Advancing the science of gardening and other stuff since 2009
Author: Linda Chalker-Scott
Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and a Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets.
Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019).
In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award.
"The Garden Professors" Facebook page - www.facebook.com/TheGardenProfessors
"The Garden Professors" Facebook group - www.facebook.com/groups/GardenProfessors
resorting to clickbait tactics to get your attention. Here’s another – “Warning!
Graphic photos follow!”
Uncorrected roots in containers or landscapes will create chronic water stress problems for trees.
If you have failing
trees on your own property or on property you manage, you need to do one simple
thing before you assume that pests or disease are responsible: you need to
determine whether the root system is healthy and functional.
Now, I know you
can’t see underground, but you can draw some informed conclusions based on
whether you can see the root flare. To find the root flare, pull away any mulch
or groundcover that’s obstructing your view. Once you can see soil, you should
be able to see the root flare. If your tree looks like a utility pole (meaning
you can’t see the flare), then it’s been planted incorrectly. This single
mistake will have myriad consequences:
If it’s buried too deeply, it’s likely the tree was planted without removing the materials surrounding the roots. Bare–rooting woody plants before planting is crucial to their survival.
Roots that are buried too deeply will not have sufficient oxygen to establish a fine root system for water and nutrient uptake, much less develop any structural roots.
Moreover, without removing the materials around the root, the roots are less likely to establish into the surrounding native soil. Neither are you able to remove poor structural roots. Check out this post for more information.
Oxygen-stressed roots will die, compounding the reduced water uptake problem.
As the crown experiences chronic water stress, it will experience dieback while opportunistic pests and disease take advantage of a tree unable to chemically defend itself.
Opportunistic pests and diseases are not the cause of tree failure – they are simply indicators of an environmental problem. Proper diagnosis is discussed here and here.
Bare-rooting plants allows you to correct defective structural roots before planting.
You should be able to confirm lack of root establishment by performing the wiggle test (that’s the secret hack). This will allow you to see whether the soil around the roots moves. If it does, that means the roots are not established. If the tree has been in the ground for more than 6 months, it’s probably not going to establish. The sooner you can dig up and correctly replant a relatively newly planted tree the better your chances that will recover and establish.
intended to write the column earlier in the year, but it’s as relevant now as
it was in the spring. This post will familiarize you with how unseasonable
weather can affect your plants. Though I’ll be focusing on my own location in
Tacoma, the phenomena are global. You just have to pay attention to what
happened last week, last month, last year in your own location.
Our spring started out wet and cool, which is nothing new. But it was REALLY wet and REALLY cool compared to normal. This meant that our trees and shrubs had plenty of water to fill their expanding leaves and blossoms – but the lower than normal temperatures affected leaf growth. These dwarf Japanese maples had lots of leaves, but they were tiny! And they stayed that way, because once the leaves begin to lay down cuticle, they don’t expand any longer, even when it gets warmer. These maples put on a second flush of growth in the summer. Look at the difference in leaf size, determined solely by ambient temperature.
We had an abundance of flowers on our fruit trees – so dense that I put off pruning some of our heritage apple trees so we could get an even bigger crop (our black Angus love apples). But summer rolled around and…virtually no apples on ANY of the trees. What happened?
Well, that cool spring ensured that most of our pollinators were late to emerge from overwintering. I had wondered about them in the spring, as I could only see a few pollinating flies and no bees. But sure enough, we had almost zero pollination. No apples this year. Next year if the weather is similar I’m going to try using my battery-powered leaf blower to pollinate these trees. I’ll take pictures.
Fast forward to summer – for us, a record-breaking drought (again). Our temperatures weren’t as high as last year, but we still had very hot weather and no rain. For our landscape it’s not a problem, as we have well water for irrigation. But those gardeners who have little or no supplemental irrigation may very well find that their woody plants and perennials don’t perform very well next year: perhaps fewer flowers or branch dieback will appear. This is due to root dieback that happened all summer in unirrigated conditions. The damage is only seen in the following spring, when there aren’t enough roots to supply water to emerging buds.
Being able to predict the impact of specific weather events on your landscape plants is key to avoiding misdiagnosis and subsequent misuse pesticides or fertilizer in a futile attempt to rescue them.
Oh, and if you are wondering about the photo at the top, you’ll have to look at a post from 2009 to see what’s going on.
Irrigation bags, often called “tree gators,” are durable plastic bags used for irrigating newly planted trees. These projects have been discussed here and here and I still don’t like them as they don’t consistently benefit trees and often create conditions conducive to pests and disease. Plus, as the blog title suggests, their aesthetic attributes are nonexistent.
Newly installed trees and shrubs generally need to have supplemental water, period. It doesn’t matter if they are “drought tolerant” species – any plant needs sufficient water to establish roots. And where automated irrigation systems aren’t possible, there are many products that promise to deliver water to the establishing root system. Unfortunately, they often deliver other things as well, including pests, disease, and early death.
be fair, many time these trees die because they were poorly planted: we know
that improperly amended soils, structurally compromised root systems, inadequate
root preparation, and/or poor installation are the leading causes of young tree
failure. But anything that covers the trunks of young trees and reduces air
flow and light exposure will, over time, create a dark, moist, and reduced
oxygen environment that’s damaging to the bark of young trees. Wet, damaged
bark allows opportunistic pests and pathogens to invade.
Until a few weeks ago, I had not seen any irrigation bags that I actually thought might work. These bags are installed on stakes away from the tree trunks, and they deliver water to the area where tree roots need to grow, enhancing root establishment. It took a trip to Malmö, Sweden to see this innovative approach and my immediate reaction was “why hasn’t anyone thought of this before?”
There are many types of irrigation bags, from sleeves to donuts, but none of them are as good for tree or soil health as a thick layer of arborist wood chips. When wood chips can’t be used for some reason, irrigation bags set well away from the tree and actually kept full of water might be a good solution.
Long-time followers of this blog know that I’ve been researching, writing, and educating on the topic of landscape mulches for over 20 years. So whenever an article comes out in a newspaper or online that directly refutes our current understanding of mulch science, on-line and real-life colleagues quickly call it to my attention. Many times I choose to ignore the article, but when it’s from a highly regarded source with wide readership I feel the need to step in. Before I discuss the problematic statements, I want to explain part of my process in determining whether an expert is really an expert.
Here are two questions I ask:
Is an expert regarded as an expert in the area of interest by other academic experts?
Is there published research provided that supports statements that don’t agree with the current body of knowledge?
If the answer to both questions is no, then the source cannot be considered reliable.
To the writer’s credit, she seeks out academic sources for her information. Her source has stellar credentials in researching and educating about compost, but has no publications on mulching or mulch materials (Question #1 = no). And there are source quotes and author statements throughout the article that are not supported with evidence (Question #2 = no).
I’ve identified the misleading or erroneous statements and quotes below with my rebuttals. I have included linked references at the end that address these points in more detail. And we have dozens of posts on mulches in this blog’s archives.
1. “In a forest…there is no big heap, just a layer of an inch or two or three, breaking down and returning to the system.” Observations of relatively undisturbed forest floors reveal deep layers of woody debris, leaves and needles, and other materials falling from the canopy. Research has shown that a minimum of 3 inches of a coarse textured mulch are needed to restrict sunlight from reaching the soil and prevent weed seed germination. Any less than this will enhance, not prevent, weed growth. Deep layers of wood chips have been repeatedly shown to suppress weeds and enhance the health of desirable plants.
2. “The process releases humates…described as ‘black, gooey liquid’…” Humates, defined as recalcitrant materials that resist further decomposition, don’t exist in natural landscapes. The only place you find humates are in the lab, where analysis of organic material with an alkaline reagent (pH = 12) produces humus as a byproduct. And on garden center shelves, where heavily marketed humic acids, fulvic acids, and humates are located.
3. “The only difference in mulches, as long as you use organic materials, is the rate at which they decompose” This needs clarification. Rapidly decomposing mulches release high levels of nutrients in a short period of time; slowly decomposing materials release low levels of nutrients over longer periods of time. Compost falls into the first category, and readily available nutrients from any source can lead to nutrient toxicity in soils and imbalances in plants.
4. “In formal beds…fine- to medium-textured material” For best oxygen and water movement, mulches should be coarse and chunky. Sawdust and compost, for example, are too finely textured to allow for gas transfer and water movement, plus weeds easily establish on top of compost.
5. “If a bed needs compost, spread an inch before mulching” This statement needs clarification. The only way you know whether compost is needed is to have the results of a soil test showing an overall low level of nutrients. Then a layer of compost could be added before chips are applied.
6. “Save…the chunks fresh out of the arborist’s chipper for pathways…Or at least pile them up to mellow before you use them.” You don’t need to compost your arborist chips. They provide a burst of nutrients during the first month, when leaves are rapidly decomposed. Using older chips is fine, of course, but why waste that early nutrient boost to your soils?
7. “If supplemental fertilizer isn’t applied when your piling on coarse, fresh, carbon-rich wood chips…it can cause some drawdown in soil nitrogen.” Fertilizer should NEVER be applied unless there is a demonstrated nutrient deficiency, and wood chip mulches do not cause a drawdown in soil nitrogen. This myth has been dispelled by years of research showing no change to soil nitrogen covered with wood chips.
8. “Generally, mulch is applied in ornamental beds at a depth of one to three inches” See point #1. This is not a science-based recommendation.
9. On volcano mulching: “In addition to promoting bark decay, it causes the tree’s roots to grow up into the mulch layer, rather than down into the soil…the tree may eventually die, and even topple.” This classic correlation-elevated-to-causation is getting tiresome. There is NO published evidence, anywhere, that proper mulches (i.e., coarse arborist chips) are going to injure bark. They do not cause bark decay. Furthermore, tree roots grow where they have water, nutrients, and oxygen. This might be in the mulch layer. Growing deep into the soil is unlikely (not enough oxygen) unless the soil is excessively sandy or otherwise well drained. Any toppling of trees can be directly correlated with poor planting techniques that prevent roots from contacting and establishing in the site soil.
10. “Keep the mulch at least several inches away from tree and shrub trunks.” Why? Does this happen in nature? No. Per point #9, a natural woody mulch is not going to hurt trunks.
11. “And don’t invite rot by smothering the crowns of perennials” A good arborist chip mulch is not going to “smother” anything. Perennials are quite capable of growing through several inches of woody mulch, which also protects the crowns from freezing temperatures.
If we are going to encourage gardeners to use nature as a guide (see point #1), then points 4-11 are, well, pointless.
A week or so ago one of my “friends” sent me a link to a new journal article that claims plants can “see.” (The use of quotes here indicates that plant vision is suspect, as is the friend status of the person who sent the article.) Of course, dissecting the claims in this article became an all-consuming task for the next several hours. And rather than writing off those hours as never to be reclaimed, I decided a blog post would at least set those thoughts down to save other skeptics the time.
The article can be found here; it reports on the ability of leaves to mimic other leaves. While the concept of leaf mimicry is not new and has been seen in agricultural weeds for decades, this article goes a step further in claiming that plants can actually see the leaves they are to meant to mimic.
But let’s back up a bit to explore leaf mimicry, which is a thing. Leaf mimicry serves to protect plants against herbivory and other types of removal (like weeding). This phenomenon was reported decades ago where agricultural weeds were shown to change their morphology to more closely resemble the desired crop. The benefit is obvious: if a weed looks like a crop plant, it is unlikely to be removed through hand weeding. Likewise, if a weed resembles a poisonous plant, herbivores that are visual learners will avoid these weeds. When some plants of a species are disproportionately allowed to survive (i.e., not eaten or removed), they reproduce better. Higher reproductive capacity means more offspring: this is the process of natural selection. We can even see this in dandelions in our lawns and gardens.
One astounding leaf mimic is Boquila trifoliolata (a tropical woody vine). This vine can be found on several host trees, where it mimics the leaves of each host and thus avoids herbivory (this short article by Gianoli and Carrasco-Urra is worth reading).
The article I’m currently dissecting doesn’t report on field observations of mimicry; instead, it looks at an indoor situation where B. trifoliolata is grown in the presence of artificial leaves. The authors claim that the leaves on the living vines began to take on the shape of plastic leaves on artificial vines located on a shelf above them. Despite Gianoli and Carrasco-Urra’s earlier speculations that horizontal gene transfer or volatile chemical signals might trigger the mimetic response, these authors propose that plants can see the artificial leaves and adjust their leaf morphology accordingly. They base this hypothesis on papers written over a century ago that suggest plants have ocelli (“little eyes”) as a way of sensing light. Of course, a century ago we were decades away from discovering pigments such as phytochrome and cryptochrome, both of which inform plants about light conditions in their environment.
There are a lot of problems with this paper; it would take me a separate blog post to critique the Materials and Methods section alone. But the biggest red flag for me was the following paragraph:
This reflects significant author bias: the experiment didn’t work in the winter, so they did it in the spring and summer to see if they got results they liked better. And apparently they did.
SIDEBAR: Other potential red flags that careful readers might note include *A lead author with no apparent connection to an academic institution *A journal (Plant Signaling and Behavior) that focuses on the questionable field of “plant neuroscience” *An experiment performed under vague and uncontrolled conditions *Typos, grammatical errors, and awkward writing throughout
I’d like to
propose a couple of different reasons that these leaves may have changed shape
in the summer and not the winter:
Summer months are hotter and brighter
than winter months. The experimental leaves were exposed to increasing heat and
water loss compared to the shaded control leaves. Newly expanding leaf
morphology changes in response to changing environmental conditions.
Under increasingly hot temperatures,
plastic releases volatile chemicals, many of which are toxic. Leaf morphology has
been demonstrated to change in response to air pollutants.
This is a deeply flawed article based on a poorly designed experiment and reflects significant author bias in the interpretation of the results.
“In the Spring a gardener’s fancy lightly turns to thoughts of…plant shopping!”
If Alfred, Lord Tennyson had been an avid gardener, I am sure he would have included the above line in his poem “Locksley Hall.” I certainly look forward to visiting nurseries and plant centers in the spring to see what new goodies await. But my enthusiasm is tempered with caution – because bad things can lurk in otherwise perfect plants. I posted a four-part series way back in 2009 (the first year of our blog) on inspecting nursery plants.
I strongly recommend you review these posts before you buy – they are 13 years old but the information is still 100% valid.
Today’s post will add some new nursery nightmares to avoid at all costs.
Free complementary gift!
Make sure you’re buying a cultivar and not a nutrient deficiency
There are lots of interesting cultivars out there with unusual foliage. This dogwood is not one of them. Interveinal chlororis is a symptom of foliar nutrient deficiency – either iron or manganese – most likely caused by excessive phosphate fertilizer.
Fusion can be innovative in music and cuisine. Not so much in plants.
You can’t say they didn’t warn you
Back to nature
The scion of grafted plants is rarely as vigorous as the rootstock. Usually you have to wait a few years for the rootstock to take over, but there’s no waiting with these weeping silver birch specimens! But given how hideously trained these trees are, maybe it’s better that they will be slowly subsumed.
Nothing drives me crazier than simplistic solutions to
complex problems. Given our changing climate, there has been an explosion of
“drought tolerant” and “firewise” plant lists in the gardening world. Most of
these lists are devoid of science and all of them are removed from reality. The
fact is that taxonomy plays a minimal role in determining whether a plant will
tolerate environmental extremes.
Let’s start with the most obvious problems with these lists.
The goal isn’t to have plants that require less additional water – it’s to have
a landscape that requires less additional water. Similarly, the relative
flammability of plants is less important than whether the landscape surrounding
those plants is protected from fire. Plants don’t exist in vacuum and unless
you are strictly a container gardener a single plant’s impact on water use or
fire resilience is negligible. So a gardener’s questions should be “How can I
make my landscape more drought tolerant? How can I reduce the likelihood of
wildfire damage?” And these are questions that can be addressed with knowledge
gleaned from applied plant and soil sciences.
First of all, let’s think about what “drought” really means:
it’s an unusual lack of rainfall. It doesn’t mean no irrigation, and it doesn’t
mean dry soil. Drought is a climatological term, not one associated with soil
water management. Fine roots and their root hairs require water to function. Without
sufficient soil water plants will go dormant or die, particularly during
establishment. Plants that are drought tolerant can tolerate seasonal lack of
rainfall, but they can’t tolerate chronically dry soil conditions.
So we need to look at the landscape factors that allow
plants to survive droughts. This includes
Root systems that are well established. This means no barriers between the roots and the landscape soil system. Barriers include soil amendments and any materials left on roots during transplant (like soilless media, clay, and burlap). Obviously proper planting is key.
Adequate water movement into and within the soil environment. Anything within the soil environment that creates a textural barrier, like soil amendments, prevents water movement. Anything on top of the soil environment that creates a physical barrier, like sheet mulches or compacted layers, prevents water movement into the soil. Sheet mulches include plastics, fabrics, cardboard, and newspaper.
Adequate irrigation to support all plants in the landscape. The easiest way to determine whether there is enough soil water is to focus on one or two well-established indicator plants that you notice are the first to show wilt in the summer. That’s when the irrigation should be turned on. For our landscape in Seattle, it was a south-facing hydrangea.
Properly mulched soil. Mulch is crucial for soil and plant health, especially in terms of soil water retention and temperature moderation. The best choice for a tree- and shrub-dominated landscape is arborist wood chips. The best choice for arid landscapes is stone mulch – but if this landscape is dominated by trees and shrubs, you need the wood chip mulch. Trees and shrubs, by and large, are not the dominant plant form in arid environments. If you are going to grow plants out of place, you need to include the mulch that matches.
These four environmental conditions are key to maintaining a
drought-resistant landscape. In terms of appropriate plants, just realize that
plants with small, thick leaves lose less water than those with broad, thin
leaves. If you want a landscape that conserves water, by all means choose
plants whose evaporative water loss is the least.
I’m not crazy about the term “firewise” as it’s not really a science-based concept. There are natural landscapes that routinely experience fires, and plants native to these landscapes have evolved mechanisms to survive moderate fires. Trees with thick bark, for example, can survive fires that are low to the ground and quick to move through. Other plants may perish in a fire, but leave behind fire-resistant seeds that are able to germinate after the next rainfall. This is not what’s meant by a firewise landscape. Instead, the premise appears to be selecting plants that are low flammability. (Jim Downer tackled this one a few years back but the message just isn’t sinking in.)
Once again, the focus of this approach is mistakenly
directed to plant selection rather than landscape resilience. The best way to
reduce the risk of fire is to have a landscape filled with healthy, hydrated
plants and a soil protected by the least flammable mulch. The two mulches
recommended for drought tolerant landscapes also happen to be the least
flammable: stones and arborist wood chips.
Despite published evidence that arborist wood chips are not very flammable when compared to all other organic mulches, many governmental groups specifically recommend against them. This is a problem. Stone mulches are great choices IF the plants in question are native to arid zones. Trees and shrubs that are not from arid zones generally require the presence of woody debris to enhance mycorrhizal and root health. Without the proper mulch, these woody plants are less healthy and likely less hydrated than their counterparts under arborist chip mulches. That makes them more, not less, susceptible to fire damage.
Most of the confusion around arborist chip mulches is probably the result of regulatory agencies confusing bark mulches with wood chip mulches. Bark mulches ARE flammable as they contain waxes and are not great choices for root and soil health. They should be avoided. Agencies associated with fire control methods need to be better informed about the significant differences between these two types of mulches and how they affect plant resilience.
And finally, it is important to understand that major
wildfires are going to burn anything that’s organic. If you live in such an
environment, the best thing you can have in your landscape is no plant material
of any sort. A buffer of stone mulch is the only logical option.
I’ve promoted root washing of containerized and B&B trees and shrubs for a few decades now. The experimental science is slowly coming along – it can take several years to determine if the practice is more successful in terms of plant survival than leaving the rootball intact. But we know how soils function in terms of water, air and root movement, and we understand woody plant physiology. So it’s pretty easy to predict what will happen when trees, whose roots are held captive in layers of stuff, are then planted, intact, into the landscape.
Early in spring 2021 I purchased a couple of Japanese maples to frame our garage. As always, I root washed these specimens. Here’s a play by play of what we did, and what we found.
After more cleaning and untangling, we have a root system ready for planting. Well, almost.
If you are still wondering why this is a cautionary tale, consider what would have happened if the rootball was planted intact:
The root flare would have been buried below grade.
There would be multiple layers of stuff between the roots and the native soil (i.e., clay, burlap, and media).
The twine circled around the trunk would girdle it eventually.
The poor structural roots would not create a stable support system.
Now, one can argue all they like that there isn’t a robust body of scientific literature to recommend this practice – and there isn’t, yet. But leaving rootballs intact creates textural discontinuities between the roots and the native soil, and poorly structured woody roots are not going to correct themselves. So why not embrace a practice that removes both the soil and root problems?
I like catchy memes as much as the next person. They’re easily memorized and passed on. But “Save the planet, plant a tree” has always bugged me for two reasons. First, and probably most importantly, this simplistic mantra absolves people of doing MORE to improve our environment. It’s a “one and done” approach: “Hey, I planted a tree today, so I’ve done my part.” That’s hardly a responsible way to live in a world where climate change is a reality, not a theory. Planting trees (and other woody plants) needs to become part of a personal ethic dedicated to improving our shared environment, and that includes reducing our carbon footprint in MANY ways.
Second, and more germane to this blog, is that most people don’t know how to plant trees (and that includes an awful lot of professionals who should know better). Planting trees properly requires an understanding of woody plant physiology and applied soil sciences. Otherwise, newly planted trees are likely to die due to one or more problems:
Poor plant species selection
Mature size too large for site.
Species not adapted to urbanized conditions. This includes insistence on using native species whether or not they tolerate environmental conditions far different from their natural habitat.
Poor/improper soil preparation
Working amendments into the soil before, during, or after planting. Your goal is to keep a texturally uniform soil environment.
Digging a hole before seeing what the roots look like. It’s like buying a pair of shoes without regard to their size.
Poor quality roots
Most roots found in containerized or B&B trees are flawed through poor production practices. If you are using bare root stock, you don’t have to worry about this problem.
Can’t see the roots? Well, that leads to the next problem.
Improper root preparation
No removal of burlap, clay, soilless media, or whatever else will isolate the roots from its future soil environment. Take it all off.
No correction of root flaws. Woody roots don’t miraculously grow the right direction when they are circling inward. They are woody; it’s like trying to straighten a bentwood chair.
Planting at the wrong time of year. It’s best to plant trees in the fall, when mild temperatures and adequate rainfall will support root establishment and not stress the crown.
Not digging the hole to mirror the root system, especially digging too deep.
Failing to place the root crown at grade (which means the top of the root crown should be visible at soil level). Look at forest trees if you are not familiar with what a root crown looks like.
Stomping or pressing the soil around the roots. That just eliminates the air space in soil pores.
Adding “stuff” like transplant fertilizers, biostimulants, etc. They are not needed and you risk creating nutrient imbalances when you add “stuff.”
Poor aftercare and long-term management
Failing to add arborist wood chips as a mulch on top of the planting area. Regardless of where you live, natural woody material as a mulch is critical for root, soil, and mycorrhizal health.
Failing to irrigate throughout the establishment period and seasonally as needed. Trees will continue to grow above and below ground, and without a similar increase in irrigation the trees will suffer chronic drought stress during hot and dry summers.
Adding fertilizers of any sort without a soil test to guide additions. Trees recycle most of their nutrients; don’t add anything unless you have a documented reason for doing so.
That’s a lot to think about when you are planting a tree – but when you understand the science behind WHY these actions should be avoided, then you can devise a better plan for planting. And if it all seems to be too much, I have created a twelve-step planting plan that might be useful. Please feel free to share it widely!
Upon reading this post’s title, you may be inclined to stop right there. (That’s why I have an eye-catching photo to lure you in.) While logic may seem irrelevant to your enjoyment of gardening, I can guarantee that reading this blog post will challenge many seemingly logical assumptions you’ve heard or read about. Recognizing unsubstantiated assumptions and avoiding their pitfalls means you can make wise choices about how you care for your gardens and landscapes.
A few definitions are needed before we get started:
Correlation refers to variables whose changes mirror one another. For instance, the addition of nitrogen fertilizer to container plants is correlated to plant growth: as nitrogen levels increase so does plant growth. You can also have inverse correlation, where the variables move in opposite directions. An example is water availability in soil and planting density: the more plants you have in a specified area, the less water is in the soil.
Causation takes correlation one step further: it establishes that one of those variables is causing the change in the other. Using the same examples, we know through published evidence that the increase in nitrogen is causing the increase in plant growth, and the increase in planting density is causing the decrease in soil water because of competing roots. These relationships are obvious to us, but what’s important is that these causative effects have been established through scientific experiments.
Sometimes scientific evidence doesn’t exist to demonstrate causation. That may be because it’s impractical or impossible to run an experiment that tests for a causative effect, or it may be because the experiments just haven’t been conducted yet. The latter is the unfortunate reality for those of us interested in managing gardens and landscapes: there is no major funding agency that supports field research for us. There is research being done, but it’s on a small scale with a shoestring budget…so the body of literature develops very slowly. In such situations, we must rely on established applied plant physiology and soil science to ask whether a suggested correlation might be elevated to causation.
Which brings me to my current source of online irritation: the constant blaming of tree failure on mulch volcanoes. Yes, tree failure is definitely correlated with mulch volcanoes – because lots and lots of newly planted trees fail. But is the mulch to blame? No one seems to care much that there is NO published work to show that mounds of appropriate mulch materials will somehow kill otherwise healthy trees. Instead, observers jump to the conclusion that thick layers of wood chip mulch kill trees. They are elevating correlation to causation in the absence of either experimental research OR known plant physiology. In fact, there is published research to show that thick layers of arborist wood chip mulch enhance tree establishment and survival. And there are many poor planting practices that increase the likelihood of tree failure. But it’s easiest to blame the wood chip mulch, though it’s merely masking a multitude of planting sins.
Not interested in mulch volcanoes? Well, there are lots of other examples of garden and landscape management practices or phenomena that fall into the logical fallacy camp. I’ve linked to appropriate references, when available, that go into more detail:
and just about any gardening product you can think of where there is NO published evidence – or appropriate, established plant or soil science – that supports any causative, beneficial effect on plants or soils. Cornmeal, Epsom salt, gypsum, and kelp products are just some of these.
All of these products, practices or phenomena are correlated with some anecdotal observation (increased yield, healthier soil, plant failure, etc.) that elevates them to causative relationships. But no science.
I’d encourage you to think objectively about your closely held beliefs about your gardens or landscapes. Are you sure that what you’re doing is actually beneficial? How do you know there’s a cause-and-effect relationship? I’m not going to talk you out of your cherished beliefs – but if you are a science-based gardener, you might talk yourself out of them instead.