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.

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

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

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

Keeping the Germs out of Germination

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

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

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

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

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

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

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

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

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

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

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

Keeping Things Light

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

When Persistence Doesn’t Pay Off

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

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

Understanding mysteries of plant diseases

(Post 1 of 3 in this blog series)

Gardeners, especially those new to gardening may find they have a “black thumb.” Plants die for no reason! “Oh well chuck it in the greenwaste recycling can and start again.” Or… “Oh I can’t grow cyclamens!… They always die in my garden for some reason.” For many gardeners it is mysterious why some plants fail to thrive or die suddenly. Plant disease processes are complicated, and it requires some knowledge of botany (anatomy and physiology), genetics, and microbiology to really understand what is happening. Also, since microbes are microscopic and most pathogens are microbial we can’t always see them at work, especially before symptoms develop. Symptoms are plant responses to the action of a disease agent. In this post I will try to describe the different kinds of diseases, and where they come from.

There are two broad categories …
of plant disease possible in gardens: biotic  diseases and abiotic diseases. Biotic diseases have a disease agent called a pathogen. The pathogen can be microbial, or a nematode or a virus, or a parasitic seed plant. Bacteria and fungi are the most common microbes. It is debatable whether viral particles are living, so also debatable whether or not they are considered microbes. Of the biotic pathogens, fungi cause  most diseases in gardens.  Many pathogens rely on environmental conditions to favor their lifestyle, this is particularly true of bacteria which like moist, warm environments.

The other category of disease is the abiotic category. Abiotic diseases have no pathogen. An environmental condition such as an excess or lack of an environmental condition causes physiological changes in plants that develop symptoms. Extremes of temperature, light, humidity, soil or water chemistry, soil physical conditions, air quality, and pesticide residue can all lead to abiotic diseases. Since there is no pathogen there is no epidemic, and abiotic diseases are not infectious. So spread, occurrence and movement of abiotic diseases are usually different than biotic disorders

So how does disease happen?
I have heard many gardeners make sweeping statements like “overwatering killed my plant” or “It just died of neglect” or “insects killed it”. Plant pathologists describe the disease process with a cartoon called the disease tetrahedron. It describes the interaction of four things: the pathogen, the environment, the host and time. Of course it is only a triangle for abiotic diseases since there is no pathogen.

For disease to occur there must be an active pathogen present that is virulent (has genes to cause disease). The pathogen must have enough inoculum present to begin the disease process. A single spore rarely leads to a successful disease (although it can in some systems).  Most importantly the pathogen must have the right genetics to recognize its host.

Next the environment must be conducive to the pathogen and its development and/or  harmful or stressful to the host. The environment can cause the host stress while favoring the pathogen.  An example would be oxygen starvation in flooded roots. The environment must favor the pathogen’s build up and dispersal of its inoculum (infective propagules such as spores, cells or seeds). Often splashing rain during the warming spring period is important for their spores to reach a susceptible host.

Finally for disease to happen, the host must be susceptible to the pathogen and possibly predisposed in some way to its attack.  Pathogens also have phenotypic synchronicity, that is the ability to produce inoculum at the same time as the host is producing susceptible plant tissues (leaves, buds or stems).

The final facet of the tetrahedron is time. Diseases do not occur instantaneously (even though we may only notice them instantly) – it takes time for them to develop.  Disease life cycles or life histories describe how pathogens survive, reproduce and disseminate themselves through the environment over time. The tetrahedron can be used to understand the factors that lead to disease but also can be used as a way to stop or control diseases (more on that in another post).

So where do diseases come from and where are they going?
Abiotic diseases are caused by environmental extremes.  Another way to look at them is that they occur when there is a violation of the adaptations of the host.  In this regard when we grow plants not well adapted to our climate or environment they can be harmed. A good example is my papaya tree. Right now it has been harmed by low temperatures. Growing a papaya in Ojai, CA is a violation of its adaptations.

Jim’s papaya tree is intolerant of Ojai winter temperatures–a violation of its adpatatons

If abiotic factors don’t cause actual symptoms, sometimes they are able to weaken the host so that a pathogen can enter, and begin disease formation.  So abiotic conditions are often predisposing factors for the development of biotic pathogens. Many of the root rot pathogens such as Phytophthora or Armillaria

Wet soils and fine texture (clay) predispose trees to root rot. Note the “root snorkels’ did not prevent the problem

require a predisposing abiotic factor such as drought, saturated soils, high salinity or compaction to facilitate disease development.

Many canker diseases such as those caused by Botryosphaeria are predisposed by drought conditions

So where do pathogens come from?

I like the hospital analogy. Where do you go to get sick? A hospital! They certainly have a difficult time controlling the spread of disease there because that is where sick people go. So where do sick plants come from? Often a nursery!  Nurseries import plants from wholesale sources, propagate from their own stock, sometimes reuse their container media, and grow many hosts in a concentrated place over time. There is no better place for diseases to occur than in nurseries.

This is especially true of root diseases because roots are inside the container and often not observed at the time of purchase

Inspect nursery stock for healthy roots before purchase

(but you always should inspect roots of all purchased plants from six packs of garden flowers to boxed trees). Also, some nurseries suppress diseases with fungicides that do not eradicate the pathogen, so when fungicides wear off (after you purchase your plant), disease can develop from now unsuppressed pathogens. Nurserymen relax! I’m not saying that all nurseries sell diseased plants (at least knowingly), but consumers should take extra care when selecting plants and when bringing new plants to their property.

While landscape mulches don’t likely spread viable pathogens they can change soil moisture status enhancing collar rots if irrigation is not adjusted. This tree was also planted deeply, another predisposing factor for disease

Once pathogens establish in the landscape, they may continue to harm new plants. Some pathogenic spores blow in on wind or inoculum moves in water courses along streams or other water paths. Animals, people and equipment can move infested soil onto a property.   Once diseases have run their course, pathogens often survive as saprophytes in the diseased tissues. They overwinter or over-summer in debris on the ground. So sanitation is critical in disease control (more on this in another post). Fruiting bodies can be moved in the greenwaste stream but there is very little research showing that disease is initiated by contaminated greenwaste, even though some pathogens may survive there. We do know that when greenwaste is chipped, it dramatically reduces pathogen and insect survival. Stockpiling wastes for as little as seven days will reduce chances pathogen survival by an order of magnitude. Certainly our favored arborist chips are very unlikely to have viable pathogens especially when sourced locally.

Understanding that diseases are not usually caused by gardening practices but by a pathogen or an environmental factor is the first step in diagnosis and control. In my next post I will talk about disease diagnosis and detection…

Your New Year’s resolution : No “alternative facts” or “fake news” in 2019!

From the Bad-Ass Teacher’s Association – a group after my own heart.

Welcome to 2019! In keeping with the tradition of a new year, I’m hoping you will join me in resolving to promote good gardening science among your friends, relatives, colleagues, and customers. One of the most important tools you’ll need is a collection of resources that are not only science-based, but are relevant to gardens and landscapes (not agricultural production). With that in mind, here’s my list of authors and institutions who are credible resources.

First off, of course, I’ll have to start with the Garden Professor faculty. While this blog is a great archive of information from all of us, some of us have also published books and articles, recorded podcasts, webinars, and DVDs.

Print and digital media – individual authors
Dr. Jeff Gillman has a nice list of books to consider, in addition to those by Joseph Tychonievich and one by Dr. Holly Scoggins. And I’ve got my collection of books and DVDs as well. Dr. Lee Reich, while not officially part of our GP faculty, has published more books for the home gardener than any of the rest of us.

So many good books!

These are popular publications rather than peer-reviewed journal articles. But the authors have solid credentials and years of experience in teaching and research. That makes them reliable sources of information, and while no one is infallible, these authors are active learners and educators. You can be sure that they present the information in their disciplines as accurately and objectively as possible.

Print and digital media – university Extension publications
Ideally, university Extension publications undergo stringent peer review and are updated regularly. In reality, not all Extension publications are equal in quality. I’m on the faculty at Washington State University and one of my jobs is to keep our Home Garden series of articles current (http://gardening.wsu.edu/). I can confidently say that the fact sheets and manuals on our site have been through peer review and are as accurate as possible. Some are getting near the end of their shelf life (five years at WSU) and need to be revised or removed.

Many of these peer-reviewed publications are relevant outside of Washington State.

Are there other universities that have peer-reviewed, current, and relevant Extension publications for gardens and landscapes? If so, please add them to the comments and I will check them out. (To save time and aggravation, please check these out yourself first. Don’t just list them and wait for me to go through them with a critical eye.)

Social media, including blogs and Facebook

The Informed Gardener website – where it all began.

I first got into social media with the construction of my Informed Gardener web page. The white papers, podcasts and other materials housed here are all science based, but they have not been through peer review. Many of them have been adapted into peer-reviewed Extension fact sheets but all of them represent a collection of relevant information that remains accurate despite being dated. Hey, there’s only so much I can do…

The Garden Professor blog – 10 years old!

The Garden Professors blog was born in 2009, followed in 2011 with our Facebook page and discussion group. Both of these have the distinction of being the first (and possibly only?) exclusively science-based gardening groups on Facebook.

The Garden Professors page, where new tidbits are shared daily.
The Garden Professors discussion group, where anecdotes and home remedies are left at the door.

 

 

 

 

 

And yes, I’ve probably left someone or something out
By now you’re probably saying “What about Dr. X’s Facebook page or Professor Y’s blog?” This post is admittedly narrow, because I only know the people that I know. I’d like to expand the recommendations in this post to include other discipline experts who have information directly relevant to the mission of the Garden Professors. (This means we are NOT including information more relevant to farming or other types of agricultural production.)

So feel free to add your suggestions as comments, keeping in mind the criteria I mentioned above. Hopefully what we can create together is a really nice resource list for all us to use.

Tuning up for Pruning Up–Care, Maintenance and Utilization of Hand Pruning Tools

A. J. Downer

Fall is passing into winter and the bare sticks in my deciduous fruit orchard are calling to my annual fruit tree pruning chores.  I can prune my entire orchard with very few tools: a good pair of bypass clippers, a similar set of loppers

Illustration 1. Tri-edge saw blades are made from stainless steel and are not easily sharpened. When dull or bent they should be replaced.

(optional) and a high quality “razor” or “tri edge” saw.  Most hand tools require some maintenance especially the clippers and loppers.   Clippers are easily sharpened but modern saw blades can not be sharpened by gardeners. I usually just buy a new saw, replacing the old one when blade eventually dulls or bends from over zealous use (illustration 1).

Illustration 2. To sharpen bypass clipper blades follow the angle of the bevel. Do not sharpen the flat side of the blade

Before using your pruning tools inspect them for signs of damage. Blades should be sharp and straight.  Loppers should have their rubber “bumpers” intact otherwise your knuckles will be smashed after exerting force on a difficult branch.  Sharp tools offer less resistance and actually decrease injury to users. One exception here is with the modern “tri-edge” or “razor” saws. These saws can cut so quickly that you may pass through the branch you are cutting and continue on to some part of your anatomy quickly ripping your flesh open. I have suffered more cuts (some serious) from these saws than from any other gardening activity (although I was recently impaled by a frog metal art sculpture!).  They should be used with careful precision, not with the wild abandon and pruning fervor of the craven academic desperate for real world pruning experiences.  A thick long sleeved shirt and gloves will also help prevent cuts from hand pruning equipment.

Bypass clippers are so termed because the blade passes by the hook. To sharpen these, find the bevel on the edge of the clippers and align a small file to the same angle of this bevel, and file the bevel until you can feel the sharpness with your finger (Illustration 2).  Never sharpen the back side of the bevel—this will create a gap, and every time you cut, a flap of tissue will remain. Back bevel sharpened clippers will require blade replacement or grinding until the back bevel is gone. The hook does not require sharpening, do not attempt to file it. Repeat this process with lopper blades.

When you are done pruning for the day, wipe the blades of your clippers and loppers with an oil soaked rag or apply a few drops of oil and rub it into the blade. Most modern saws blades are made from stainless steel and require no oil protection.

As a Cooperative Extension Advisor, one of the most common questions I receive is: “Should I sanitize my clippers between cuts or between uses on various plants?”.  Indeed, many publications, extension leaflets, gardening columns, and other sources make broad recommendations to sanitize clippers after every cut. Some articles even compare various products for their killing efficacy.  Blind recommendations are often made to sanitize clippers when the pathogen is not  known or specified.  It is not necessary to sanitize your clippers when pruning most garden plants and fruit trees.  There are a few pathogens that are spread by moving plant debris, but published evidence that they are spread by hand pruning equipment (especially clippers) is nil. One exception is palm wilt caused by Fusarium oxysporum f.sp. canariensis which is easily spread by saws. Some of the canker fungi caused by Botryosphaeria can also be spread by pruning equipment. With many of these pathogens, a wound is required for infection so it may not be that the clippers are spreading disease so much as providing an entry point (infection court) so that pathogens have a way to enter.

If diseases are present in or near the plant already, sterilizing pruning equipment will simply provide a clean entry port for the pathogen—infection can still follow after the cut is made with a sanitized tool.

In my garden, I never sanitize clippers between cuts.  However, conditions vary across the US, and in some places rain, humidity, and temperature are more favorable for disease development.  If you have concern about spreading pathogens, prune during the dormant season, when the likelihood of pathogen activity is lowest. Apply dormant sprays containing copper to limit the onset of new fungal diseases that may enter pruning wounds.  If you still feel you need to protect wounds from dirty clippers I like to use the flame from a plumber’s torch to sanitize.  A few seconds along the cutting edge front and back kills all pathogens (Illustration 3).  The process is similar for a saw but efficacy is increased if the saw gullets are wiped clean with a cloth and then the flame applied. The only time I take these measures is when I know I am working with plants that can be inoculated by pruning (which is rare).


Illustration 3. A plumber’s torch will rapidly sanitize saws and blades when pathogens are present in plant tissues.

When pruning garden plants, there are a plethora of recommendations on how to make cuts. Rose experts have extolled the virtues of an angled cut so water runs away quickly, flush cuts used to be recommended by arborists as the highest quality cut. These examples are without research foundation. Cuts on woody plants should made to create the most circular exposure that leaves the smallest surface area possible. We abandoned flush cuts many years back because they cut into protective zones that limit decay in trees. Some gardeners feel compelled to cover their cuts with a pruning paint and there is a similar paucity of research to support this practice. Leave pruning wounds unpainted.

The Myth, the Legend, the Parasite: Romance, Lore, and Science beneath the Mistletoe

As we hurdle ever closer to the holidays and the end of the year, there’s lots of plants we could talk about – amaryllis, poinsettias (and the abuse thereof with glitter and paint), whether or not your cactus celebrates Thanksgiving, Christmas, Easter or is agnostic, and on and on.  Each of these plants have an interesting history and connection to the holidays, but today we’re going to be a little more naughty…but nice.  We’re going to talk about mistletoe.

Now, mistletoe is one of those holiday plants that you don’t really want growing in your own garden. That’s because, even though it is a symbol of love and even peace, it truly is a parasite … and poisonous. It has been celebrated and even worshipped for centuries, and still has a “naughty but nice” place in holiday celebrations.

Burl Ives, as the loveable, banjo-playing, umbrella-toting and story-narrating snowman in the classic “Rudolph the Red-Nosed Reindeer” claymation cartoon tells us that one of the secrets to a “Holly Jolly Christmas” is the “mistletoe hung where you can see.” But where does this tradition of giving someone an innocent (or not-so-innocent) peck on the cheek whenever you find yourselves beneath the mistletoe come from? And just what is mistletoe anyway?

While mistletoe specialists need mistletoe, the reverse does not hold—mistletoe in many regions is dispersed solely by dietary generalists.
Distribution of mistletoe (and mistletoe specialist birds). Source: Mistletoe Seed Dispersal. Watson, D.M.

There are around 1500 species of mistletoe around the world, mainly in tropical and warmer climates, distributed on every continent except Antarctica.  In North America, the majority of mistletoe grows in the warmer southern states and Mexico, but some species can be found in the northern US and Canada.  A wide variety of birds feed on the berries of mistletoe and thus disperse seeds.  These birds include generalists who opportunistically feed on mistletoe, and specialists who rely on the berries as a major food source.

Mistletoe Haustoria from from Julius Sachs’ 1887 Lectures on Plant Physiology. Source: The Mistletoe Pages

First, we’ll cover the not-so-romantic bits of this little plant.  Mistletoe is a parasitic plant that grows in a variety of tree species by sinking root-like structures called haustoria into the branches of its host trees to obtain nutrients and nourishment. It provides nothing in return to the tree, which is why it is considered a parasite.

 

A heavy mistletoe infestation.                        Source: Pixabay

Mistletoe grows and spreads relatively slowly, so it typically does not pose an immediate risk to most trees.  While a few small colonies of mistletoe may not cause problems, trees with heavy infestations of mistletoe could have reduced vigor, stunting, or susceptibility to other issues like disease, drought, and heat. So be on the lookout for mistletoe in your trees and monitor it’s progression.

This little plant does have a long and storied history — from Norse mythology, to the Druids, and then finally European Christmas celebrations. Perhaps one of the most interesting things about the plant is the name. While there are varying sources for the name, the most generally accepted (and funniest) origin is German “mist” (dung) and “tang” (branch). A rough translation, then, would be “poop on a stick,” which comes from the fact that the plants are spread from tree to tree through seeds in bird droppings.

“Baldur’s Death” by Christoffer Wilhelm Eckersberg (1817)

In Norse mythology, the goddess Frigga (or Fricka for fans of Wagner’s operas) was an overprotective mother who made every object on Earth promise not to hurt her son, Baldr. She, of course, overlooked mistletoe because it was too small and young to do any harm. Finding this out, the trickster god Loki made a spear from mistletoe and gave it to Baldr’s blind brother Hod and tricked him into throwing it at Baldr (it was apparently a pastime to bounce objects off of Baldr, since he couldn’t be hurt).

Baldr, of course, died and Frigga was devastated. The white berries of the mistletoe are said to represent her tears, and as a memorial to her son she declared that the plant should represent love and that no harm should befall anyone standing beneath its branches.

The ancient Druids also held mistletoe in high esteem, so high that it could almost be called worship. During winter solstice celebrations, the Druids would harvest mistletoe from oak trees (which is rare — oak is not a common tree to see mistletoe in) using a golden sickle. The sprigs of mistletoe, which were not allowed to touch the ground, would then be distributed for people to hang above their doorways to ward off evil spirits.

While the collecting and displaying of mistletoe was likely incorporated into celebrations when Christmas became widespread in Europe in the third century, we don’t really see mention of it used specifically as a Christmas decoration until the 17th century. Custom dictates that mistletoe be hung in the home on Christmas Eve to protect the home, where it can stay until the next Christmas Eve or be removed on Candlemas (which is Feb. 2). The custom of kissing beneath the parasitic plant isn’t seen as part of the celebration until a century later.

Washington Irving, who more or less reinvigorated the celebration of Christmas in the United States in his day and whose writings still define the idyllic American Christmas celebration, reminisced quite humorously about mistletoe and Christmas from his travels to England. He wrote:

“Here were kept up the old games … [and] the Yule log and Christmas candle were regularly burnt, and the mistletoe with its white berries hung up, to the imminent peril of all the pretty housemaids.”

Whether or not your housemaids will be in peril, the hanging of the mistletoe can be a fun Christmas tradition. Look for it at garden centers and Christmas tree lots this season.  Or maybe you can find some growing wild and harvest it for your own decor. However, I would recommend not getting it out of the trees the “old Southern way” — shooting it out with a shotgun.

Sources:

  • Tainter, F.H. (2002). What Does Mistletoe Have To Do With Christmas?  APSnet Features. Online. doi: 10.1094/APSnetFeature-2002-1202
  • Briggs, J. (2000). What is Mistletoe? The Mistletoe Pages – Biology. Online. http://mistletoe.org.uk/homewp/
  • Watson, DM. (n.d.) (accessed). Mistletoe Seed Disperal [Blog Post]. Retrieved from https://ecosystemunraveller.com/connectivity/ecology-of-parasitic-plants/mistletoe-seed-dispersal/
  • Norse Mythology for Smart People. (nd) The Death of Baldur. Retrieved from https://norse-mythology.org/tales/the-death-of-baldur/

 

What’s wrong with my plant? You’re not going to find pat answers.

As many of you know, the Garden Professors host a Facebook group dedicated to the discussion of science-based practices for gardens and landscapes. (Side note – if you haven’t joined us please do!) Recently we’ve had a spate of “what’s wrong with my plant” posts, usually focusing on some leaf issue and little other information. And far too often an eager group member will jump in with a fertilizer recommendation.  So today’s blog post has two objectives: explaining why you can’t reliably diagnose problems from a picture of suffering leaves and why blanket fertilizer recommendations should be avoided.

To illustrate the problem with armchair diagnosis, consider this photo below.

Interveinal chlorosis in Rhododendron.

Now there are two ways to ask a question here: the first is “what’s wrong with these leaves” and the second is “what’s wrong with my plant.” We can easily answer the first one: there is nutrient deficiency in the leaves, most probably iron or manganese. But that does NOT mean there is a deficiency in the soil. So we can’t address “what’s wrong with my plant” because we don’t have enough information.

How can we determine what’s wrong? My first question to the poster is invariably “have you had a soil test?” Soil test results will indicate whether the element in question is actually deficient, and will provide levels of other nutrients that could interfere with root uptake.  If there’s no deficiency of the nutrient in question, then adding fertilizer is not going to help! And adding fertilizer unnecessarily can create further soil nutrient imbalances and contribute to environmental pollution.

Lots of iron – no deficiency here!

Once we have the soil test results, we can then begin to address “what’s wrong with my plant.” But not from the original picture. (If you are curious about what else could be causing this problem, check out this blog post from 2011.)

Let’s try another. Consider the leaves in this photo:

Another unhappy Rhododendron

We now know to ask “what’s wrong with these leaves?” Ignore for now the deficiencies in the older leaves and look at the size of youngest ones compared to the older. The answer is fairly straightforward here: there was too little water available when the newly emerging leaves were expanding. Leaf expansion depends on turgor pressure – the higher the turgor pressure, the larger the leaves get. Once expansion stops, protective plant biochemicals are laid down which prevent further expansion. By comparing the youngest leaves to the leaves from previous years, you can see that they are significantly smaller. But why?

Again, we need more information before we can answer “what’s wrong with my plant.” Was there too little available soil water during leaf expansion? It’s possible, but this example is from western Washington State, a climatic region with wet springs. Most likely there is an issue with the roots. My first question with these cases is “can you easily move the plant in the ground?” This is my “wiggle test” – a way to determine if roots are established. In this case – and in nearly every case like this that I’ve seen personally – the roots are NOT established. Often this is because the plant (1) was not bare-rooted at planting and/or (2) was planted too deeply. Without decent root establishment there is not enough water uptake to support full turgor in expanding leaves.

It may be quick and easy, but “pop and drop” is not a good planting method.

Lack of an established root system also account for the interveinal chlorosis you can see in the oldest leaves. These leaves are fully expanded, probably because the plant was still at the nursery when these leaves emerged. But their color is off. A root system that doesn’t supply sufficient water for leaf expansion is by default not going to provide sufficient nutrients, either.  Adding fertilizer to this plant is not going to help! It needs to be dug up and replanted correctly or replaced. It is never going to thrive under the current conditions.

Armchair diagnosis can be accurate and fun if you follow a set of guidelines to extract more information. But simply recommending a fertilizer based on leaf appearance is neither science-based nor environmentally responsible.

No. Just….no.

“Fun-guy” in your Mulch

By Jim Downer

Fresh wood chips!

As many avid GP readers are aware, mulches are a common horticultural tool that help gardeners maintain soil moisture, nutrient content, weed suppression and assist in disease prevention.  The best mulch is made from chipped tree trimmings wastes and has a large wood content.  Coarse “arborist chips” mulch is fast becoming one of the most frequently sought after mulches for residential landscapes.  It is very effective and contributes to significant soil improvements over time.  As chip mulches decompose, the fruiting bodies of fungi are often seen growing up through mulch.  Sometimes, as gardeners work in previously mulched beds, they see mycelium or cordons (rhizomorphs) of mulch fungi growing through the mulch.  Some gardeners are not fond of finding mushrooms growing in their mulch and have termed these as “nuisance fungi”.  There have even been extension leaflets on nuisance fungi and how to rid them from your garden!!  Fungi are a natural part of mulch breakdown and their presence in mulches is desirable!

Phanaerochaete chrysorhiza invading Eucalyptus globulus mulch

The first encounter many gardeners have with mulch fungi is when they see “mold” growing in the chips or at the interface of mulch and soil.  Mold gets a bad rap with many homeowners when they find it after water damage in their house, so perhaps they assume it is also bad for their gardens.  Mold abatement in homes has become a specialized industry, and while the spores of some fungi can be human pathogens, fungi are not to be feared in gardens unless your immune system is damaged or otherwise compromised.   Unlike houses, gardens are a good place for fungi to grow and thrive.

Fungi absorb water and nutrients from their hyphae which grow into their food (mulch particles).  The absorptive lifestyle of fungi is unique.  Since fungi have no internal digestive systems, they rely on excreting enzymes outside their bodies and into their food which breaks down the substrate so they can absorb it.  By doing so, they also release minerals, sugars, amino acids and many other compounds for other microbes and plants to utilize.  Fungi are mostly saprophytes or decomposers, and their role is to release organic nutrients to soil so they can be recycled.  This is why mulches are so beneficial to woody plants.  Without fungi, forest litter would pile up largely undecomposed because bacteria and other microbes are less efficient in breaking down cellulose.   Some fungi are mutualistic partners with woody plant roots.  Ectomycorrhizal (EM) fungi rely on interactions between trees themselves and the litter or mulch layers under trees.  Fruiting bodies of EM fungi may appear as mushrooms or puff balls in or next to mulches.

Lepiota spp. an ectomycorrhizal species
Amanita muscari , another ectomycorrhizal species. Just don’t eat it.

 

 

 

 

 

 

 

 

Sometimes fruiting bodies (mushrooms) push through mulch, but are not the result of mulch presence.  Pathogens such as Armillaria mellea (oak root fungus) can form through mulch layers or turfgrass, but they are fruiting off the dead roots of their tree host.  Similarly, the inky cap mushrooms (Coprinus spp.) often grow saprophytically on dead roots (they are not the cause of root death) and will push through litter layers.  Coprinus are good indicators that a tree has dead roots.  Coprinus is not a plant pathogen, and mulch does not increase prevalence of pathogens in landscapes.  As we have discussed many times in the blog, mulches are unlikely to spread or support plant pathogenic fungi.

Coprinus spp., which feed on dead roots

Another way to view the role of fungi is the chemistry that they facilitate in soil.  Mulch is organic matter, which has a high concentration of carbon.  Carbon is transformed from a solid form into a gas – carbon dioxide – through the action of microbes (mostly fungi).  So oxidation of carbon is driven by fungi growing through their substrate (forest litter in forests or mulch in gardens).  In mulching systems this is a slow process taking a few years.  In composting systems it is rapid, taking months with the added energy of mechanical turning etc.   Slow decomposition of organic matter is useful, as the benefits of mulch in suppressing weeds, slowing evaporation from soils etc. are maintained over time. Slowly oxidizing carbon means that it will be around longer, creating less greenhouse gasses than in the composting process.

In publications that recommend ways for “dealing with nuisance fungi” it is suggested to let mulch dry out, which stops the action of the fungi.  This is one of the most harmful things that can be done for active mulch zones.  Killing the fungi in mulch also stops their oxidation of carbon, subsequent nutrient release and support for the high microbial activity in mulches that benefit both plants and disease suppressing fungi that plants rely upon to maintain their health.  While fungi can reactivate when dry mulches are moistened, their biomass is damaged by severe drought which also injures plant roots as well.

All good things come to an end or as our physics friends say, “Entropy increases!!”  So as labile (easily metabolized) carbon is used up in fresh mulches, fungi go into spore bearing or reproductive phases and begin to make fruiting bodies.  As long as there is labile carbon, fungi will thrive and grow mycelium and hyphae into their food.  When carbon is being used up (or when there is sufficient mycelium), fruiting bodies start to form.   To maintain these processes, it is important to add fresh mulch over the old decaying mulch.  Once or twice a year depending on temperature and moisture levels.  Along the way, some mulch may develop fungal fruiting bodies.  Fruiting bodies may resemble mushrooms, puff balls, earth stars, bird nest fungi, or simply resemble paint that has been splashed on the wood chips.  They are only trying to survive by developing spores which will later spread onto fresh mulch materials.  Most mulch fungi have very ephemeral fruiting bodies, so even if they are seen to be a “nuisance”, they will only be around for a very short time before they also decompose and become part of the remaining mulch layer or soil.

Ceraciomyces tessulatus, a paint-like fungus

One very common group of organisms seen in mulch and mistaken for fungi are the slime molds.  They are not related to fungi, but do develop spores and have a mobile (plasmodium) phase where they can be seen to slowly move from one spot to another.  Eventually, when the plasmodial stage is done feeding, the sporangial phase is made and they turn into spores.  The most commonly encountered slime mold in mulch beds is the dog barf fungus, a slime mold called Fuligo speticaFuligo is dramatic because it can appear overnight and is large (a patch of the sporangium can be several inches across).  When kicked, Fuligo bursts into dark spores that will fly up into the air.  Slime molds are also saprophytes and live on the decomposing organic matter in mulch.  They pose no threat to humans or garden plants.

Yellow slime mold, aka dog vomit fungus (photo from Wikimedia)

Fungi in the mulch are a good thing and indicate that moisture, temperature and organic matter are at the correct levels for high microbial activity!  This is what creates a healthy soil and ensures healthy garden plantings.

Thanksgiving: A celebration of the native plants and indigenous crops that grace the table

Native vs. non-native – that a subject that is brought up frequently on our forums and one we have to discuss at length.  However, I thought I’d take it from a different direction this week, a little diversion if you will, seeing as we are just a week away from our American celebration of Thanksgiving that centers around food – much of it native to the United States.

It is a holiday that is quintessentially American (or North American, since our Canadian friends also have their own Thanksgiving). A commemoration of not only the arrival and survival of the pilgrims in Plymouth in 1621, but of our thankfulness for what we have. It is a time for us to gather with family or friends and reflect upon our blessings.

While, much to my chagrin (and that of many others), Thanksgiving seems to have been swallowed up by the Christmas “season” and you can even go shopping for more stuff (an abomination, for sure) on a day when we are supposed to be thankful for what we have, it is still a day celebrated by many.

Turkey, dressing, potatoes, fresh bread rolls and pumpkin pie are the traditional fare for the celebration these days, but they are a far cry from what the original feast shared by the pilgrims and American Indians would have featured.

Historians agree that, while the feast was probably meat-heavy, turkey was probably not on the menu. It just wasn’t as popular a food item as it is today. Most agree that the original feast featured venison, with some waterfowl (goose or duck) and seafood (shellfish like oysters are a definite, maybe even eels or other shellfish).

I don’t think I’m alone in saying that I like the side dishes better than I like the actual turkey. There’s the dressing (or stuffing, depending on your preparation or colloquial terminology), mashed potatoes, sweet potatoes, and my aunt’s seven-layer salad that’s usually more mayo and bacon bits than vegetation.

The produce dishes at the first Thanksgiving would have been vastly different than the modern day smörgåsbord that we prepare. Experts agree that the majority of dishes would have been from native plants and indigenous crops grown by the local tribes, with a few ingredients showing up from the pilgrims’ gardens.

First off, the absence of wheat flour, sweetener and flour would mean the lack of the classic dessert…pumpkin pie. It is hard to imagine a lack of pumpkin while we live in a time in which we are surrounded by pumpkin spice everything (though mostly artificially flavored).

Sugar would have been too expensive to purchase for the voyage, and other sweeteners would have been limited to maple or other tree syrups. (Colonists had not yet brought over the honey bee, which is a European immigrant itself).

This is not to say that there wasn’t squash. There were squashes, including pumpkins, as part of the native diet at the time having spread from their origins in Mexico and Central America  . They were likely included in the feast, but either boiled or roasted, and unsweetened.

Beans would have probably been one of the dishes, as well. The Natives Americans ate beans both in dry and green form, but at a fall feast, the beans were likely the dried variety and cooked into a soup or stew. Corn was also a feature of the first Thanksgiving, but not sweet corn (which didn’t make an appearance until much later). The corn would have been a flint type (similar to popcorn) that would have been cooked into porridge or used as a bread.

Native tree nuts, such as walnuts, chestnuts and beech nuts could have also been used in the preparation of dishes. There isn’t any written record of the native cranberry or blueberry being used, either, but they would have been abundant in the area. They likely wouldn’t have caught on in popularity until sweeteners such as sugar from Europe or honey was available to dull their acidic bite, but the dried fruits could have been used in preparations of some of the meat. If there was a salad, watercress could have been used if an early frost hadn’t wiped it out.

The pilgrims had brought with them from Europe various seeds, including herbs and onions, that could have been used to flavor some of the dishes. They may have also brought things like turnips and carrots that could have been available for the first feast (though there isn’t any direct written proof).

One native food that would have most likely been on the first Thanksgiving table is the sunchoke (Helianthus tuberosus), or Jerusalem artichoke. Fallen out of favor for some time, the sunchoke is making its return to many gardens.

Image result for jerusalem artichoke
Jerusalem artichoke/sunchoke flower Wikimedia Commons

A true native food source, the sunchoke is the tuberous root of a species of sunflower (you may even see them growing on roadsides in the fall). The rhizome is roasted or boiled and has a nutty, starchy, potato-like texture and flavor. If you want to grow it, just remember that it is a perennial that will readily spread in the garden. These would have been the closest things to a potato dish the first celebrants would have eaten — we were still a long way away from bringing the potato from South America and the sweet potato from the Caribbean. (Botanist’s note: What we eat are sweet potatoes [Ipomea batatas], not yams [Dioscorea sp.], despite the insistence of canning companies. They aren’t even in the same family.)

So as you sit down for your Thanksgiving feast, be thankful for the blessings in your life and for the leaps and bounds our food options have improved over the past 400 years. Also be thankful for butter, flour, and sugar so you can have your pumpkin pie.