The nitty gritty on how water moves in plants, part 1

Maple sap – is it in xylem or phloem? Photo courtesy of PXHere.

It’s still too cold here in the Pacific Northwest to see much happening outside, so it seems a perfect time to write about something you can’t see anyway. That “something” is the movement of water and dissolved substances through two pathways: the xylem and the phloem. And before you roll your eyes and go watch TikTok videos, keep in mind that learning about these transport systems is critical to understanding how plants work and caring for them appropriately. To prevent brain overload, we’ll focus on how xylem works this month and tackle phloem next month.

Functional xylem is composed of dead, lignified cells connected into a series of tubes that move water one way – from the roots to the leaves. You can think about xylem like a giant straw sucking water out of the soil and moving it into the atmosphere. You’ll find dissolved substances in xylem water, such as soil minerals and root-stored compounds including growth regulators and sugars. Since this is a one-way highway, everything in the xylem ends up at the end of the straw, which is primarily the leaves. Most of the water dissipates into the atmosphere through the stomata (a process called evapotranspiration) and the dissolved substances are left behind.

Water movement through plants. Photo courtesy of Wikimedia

I mentioned that sugars can be found in the xylem, which will confuse gardeners who correctly associate sugars moving through the phloem. That’s generally true except during late winter when some trees, most famously maples, will produce a sugary sap in the xylem. While the exact mechanism of sap production remains unclear, we know that the sugars are coming from storage sites in the trunk and require a freeze-thaw cycle to enter the one-way xylem highway.

The temperature in the canopy of palm oasis can be much lower than the surrounding air, thanks to evaporative cooling. Photo courtesy of Flickr (Laura Hamilton)

While many people see this process as the plant “wasting” water, it is the only way that soil minerals can reach the leaves. In the summer, evapotranspiration lowers leaf temperature through evaporative cooling. Thus, doing anything to interfere with xylem function (like using antitranspirants) will have a long-term, negative effect on plant health. Likewise, anything in the soil that’s taken up by roots may end up in the leaves – for better or worse.

Gardeners need to think about this last caveat carefully. Plant species are highly variable in their abilities to regulate what goes into the xylem and what is left behind in the root tissue. Regulation is controlled by a barrier called the Casparian strip, which is a ring of living cells that require water (and its contents) to pass through their membranes to enter the xylem. You can think of the Casparian strip as a customs office at a country’s borders: some things are allowed in, and others are forbidden. Depending on how selective this border crossing is, soil contaminants can be left behind in the roots or carried through the plant. This is why it is so very, very, important to have your vegetable garden soils tested for heavy metals and other contaminants, and to take precautions if contaminants are found.

Arsenic is only one of many heavy metal contaminants that might be in your soils.

Another “drainage solution” that makes problems worse

Lack of surface drainage suggests problems below ground

I received an email this week from an arborist colleague who had been sent an “engineering solution” which claims to help with rooting issues in clayey soils or areas where root area is reduced. There was a spiffy diagram accompanying this which I’ve reproduced below.

I could dissect this for you and point out all the problems right now, but instead I’d rather supply you with some factual information and let you apply it to this “engineering solution.”

  1. Planting hole material that is not the same as the surrounding soil will have reduced water, air, and root movement due to the abrupt changes in texture. The hatched material in the pit appears to be different from the surrounding soil, leading to the assumption we’ve got modified backfill. Here’s a peer-reviewed journal article that discusses the fallacy of soil amendment.
  2. “Augured sump drain/root channel bores” are simply modified French drains. French drains serve to move free water (i.e., water that is not in soil pores) somewhere. Where “somewhere” is in this case is unclear.
  3. French drains and other drainage systems do NOT reduce the amount of water that soil holds. Field capacity is the term used to describe a saturated soil. A sandy soil has a low field capacity and drains quickly. The higher the clay content of a soil, the higher the field capacity and the slower the drainage.
  4. “Drainage material” placed beneath the root zone will slow water movement and create a perched water table above the “drainage material.”
  5. Drains as well as soils that are full of standing water have no oxygen. Roots will not grow where oxygen is unavailable.
Stop. Just…stop.

I don’t think I need to belabor these points any further. The bottom line is that you are going to create textural discontinuity problems in the planting pit if you follow these guidelines.  

“This one secret hack will save you time and money!”

I’m 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:

No flare = no chance

  1. It’s buried too deeply – the flare needs to be at the surface.
  2. 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.
  3. 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.
  4. 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.
  5. A structurally flawed root system, stressed for oxygen and encased in layers of clay (or potting media) and various combinations of burlap, twine, and wire baskets, is not going to establish quickly or well. Increasingly, it’s not able to supply sufficient water to the growing crown.
  6. Oxygen-stressed roots will die, compounding the reduced water uptake problem.
  7. 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.
  8. 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.

The wiggle test!

Understanding how weird weather affects our plants

Nutrient deficiency? Or something else?

I’d 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 two potted Japanese maples

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?

Normally, our apple blossoms are opening when there is lots of insect activity

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.

This chart only goes through August 28. We had no rain until October 20th.

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.

Crown dieback from water-stressed roots.

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: the good (rarely), the bad (frequently) and the ugly (all of them)

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.

Surprisingly, camoflauge green doesn’t actually camoflauge anything.

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.

To 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.

Arborist wood chips provide a highly absorbant matrix that releases water slowly into the root zone.

Maddening mulch myths

A good example of bad mulching

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:

  1. Is an expert regarded as an expert in the area of interest by other academic experts?
  2. 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.

This free, downloadable, peer-reviewed resource can help you learn how to differentiate bewteen credible and not-so-credible information.

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.

Just type “mulch” in the search box and find all kinds of good stuff!

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.

This is what an inch or two or three will do for you.

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.

Interveinal chlorosis is often associated with excessive soil phosphorus.

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.

Compost used as a mulch is a weed magnet.

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.

If your nutrients are off scale, don’t use compost!

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?

There’s nothing better than fresh arborist mulch straight out of the chipper.

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.

If wood chips cause a nitrogen deficiency, then plants apparently haven’t gotten the message.

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.

The soil of this lush landscape is completely covered with a thick layer of arborist chips.

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.

Our perennial rhubarb thrives in its deep arborist chip mulch.

If we are going to encourage gardeners to use nature as a guide (see point #1), then points 4-11 are, well, pointless.

Literature

Chalker-Scott, L. 2007. Impact of Mulches on Landscape Plants and the Environment — A review. Journal of Environmental Horticulture 25(4) 239-249.

Chalker-Scott, L., and A. J. Downer. 2020. Soil Myth Busting for Extension Educators: Reviewing the Literature on Soil Nutrition. Journal of the NACAA 13(2).

Chalker-Scott, L., and A.J. Downer. 2018. Garden myth busting for Extension educators: reviewing the literature on landscape trees. Journal of the NACAA 11(2).

Lehmann, J., Kleber, M. The contentious nature of soil organic matter. Nature 528, 60–68 (2015). https://doi.org/10.1038/nature16069

The plants have eyes! Another foray into B(ad) S(cience).

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).

Boquila trifoliolata [courtesy of Wikipedia]

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.

Figure
Image from White and Yamashita, 2022

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:

  1. 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.
  2. 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.
Leaves can orient themselves vertically to reduce exposure to high light intensities. Morphology can change, too.

This is a deeply flawed article based on a poorly designed experiment and reflects significant author bias in the interpretation of the results.

And just for more cowbell, here is Christopher Walken’s take on plant ocelli.

Shopping for landscape plants – an illustrated cautionary tale

Flower shows, like this one in Philadelphia, get gardeners excited about buying new plants.

“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.

Part 1: inspecting the root flare and trunk.

Part 2: inspecting the roots.

Part 3: avoiding suckers.

Part 4: avoiding poorly pruned young trees.

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

It may be striking, but it’s not healthy.

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

It’s two…two…two trees in one!

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.

Just don’t do it. Please.

Plant lists that shouldn’t exist

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.

Lack of irrigation and mulch guarantees a drought-stressed landscape regardless of the selected species.

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.

Drought Tolerance

Arborvitae can tolerate droughty summers, but they don’t tolerate improper planting and management.

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.

Even “drought tolerant” species like Sempervivum will die if there’s not enough soil water.

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.
The broad, thin leaves of hydrangeas lose water rapidly and make a good indicator plant for water stress.

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.

Firewise Landscapes

Jack pine (Pinus banksiana) produces cones that require fire to open and release their seeds.

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.)

Failing trees of any species are more flammable than living trees of any species.

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.

A stone mulch in a southwestern desert landscape.

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.

The wildland-urban interface faces the risk of extreme fire danger. [Photo byAnthony Citrano]

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.

Why root washing is important – an illustrated cautionary tale

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.

Maple newly purchased from nursery.

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.

Container removed, exposing fine roots. Some of the media has fallen away and is at the bottom of the wheelbarrow.
Since we can’t see the root flare, we mark the point at which the trunk and soil meet.
As we remove the container media, we find burlap and twine. And under that, a clay root ball. There is a root crown somewhere…
Some beating on the clay rootball helps create some cracks where water can then help with the process.
Into a nice soaky bath to loosen up that clay. The longer it sits, the more clay will slough off.
We speed the process along with a directional spray of water.
Jim gets his fingers into the wet clay to pry it away from the roots. Still no root crown, but you can see the Sharpie line on the trunk a couple inches above the clay.
Eureka! A root flare several inches below the original media level.

After more cleaning and untangling, we have a root system ready for planting. Well, almost.

We have roots, but we still have some problems.
It’s got some pretty crappy roots (from not being potted up properly at the nursery), and the remanent of a stake next to the trunk (about 4 o’clock). But there is a nice structural root to the left, with healthy fibrous branches.
“Knee roots” have to go (I call them “knee roots” because they are at 90 degree angles). They have poor structure and will only continue that downward growth pattern, rather than growing outwards. The easiest thing to do is sever them when they turn downward at 90 degrees – don’t worry about removing them if they are too tightly entwined. New root growth at the cut will be directed outwards.
We neglected to get “beauty shots” of our maples through the summer, but you can see one of them to the left of the New Zealand flax plant in the pot. Both maples established their root systems quickly and grew vigorously throughout the summer.
Now in late October, the maples are turning color. Note the distance between the trees and the garage – this ensures that we will have little branch/building conflict as the trees grow in height and spread.
Here’s one of our beauties getting ready to shut down for the winter. They thrived throughout the summer, even when we reached record high temperatures. We look forward to their continued success in years to come.

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?

Caveat emptor!