Problems with Planting Trees


Ok. I admit this blog is going to turn into a rant pretty quick because there seems to be a lot of ways to screw up a fairly simple horticultural practice—tree planting.  Since Arbor days are happening/happened everywhere around now, its a good time to talk about how to plant trees.   First let me state some simple and useful guidelines for a successful tree planting.

-When at all possible, plant trees bare-root. Even washing the container media away. This allows for inspection and removal of root defects.
-Select trees carefully that are free of defect and disease and that are adapted to your climate and soils
-Plant the youngest tree you can
-Take care in choosing the planting site.
-Avoid Root Barriers
-Plant trees so that the root flare is above ground slightly
-Plant trees in a hole only deep enough to contain the root system, no double digging.
-Plant trees in a hole wide enough to contain the root system, no wide holes (unless there is a reason for using one)
-Fill the hole with soil removed to make it. Do not amend the backfill around newly planted trees—Do not put rocks in the bottom of a planting hole!
-Plant trees without staking unless there is a reason to stake them
-Plant trees away from turfgrass or other groundcovers.
-Plant trees under the cover of a fresh layer of arborist chips.

-Irrigate newly planted trees from the surface—Do not install U tubes or tree snorkels to irrigate deeply.

An old planting detail from “a book of trees” . Several myths here: rocks at the bottom of the hole, amended hole, nursery stake still there when it does not need to be, etc.

I guess this rant comes from the variety of tree planting specifications I have seen over the years used by municipalities, landscape architects, nurseries and others. There seems to be a need to use the latest product, method or modification to site soils in order to make a fancy planting detail. Simpler is better and research by Universities has not verified most of the “innovative” approaches seen in planting details.

The first step in planting a tree is to chose the tree you want to plant. While this seems simple there is a lot that goes into tree selection. Setting aside personal choices, it comes down to selecting a tree that is healthy and free of defect. The potential candidate tree should have no signs or symptoms of disease, a naturally developed canopy unfettered by nursery pruning (especially heading cuts), and has few or no root defects. Initial superficial examination of the root collar in the nursery can eliminate some trees with circling or girdling roots. However, when the tree is planted root washing will reveal the entire root system and as Dr. Linda Chalker Scott has shown in this forum, root washing allows for rapid establishment in site soil. When at all possible chose the youngest tree you can for the new site. Young trees have fewer root defects, and we have the advantage of training them (structural pruning) from an early age. Young trees establish rapidly and will often outgrow older, boxed trees. The larger the specimen that you plant, the more chance for establishment problems such as settling, drying out, root rot or just slow growth. Planting trees from seed is ideal but most gardeners don’t have the patience to wait and seedlings, and seedlings do not give the option of using cultivated varieties that impart horticultural value, such as predetermined flower color, disease resistance, and known form (canopy shape and size).

Once the tree is selected, purchased and root washed, it is time for setting it in the ground. The first step is choosing a good planting site. A good site for a tree is somewhere that provides adequate soil volume for its roots to expand and for its canopy to expand. Many trees in urban settings fail to achieve their potential because they have restricted spaces to grow in. Chose a location in full sun. Unless you are planting a species that grows well in shade or needs protection from the environment, most trees will grow best in a sunny location. While trees are forgiving of most soil conditions, they will not grow well in compacted soils. If this is all that is available, break up compacted soils before planting. Consider the ultimate size of the tree you are planting, and imagine it attaining that size in your planting site. Avoid sites that have close proximity to buildings or hardscape. One of the most frequent problems with trees is that as they attain mature size they conflict with the infrastructure at the site.

Dig the hole for your tree so that the roots are very slightly above the grade. Do not double dig! While double digging has its proponents, there is no research-based reason for destroying soil structure– it is a disaster for tree planting. When a hole is dug too deeply soil will always settle after planting and irrigation resulting in the tree being planted too low in the ground. The root collar is buried and this is a predisposing factor for disease. The hole should have undisturbed soil under the roots. The hole only needs to be as wide as the root system. While many planting details show wide holes these are not necessary in most garden sites. If the site is compacted, wide holes can give temporary advantage to a newly planted tree, but the width of the hole will be the size of the “pot” the tree will have to grow in. So it is better to modify the site first to take care of compaction and then you will not need a wide hole.

Root barriers do not function well in most landscapes and lead to the development of landscape trash. They can also create root defects

Root barriers were very popular and are still specified today.  They actually do not usually achieve thier goal of preventing surface roots and protecting infrastructure.  Trees outgrow root barriers and they result in increases of landscape trash/pollution.  Root barriers can also create root defects such as circling and girdling roots.  Do not install root barriers, if you are tempted to do so you are likely not choosing a good site to plant a tree.

Cover the roots with backfill from the hole. Do not modify the backfill. Research does not support adding amendments to planting holes for trees. The native soil is what the tree will be growing in ultimately, and there is no reason to modify it. If the soil at your site is so bad that it needs to be changed, this should be a site-wide soil modification that will cover all the area the tree roots will explore up to its maturity. Most gardeners are not able to do this. Roots rapidly expand beyond the planting hole within months, so the time and benefit derived from an amended planting pit is minimal. Adding amendment, especially organic amendments to backfill can also be disastrous for trees. The organic material may utilize nitrogen in the soil and lead to a deficiency in the newly planted tree, worse, it may break down and cause anaerobic conditions in the bottom of the planting pit. Avoid amending planting holes! Never place rocks in the bottom of the hole—this does not create drainage, but creates an interface that prevents it.

A “lollipop” Tree.  Note the very skinny un-tapered stem, lack of temporary lateral branches and retention of the nursery stake–all bad…. Also note the tree snorkel lurking to the left. Kudos for keeping turf away but not far enough away.

If you have selected a good tree, it will stand without staking. There are three reasons for staking: support; anchorage; and protection. Support is sometimes necessary when a tree is cultivated with a long un-tapered trunk and a lollipop crown. Lollipop trees are often sold in nurseries as they resemble small trees. Trees trained in this manner, will not stand without staking. Loose staking allowing trunk movement will foster development of caliper so the tree can eventually stand without supportive staking. Anchor staking is used for trees that experience high winds and “staked out” with guy wires and a non-constrictive collar. Protective staking is analogous to placing bollards around a tree prevent impact from machinery or cars. Always remove the nursery stake at the time of planting and provide any additional support the tree may need. Many Cooperative Extension services have publications on how to stake a shade tree.

Providing a No Turf Zone around trees will aid in their establishment

Avoid planting trees in lawns. Turfgrass and trees conflict with each other. Trees shade turfgrass which results in a thinning sward and increased disease prevalence. Turfgrass slows the growth of trees in an attempt to limit their shading effects. Turfgrass is a very competitive water user and trees will be deprived of moisture and nutrients if turfgrass is present.  If trees must be planted in lawns, maintain at least a 1 yard radius around them with no turfgrass.

Aeration/Irrigation snorkel tubes do not help trees and result in landscape pollution. Note the original nursery stake still in place and the supportive stakes should have been removed long ago. Mulch needs to be replenished.

It has become a common practice to add irrigation or aeration devices to tree plantings. Sometimes called a tree snorkel these plastic 4 inch U tubes are buried below the root zone. Kits can be purchased from Box stores, and architectural details have been drawn specifying their use. Work by UC researchers showed that oxygen does not diffuse far from aeration tubes. So utilizing tree tubes to increase air flow is suspicious. Some planting details specify adding irrigation to the tubes to force a deep rooted condition in the tree. This places water below the root system, which can dry out and compromise establishment—not a good idea… Worse of all tree snorkels are sometimes installed with no purpose at all other than that was what the planting plan indicated. This is a needless practice and results in landscape pollution. Long term, tree snorkels are ugly, easily broken and provide no useful function to an establishing landscape tree. It is not in the nature of trees to proliferate absorbing roots deep in soil and snorkels will not change a tree’s genetics.

After the tree is set in its hole, and backfill settled in with water, apply a 4 inch layer of arborist chips as far out from the trunk as feasible—at least several feet. The chips will modify the soil improving, chemical, physical and biological properties while conserving moisture from evaporation, preventing runoff, and germination of annual weeds. Generally trees thrive under mulch as it simulates litterfall, or accumulation of organic matter under their canopies. Replenish the mulch as it deteriorates. Finally apply irrigation as needed through the mulch from the surface of the soil. This will help establishing roots, leach salts, and move mulch nutrients into the soil profile.  Avoid companion plantings near the main stem of the tree and avoid piling mulch around the tree stem. Following these guidelines will lead to a healthy and useful shade tree that provides its many services for decades.

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.”

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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.

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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.


Rethinking the monocultural lawn (again)

Over the last couple of weeks I’ve been in London having some unforgettable garden experiences. Thanks to the generosity of my UK colleagues Glynn Percival and Jon Banks I was treated to Kew Gardens, RHS Wisley Gardens, and Windsor Castle. I hope to construct several blog posts from these visits, but today’s post is an homage to the English garden meadow. Instead of monocultural turf lawns, mowed and sprayed into submission, why not consider a more biodiverse and visually pleasing approach to groundcover?

RHS WIsley meadow. Photo by Charlotte Scott.

As the title of this post suggests, this is not a new topic in our blog. (You can read other related posts here, here, and here.) What was so stunning about these garden meadows (meadow gardens?) was the scale and effortless beauty. For instance, consider this tree-lined parkway at Kew, covered with English daisies.

Kew Gardens parkway. Photo by Charlotte Scott.

I saw my first honest-to God cowslip in a meadow garden at the British Museum of Natural History.

Primula veris (cowslip). Photo by Charlotte Scott.

How about these adorable tiny daffodils and checker lilies?

Daffodils and Fritillaria. Photo by Charlotte Scott.

And here they are en masse.

Masses of spring bulbs transform this lawn. Photo by Charlotte Scott.

This isn’t to say that the formal lawn isn’t a thing in England, It is.

Windsor Castle. Photo by Charlotte Scott.

But unless you have a castle, a baseball diamond, or a putting green to manage, why not consider something more appealing, not only to the eye but to your beneficial wildlife?

Rivers of daffodils bisect more traditional grass lawn. Photo by Charlotte Scott.


Understanding the mysteries of plant diseases: Prevention, Control and Cure (Part 3 of 3 in this blog series)

Understanding the mysteries of plant diseases: Prevention, Control and Cure (Part 3 of 3 in this blog series)

What next?
You’ve done your research and made a diagnosis—now what? Sometimes the plant has to be removed and never planted there again. Start over, do something else.

Some diseases are difficult or impossible to control (virus diseases) especially when they are new or unknown to science.

Controlling plant pathogens or abiotic disorders can be daunting, frustrating, even impossible. As I mentioned in the last blog early detection gives more options for control because the disease has not advanced to a degree where it can not be controlled. Controlling plant diseases is not just palliative (treating your plant’s pain) it involves understanding where pathogens come from, stopping their movement, arresting their development and preventing their spread. Understanding genetics of resistance can offer amazing control of diseases, and finally biological control limits the development and spread of many pathogens.

Virus particles can only be seen with an electron microscope, since signs are hard to visualize, early detection is very difficult. Shown are TMV virions

What Can What Can’t?
There are some battles that can’t be fought or fought easily with plant pathogens. When plants are infected with viruses, there is almost no control option but removal (roguing). All plants likely contain some kind of plant virus; but not all viruses in plants cause symptoms of disease. Dangerous viruses like tomato spotted wilt, impatiens necrotic spot virus, cucumber mosaic virus, or many others, are devastating to their hosts and once infected there is no controlling these. Removing infected plants at the first symptom of viral involvement is prudent but often infections have already spread. Viral pathogens almost always infect without significant symptoms,  and become systemic in the plant before their more

Once trees with root rot show symptoms it is too late for control measures, the tree is dead and will not recover. Here Phytophthora spp. has killed a eucalyptus tree.

devastating effects become visible.  By late season, in most vegetable gardens, viral titre (concentration) is very high in solanaceae plants (pepper, tomato etc) and in cucurbits, both groups are highly susceptible. When perennial plants get viral pathogens there is no cure, and symptoms will increase over time. In orchids, viruses can sometimes be avoided by tissue culturing the meristem (which is usually virus free) to clean up a rare plant worthy of salvation. Sometimes plants are already dead but don’t look it. In the case of root rotted trees and shrubs, leaves may still hang from the tree, may still be green but the tree is beyond salvation, control may not be possible. In general control measures are best conducted early.  And by early I mean before you obtain your plant!

An old adage goes: “An ounce of prevention is worth a pound of cure” This is especially so when there is no cure!.  Prevention as a control technique really involves several factors such as exclusion, quarantine, and maintaining plant health so that plants are not predisposed to disease. The first tenet of control is exclusion. Don’t bring pathogens to your garden. Gardeners are their own worst enemy where plant diseases are concerned. Since pathogens can be seed-borne, come with insects, be already infected in the nursery, or resident in soil, care must be exercised when new plants are selected for your garden. Practice safe plant swapping!  Gardeners sharing plants with each other may also be sharing their respective plant diseases! Be careful where you buy plants, sloppy nurseries with their plants on the ground in standing water is a red flag.  Also be on the look out for weeds in nurseries since they can harbor insects that vector virus diseases. Plant debris left on the ground and not cleaned up, can be a source of fungal spores.  So consider the source when selecting plants for purchase.  Inspect plants carefully before purchase, especially slipping the container off to look at the root

All plants should be inspected carefully, especially root systems. Always remove a sample from containers to inspect. Here cyclamen are shown with root rot.

system. I don’t purchase anything (even boxed trees) without doing this first. If you are satisfied that you have a healthy plant then you are ready for the next phase of disease control.

Plants are often quarantined before they are released for cultivation or planting. When you bring your plant home, leave it in the pot for some time. Even bedding plants if purchased young can grow for a bit in containers. Remember lack of growth is a symptom of incipient disease.  Observe your new purchase of a few days or even weeks depending on the plant. If normal growth is occurring then move on to garden placement and planting. A little time set apart from other plants, and careful observation, will possibly prevent bringing something bad to your garden.

Once disease is established, and symptoms are apparent, gardeners often turn to pesticides to try to provide therapy. Sometimes fungicides applied to a plant post infection will slow down the spread of the pathogen within or on a plant. Therapeutic approaches can also turn on plant defense systems or enhance them so that the plant can limit the progress of disease. Therapy is usually not an option with most diseases because the pathogen has often gone beyond the point of stopping it by the time disease is recognized.  Some fungicides applied early, can be very therapeutic in turfgrass diseases, blights and powdery mildew diseases. The key to therapy as a control option is to detect the disease early and use an efficacious material that is labeled to control the pathogen you think is causing the disease on a given plant.  All this should be on the label.

Pruning out branches with cankers is a form of eradication.

An immediate response of many gardeners when disease is discovered is to kill the pathogen. This is eradication. Eradication takes several forms. There are eradicant pesticides, that kill the pathogen on contact. Usually these cause some degree of harm for the host since most pathogens have a host relationship that is destroyed when the pathogen is killed. Eradication can also be  or removing plants from the garden that are a source of disease. Picking up and disposing of fallen plant debris is eradicating a source of potential inoculum from the garden. Pruning cankered branches from a tree is a form of eradication.

One of the best forms of disease control is resistance. Selecting plants that resist disease is built in control. Diseases such as rust and powdery mildew have wide ranges of interaction with their hosts. By selecting plants that are resistant, there is no need for other control measures such as sprays. Resistance to plant disease comes as two types. Horizontal or multigenic resistance is partial or incomplete resistance and is conferred by several genes or their interactions in the host with the pathogen. Vertical or complete resistance is resistance conferred by a single gene in the host. Plants with horizontal resistance will get some of the disease but it won’t be overwhelming, often in grains, lack of resistance will result in complete crop failure. Plants with vertical resistance show no symptoms and are completely immune to the pathogen. While this complete resistance is appealing, it is only conferred by a single gene, and the pathogen can easily break down this resistance and cause disastrous disease. Horizontal resistance while not complete, is called “durable” resistance because it takes more time to overcome resistance conferred by multiple genes. The nature of resistance in garden plants is rarely detailed by growers or seed sellers. Often we are lucky to see any labeling for resistance. Crops like rose, crape myrtle and snap dragon are often sold as resistant to powdery mildew or rust and in some cases plant breeding programs strive to incorporate disease resistance into their breeding lines but then fail to label the product as disease resistant!

Cultural control often involves doing the right horticulture. Keeping turfgrass away from the stem of trees, applying arborist chips, planting at the right depth, are all cultural controls for root disease.

Cultural control is using good horticultural practices to limit the development of disease. Since many plant pathogens require a host to be predisposed, we have the opportunity through good horticulture to avoid the disease development. Planting woody ornamentals at the right depth is a cultural control of Phytophthora collar rots.  Appropriate application of water, reduces stress and prevents plants from being predisposed to both root rots and canker diseases. Correct pruning cuts limit the development of decay in trees.  Appropriate horticulture as discussed in the Garden Professors page will go far toward cultural control of common garden maladies.  Proper plant selection is also a form of cultural control. Choosing plants adapted to the growing area climate, and soils selects plants that are less likely to be predisposed to disease. Poorly adapted plants are more susceptible to pathogens and thus more likely to become diseased.

Biological control is the effect of one organism limiting the development of another thus preventing disease. Classical bio-control is when an exotic pest is introduced and there no natural enemies or parasites to regulate it. The pest/pathogen multiplies rapidly killing or affecting a large plant population. Research in the native range of the host looks for native organisms to control the pest. They are brought to the infestation, released and the pest/disease is brought into control. This works well with insects and the damage they cause. It has also been achieved with exotic plant pathogens.  For our native pathogens, there may already be a community of organisms that limit its development.  This is especially true for soil-borne pathogens. This is why the GP professors so often recommend fresh wood chips as mulch.  Fresh wood chip mulches supply carbon for organisms in soil that interfere with soil-borne pathogens; a kind of mulch-mediated bio control for root diseases.

I find controlling diseases is a lot more difficult than understanding or identifying them.  Usually by the time you have observed disease in the garden it is too late to stop its progress. You can take mental notes not to plant that variety again, or prune more diligently etc. but diseases are largely regulated or advantaged by the environment and our good or bad gardening practices. Of course the pathogen has to be present for biotic disease to happen, as we know that organisms don’t spontaneously generate.  Disease control starts with identification then research and finally gardening actions that help prevent, limit or eradicate disease propagules.

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 lights.  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.

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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.


Is it good advice? Or is it CRAP?

In my educational seminars I’ve long shared a version of the CRAAP test (currency, relevance, authority, accuracy, and purpose) for analyzing information related to gardens and landscapes. My version is CRAP (credibility, relevance, accuracy, purpose), and we’ve published an Extension Manual that explains in detail how to apply it. This past week I was at the Philadelphia Flower Show participating in Bartlett’s Tree Care Update panel. Given that the theme of the show was “Flower Power,” I figured that a talk on Magical Mystery Cures was in order. And the 1960’s was the decade where the late Jerry Baker gained prominence as a garden authority – and whose presence is still widely felt nearly 60 years later.

And anthropomorphizing of plants begins….










Now, I could spend the rest of the year discussing all of Jerry’s advice, tips, and tonics for gardens – but it’s more useful to determine whether he is a credible source of reliable information. So let’s apply the CRAP test.

C = credibility. What are Jerry’s credentials as a garden expert? It’s easy to find this information from the internet, including the Jerry Baker website. He had no academic training in plant or soil sciences but started his career as an undercover cop who often posed as a landscaper. His books are all popular publications, meaning they have not gone through critical review by experts before publication.

R = relevance. For our purposes, his information is relevant to our focus of managing gardens and landscapes (as opposed to production agriculture, for instance).

A = accuracy. Jerry’s advice is not based on any scientific source. He relies on common-sense approaches, folklore, and his grandmother’s advice. In fact, many of his assertions are at odds with published scientific evidence. Now, science evolves, and older scientific publications are sometimes found to be inaccurate after new information comes to light. If Jerry’s books were meant to be accurate sources of information, they would be updated with new findings as subsequent editions were published. This is what happens with textbooks, for example.

P = purpose. What is Jerry’s ultimate purpose? It’s sales. There’s no way around this conclusion. Over twenty million copies of his books have been sold, and during his career he became the spokesperson for several gardening products. Probably the most well-known of these was the Garden Weasel (which parenthetically is a great way to destroy fine roots and soil structure). There’s no doubt he was a brilliant self-promoter and marketer. But he was not a reliable resource, and many of his “tips and tonics” are extraordinarily harmful to plants, pets, and the environment.

“Garden Weasel” courtesy of Wikipedia

While I was wrapping up my research on Jerry Baker I was particularly taken by a chapter in one of his books (one of his Back to Nature Almanacs) called “The Tree Quacks.” I thought some of these quotes were particularly ironic:

The source…
…and the quotes








Imagine my surprise when I discovered that these quotes were actually not his own. In fact, the entire chapter was plagiarized from a 1964 article by John Haller in Popular Science, which is online. This action is uncomfortably similar to his 1985 trademarking of the phrase “America’s Master Gardener,” 12 years after the Master Gardener program was formed (but not trademarked) at Washington State University.

Text from 1964 Popular Science article

I hope this post has helped you learn to analyze the credibility of information and information sources. If so, you can claim the of America’s Master CRAPper ™!

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.

Image of graph showing relationship between soil temperature and seed germination.

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.

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

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.

Understanding mysteries of plant diseases: Diagnosis and Detection (Part 2 of 3 in this blog series)

Something is wrong?

Sometimes its subtle sometimes its not–like here with powdery mildew on coast live oak.

Do you ever have a feeling that there is something wrong with a plant? It’s just not healthy looking, or it has not grown for awhile? As we discussed in the last blog, disease is a process–it occurs over time. When in the disease time-line you notice the process, can be quite varied. Some astute gardeners may know something is wrong before there are symptoms, others may not take notice of the process until the plant is dead. Your disease detection acuity, or disease intellect, is largely dependent on your ability to recognize when the disease process is happening.  Early recognition gives you a chance to interrupt or limit the progress of disease or “control” it.  This blog is all about enhancing your disease detection acuity.  In the last of the series, I will cover what we can do about plant diseases, their prevention and control.

Sycamore anthracnose disease has vein-following necrosis as one of its foliar symptoms.

Symptoms and signs
Plants respond to challenges from a disease agent by producing symptoms. Symptoms are physiological changes in plants which we can see. Yellowing leaves, necrotic (dead) areas of leaves, stems, flowers or roots are common for many diseases. Some symptoms are very subtle. Slowed growth may be the first symptom of a systemic disease that is spreading within a plant’s vascular system or destroying its roots. Some plants can have 75% of their roots killed by pathogens without any visible symptoms on foliage. Most of the time when symptoms are this subtle, the plant is not growing at the same rate it would if it were healthy. Absence of new foliage, short internodes (distance between leaves), lack of initiation of flowers can all be symptoms of disease. Another subtle symptom is an overall color change that takes away a Plant’s “brightness” or healthy glow. I think most gardeners recognize this, but may not associate color dullness with disease. When subtle symptoms are detected, it is always a good idea to check the roots to see if they are functional (not rotted).

Overt symptoms are easy to distinguish. Fire blight is a good example—the bacterium Erwinia amylovora is spread by bees to flowers where it invades the floral nectaries. Bacteria migrate into shoots and stems and turns them pure black.

Early infections of the fire blight bacterium kill the peduncles of flowers

The disease proceeds rapidly in springtime during bloom and the symptoms (necrosis) are striking. Blights, anthracnose diseases, and canker diseases all produce necrotic tissue symptoms that are easily distinguished from healthy tissues. Even root rot is overt if you take the time to look at the roots!

Fungal hyphae of Armillaria fuse into a mycelial mat under the bark of Peruvian pepper.

Signs are the causal agents of symptoms. Fungal hyphae (collectively mycelium) growing under bark or on plant surfaces are easily observable signs. Just like symptoms, signs can be overt or cryptic. Armillaria mushrooms form in large clusters around the bases of infected trees and are easily identified, but the fruiting bodies of canker diseases (pycnidia) are very small and look like small pepper granules on the surface of a dead twig or branch. Plants often form galls (a symptom) that form around insects or bacterial pathogens (signs). Observing plants carefully to look for signs can be quite diagnostic. For instance if you observe the symptom of distorted new growth on your grape or rose and then carefully examine the tissue with a hand lens you may find the sign of mycelium from powdery mildew. Symptoms often develop after signs and many signs only form in the dead tissue or after the disease has produced much damage.

Fruiting bodies appear as black dots in this necrotic coast redwood tissue. Also note signs of white mycelium

Since signs are often reproductive structures of a pathogen, they are very helpful in pathogen identification. Many microbes have signs and cause significant disease but their signs are microscopic and thus hard

Phytophthora mycelium and chlamydospores are impossible to see in garden soil. These signs are not visible in-situ.

to observe.  The mycelium and spores of many Phytophthora spp. that cause root rot of trees, crops and flowers are invisible in-situ.  They can only be visualized by isolation in the lab.

Internet searches and labs
So you have observed symptoms, you think you have signs but are not sure. What next? There are thousands of plant diseases, and we see new diseases at an ever increasing rate as we explore growing new plants in new places.  Accurate disease diagnosis is beyond most gardeners.  Certainly you can narrow things down by looking at google images of diseases listed for the plant in question. But you can also be misled by google searches.  I would trust only University-based web pages, as there is a lot of mis-information from other sites that are inaccurate or outright incorrect in their diagnoses. In many states Cooperative Extension offices have personnel that will look at samples for you, or may refer you to a diagnostic lab that can examine or isolate the pathogen from your plant sample for a fee.

If you go to a lab for diagnosis, it can rapidly degrade into incorrect or inconclusive findings. Lab analysis, isolation and pathogen identification work well if you already have a suspicion of what your pathogen is.  You are just seeking confirmation.   Samples sent to a lab found without pathogens may not not indicate the plant was not diseased.  Samples degrade as soon as they are taken, they may not be examined right away at the lab or they may not have been transported correctly (ice chest away from sunlight). Sometimes pathogens just don’t survive well in samples, and will be hard to detect in lab settings. And most frequently, the lab usually does not get a sample with the pathogen in it.  A good example is branch die-back symptoms on a tree.  So the gardener brings in dead twigs, but the twig dieback is actually caused by extensive root rot.  The gardener never even thought to look at roots, because the twigs were the dead part. The lab of course finds no pathogens, only saprophytic organisms, which it lists and leaves the sample submitter confused and wondering if they are pathogens.   Labs are best used to confirm something you already have strong suspicions about. You have the fruiting bodies (signs) the symptoms match on-line versions of the disease you are looking at, on the same host, and everything seems right, but you want to be sure. Then a lab is useful. Especially if they get a good sample with signs present.

There are some useful test kits that home gardeners can use to confirm their diagnoses.  Lateral flow test strips are available that detect pathogen analytes. These are especially useful in testing for plant viruses and the diseases they cause.  While the cost of each test is low, there is usually a requirement to purchase a number test strips, so the cost can be over $250 to purchase a number of lateral flow test strips. The test for Phytophthora (root rot organism) is quite effective, gives results in five minutes and requires no special chemistry or long incubation periods. Some of these diagnostics are species specific, some like the Phytophthora kits, only detect the genus, not the species of Phytophthora involved. Test strips are specific to the disease at hand, so you would already need to be pretty certain of what you have if you are using these. Like a lab, they can confirm what you suspect.

Another handy way to diagnose disease is to use a host index. This is basically a list of diseases occurring on a list of different plants. Cynthia Westcott published the most important host index for ornamental plants, but it is long out of print now, and no longer published. Her plant disease handbook can still be found occasionally at Library book sales. The host index by Farr and others, “Fungi on plants and plant products in the United States” produced back in 1994 by the American Phytopathological Society is still in use by most plant pathologists because in its twelve hundred pages, you can likely find what you are looking for.

So after looking at symptoms, perhaps some testing and examining a host index, you think you have your diagnosis. So what? What can you do with a diagnosis? Well this is a jumping off point for reading the literature on a particular disease and its causal agents. Understanding the disease, its processes and timelines for disease progression will assist you in building an effective control program for your plant or garden.  At least you can decide if you dig it up and start over, or weather there is a chance of saving your plant and helping it to resist and recover from the pathogen at hand.  Astonishingly, many plants are treated (even by professionals) without an accurate diagnosis.  Know your pathogen and you will know the range of its effects on your garden plant and you can research ways to limit its damage and spread.  Next time I will talk about actions to keep garden plants healthy.

A Gardener’s Primer to Cold Hardiness, Part 2


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.