezer. The pressure can blow off the lid or split the container – and the same thing happens to animal cells: the membranes are distended until they burst. But plant cells are different: there are cell walls outside the membrane which are rigid and prevent membrane rupture. However, ice crystals are sharp and can lacerate membranes, including those in plant cells.

Frozen bottles of water will either leak or explode (PxHere)

How cold hardy plants avoid freeze damage

Woody plants have evolved a mechanism to survive winters that allows ice formation in certain areas and prevents it in others. This process takes advantage of the fact that plant cells have walls, and that the area between the cells – called the extracellular space – is not alive. Extracellular space is filled with gases and liquids – including water. Water can freeze in these spaces without causing damage because there are no membranes in extracellular spaces, only cell walls. As ice freezes in these “dead” spaces, more liquid water is drawn into them by diffusion from the adjoining cells. There are two outcomes of this: one is that ice only forms in the dead space, not the cells themselves, and two is that the liquid inside the cells becomes more concentrated.

Water that is full of dissolved substances (like sugars and salts) is less able to form ice crystals because there are relatively fewer water molecules in concentrated solutions. We can see this when we add deicers to frozen walkways and roads. The ability of water to stay in liquid form at temperatures below freezing is called supercooling. Plants that are cold hardy are able to tolerate ice formation in dead tissues and avoid ice formation in living tissues by supercooling.

Salt allows water to stay in liquid form at temperatures below freezing (BU News Service)

Supercooling is different than flash freezing

We need to discard any comparison of supercooling to flash freezing, a process used for cryopreservation. Flash freezing rapidly lowers the temperature of the tissue or organism being preserved at rates far faster than what happens in nature. The water molecules don’t arrange themselves in a crystalline lattice as they freeze. Instead they form small crystals in an unstructured form, which don’t take up more space than liquid water. This means that ice doesn’t damage the cells, which are still viable once thawed.

Supercooling allows water to remain in liquid form at temperatures below freezing…but eventually everything freezes (Wikimedia)

Supercooling is a process that occurs under natural conditions, which usually mean slow decreases in temperature. This allows water to continue to move out of the cells into the extracellular space where it freezes. (There are exceptions to this naturally slow rate, and I’ll discuss those in a follow up post.)

There is a limit to supercooling

Unfortunately for plants (and gardeners) there are limits to supercooling. These limits vary with species but even the most cold hardy plants will eventually experience injury and death. The reason this happens, however, isn’t from the freezing itself, but from drought stress. Let’s look at what’s happening inside the cells during supercooling.

A schematic diagram of plant cell plasmodesmata (Wikimedia)

As water continues to diffuse into the extracellular spaces, the cell becomes less turgid; this is called freeze-induced dehydration. Without water forcing the cell membranes against the walls, the membranes start to pull away as water is lost. Eventually the membranes and plasmodesmata (which connect living cells to one another) are stretched and break. These cells are now dead – they are isolated from the rest of the plant and the torn membranes allow liquid to seep out. So cells, tissues, and entire plants that die from low temperature stress are usually killed by drought stress!

And a photomicrograph of plasmodesmata connecting plant cells (Wikimedia)

In my follow up post, I’ll discuss the practical significance of this phenomenon, including rapid temperature changes in natural and the influence of wind. And, of course, some suggestions on how to help plants survive these stressful conditions.