Author: Linda Chalker-Scott

The emotionally-charged native plant debate only seems to be growing. Well-meaning but misinformed decision-makers continue to institute native plant policies with pressure from special interest groups. Most recently, North Carolina’s General Assembly weighed in on the side of emotional appeal rather than research-based information in mandating “that native trees, shrubs, and other vegetation are [to be] used for landscaping at state parks, historic sites, and roadways.”

Don’t get me wrong – I love native plants and recommend the use of well-suited native plants in gardens and landscapes. I’m co-author of a book that helps gardeners in the Pacific Northwest choose native species that are likely to thrive in their gardens. But the belief that native plants are superior to introduced species in urban and other unnatural areas is just a knee-jerk reaction to the very real environmental and ecological problems we face. It gives believers a false sense of accomplishment in that they can reverse significant threats such as climate change, wildlife extinction, and pollinator decline simply by using native plants rather than introduced species.

Supporters for this native-only policy list the same tired (and false) reasons that native plants are superior to introduced plants. Here are some of those reasons cited in the North Carolina decision, along with my commentary:

“There are many environmental benefits to native plants, and they are much more likely to thrive in our weather and soils” (North Carolina Department of Natural and Cultural Resources Secretary D. Reid Wilson)

• The concept of nativity is subjective and many scientists argue that such a subjective division makes it difficult to study, much less discuss, the benefits and drawbacks of introduced plants .
• This post by Dr. Bert Cregg bursts the bubble on some of the native plant superiority myths.
• Native soils are not the same as compacted, amended, and otherwise disrupted soils found outside natural ecosystems.
• There is no research to support that native plants thrive in soils that have been disrupted by development and urbanization.

“Native plants are adapted to the state’s environment and more likely to thrive, especially during drought.”

• Roadways, state parks, and historical sites are not natural environments (though some parts of parks and historical sites could be).
• Plants that can adapt to disturbed environments are most likely to thrive. Some of these are called weeds.
• Plants that can survive periods of drought have morphological and/or physiological adaptations for doing so. It has nothing to do with their nativity.

“They support pollinators essential to food production and ecosystem health and boost otherwise declining bird populations that depend on insects associated with native gardens.”

• One of the basic tenets of ecology is that new resources are exploited by existing members of a food web. What happens with one species of insect or bird or plant is not the big picture – ecology is the big picture.
• This blog post by Dr. Bert Cregg discusses a paper showing that exotic species can grow more quickly than native plants, but they are eaten more by herbivores.
• This  blog post looks at some of the research on insectivorous birds that contrasts with the claim that native birds require native insects.
• The most biodiverse landscapes are those with a high diversity of plants. The vertical structure of a landscape, created by the varying heights of trees, shrubs, and other plants, is crucial for bird habitat. I’ve published both a research article and fact sheet on this topic.

“Native plants, especially grasses, are better able to store carbon, thereby reducing greenhouse gases.”

• Native plants have supercharged photosynthesis? There’s a Nobel Prize waiting for someone to demonstrate that.
• Trees and other long-lived woody plants are best for storing carbon. Certainly not grasses. And the nativity of the woody plants is irrelevant to carbon storage.

“Native plants provide habitat for birds and other pollinators, are more resilient, and require less fertilizer and other maintenance.” (Brian Turner, policy director at Audubon North Carolina)

• Birds and plants have complex and often unexpected relationships. This post discusses a review article on the interaction between birds and those plants who depend on them to spread their seeds.

In June 2023, North Carolina’s Department of Cultural and Natural Resources installed a new 100% native plant garden in front of their DNCR headquarters. In comparing the before and after photos of the site, I’ve got a few observations.

• If storing carbon is important (as stated earlier), then cutting down all those trees and shrubs (which don’t appear to be invasive species) was an interesting decision.
• Why not just add a native garden to the existing landscape? That would have increased the plant diversity and retained the vertical structure, which is highly important for biodiversity.
• If we want stable, biodiverse landscapes in our urbanized environment, we must include the use of introduced species – especially trees. 

“This policy is a big win for birds and everyone who cares about North Carolina’s wildlife. It just makes sense. ” (Brian Turner, policy director at Audubon North Carolina).

• Nope. It’s a big win for dogmatic belief systems.

There are many things that we can do in our gardens and landscapes to maximize biodiversity. Spouting false claims about native plant superiority, garden shaming those who don’t eliminate introduced plants, and forcing communities, cities, and states into lock-step on what can and can’t be planted is not part of that process.

I’ve been doing horticulture myth-busting for almost 25 years now – and what I’ve learned is that myths are zombies. Not only do myths not stay dead, but new zombie myths are also continually created. One of the newest bright-n-shiny distractions is electroculture. It’s EVERYWHERE.

What is electroculture, you might ask? Well, Jaccard (1939) described it as “the stimulation of growth in plants by means of electricity passed into the atmosphere surrounding them or into the soil in which they are growing.”

There was a surge in research in the late 1800s through the early 1900s, partially due to earlier observations which tied electrical storms to improved plant growth. (Further research determined that lightning fixes atmospheric nitrogen into a solid form (nitrate), which dissolves in raindrops and enters the soil system. This was undoubtedly responsible for the reported improvement in plant growth after electrical storms.)

Scientific interest in electroculture tapered off with advances in plant physiology and the development of commercial fertilizers. Furthermore, the few scientific publications that came from early studies showed no consistent benefit from electroculture:

• “Favourable results in increased growth and yield have been obtained from time to time, but they are uncertain and largely dependent on weather conditions.”
• “Plants on poor soil are little influenced, since electricity…does not provide either nutrients or energy.”
• “A current of 10 milliamps inhibited growth in 5 plots, but in 10 others yields increased.”
• “Electroculture experiments produced no differences in the treated trees in growth or yield.”

In the 21^st century, a desire to use fewer chemical fertilizers has spurred renewed interest in electroculture. There are vast numbers of websites proclaiming improved plant growth from sticking copper wires in the ground. None of these are backed with any reliable evidence, but proponents argue that new research (since 2000) supports this practice.

I did a search of the scientific literature through AGRICOLA, CABI, and Web of Science/BIOSIS. There were zero publications after 1968. However, Google Scholar lists several. Google Scholar searches for publications in any form on the internet that have been authored by scientists. Here’s an example of what you will find:

• A 2021 conference report for the IEEE 13th International Conference on Humanoid, Nanotechnology, Information Technology, Communication and Control, Environment, and Management (HNICEM).”
• A 2021 Research Centre Lab report from the Late Ramesh Warpudkar ACS College Sonpeth, India.

These are not peer-reviewed, scientific journal publications. Even earlier ones from the 1980s are in irrelevant journals such as Journal of Biological Physics. Plant scientists publish in plant science-related journals. Researchers outside the field of plant sciences really have no clue how plants function, nor do they have the expertise to design experiments which consider the variables that affect research in applied plant sciences.

Yet, proponents of permaculture push this concept without evidence, using anecdotes as equivalents to scientific data. Conspiracy theories abound, with accusations that chemical companies have forced scientists to cease electroculture research.

Proponents of electroculture “rely on theories of geobiology and use light devices, antennas, or magnets intended to act on the cosmo-telluric electromagnetic fields of the universe, as in dowsing (Wikipedia).”

Worldwide, scientists consider electroculture to be a pseudoscience, particularly because it does not propose any plausible scientific mechanism to explain how electricity would stimulate plant growth.

If and when this changes – when recognized plant science experts publish positive results that are confirmed by other plant researchers – those results will be in bona fide plant science journals and be worth discussing. Right now, don’t waste your time with another horticulture myth that refuses to die.

If you are interested in honing your BS detector, please take advantage of this peer-reviewed Extension Bulletin.
My thanks to Sylvia Hacker for finding great vintage photos to help illustrate this post.

Last week there was much ballyhooing over a new article on the benefits of native plants in supporting insect populations. I’ve posted on the fallacy of native plant superiority before, pointing out that landscape biodiversity not plant provenance, is most important for supporting all types of beneficial wildlife. Despite robust, published evidence to the contrary, more people and governing bodies believe that native plants are the magic bullet for urban landscapes. (Never mind the fact that there are no plants native to urban environments.)

Using CRAP analysis to assess information:

• C = credibility. Are the authors experts in the field of interest?
• R = relevance. Is the information applicable to the field of interest (in this case, management of plants in urban landscapes)?
• A = accuracy. Is the information grounded in current, relevant science?
• P = purpose. What is the underlying reason that the information is being shared?

This most recent paper warrants a careful dissection as it has gone viral online. For me, the first red flag is that there are no plant or soil scientists on this team. The first two authors, who were responsible for developing the main ideas and designing methodologies, are both ecologists by training. The other authors are involved in insect collection and identification as well as ecological modeling. Not having plant and soil scientists on the team to ensure science-based practices are followed during landscape modification is a serious oversight.

The methods section regarding the study site is astonishingly vague, given this is essentially a landscape plant installation and management project (i.e., applied plant science, not ecology). A well-designed experimental project would include control plots, replication of treatments, site analyses (including soil type and texture as well as soil testing for nutrients and organic matter), and detailed explanations of how the site was prepared, how plants were selected, prepared, and installed, and what site management occurred post-installation.

Here is the section on how this “experiment” was designed:

“In mid- April 2016, 80% of the site was substantially transformed through weeding, the addition of new topsoil, soil decompaction and fertilisation, organic mulching and the addition of 12 indigenous plant species…Selected plant species met the criteria of being locally indigenous to the City of Melbourne and represented a range of growth forms— including graminoids, lilioids, forbs and trees— requiring no ongoing management such as watering and fertilisation.”

The purpose of the methods section is to provide detailed explanations on how the study was conducted so that it could be replicated by other scientists elsewhere in the world. There is no way to replicate this study properly, as the methods are vague and very possibly not based on applied plant and soil sciences:

1. “Addition of new topsoil” As we’ve pointed out in this blog numerous times over the past 14 years, you don’t add new topsoil to landscapes.
2. “Soil decompaction” What is this? Does it involve tllling, which would directly affect the health of the two existing trees?
3. “Fertilization” What is the fertilizer? When was it applied in the process? At what concentration and based on what data was it applied? You need to know baseline levels of nutrients before you can rationally add any fertilizer.
4. “Organic mulching” What material? Compost? Cardboard? Bark? How deeply was it applied?
5. “Addition of…plant species” Were these bare-rooted or simply popped out of the pot and dropped in the new topsoil?
6. “Requiring no ongoing management such as watering” News flash: newly installed plants REQUIRE irrigation during the establishment period regardless of their nativity. And this site now contains substantially more plants than before, meaning increased competition for water and other resources.

The problems with this nonscience-based approach to landscape plant management can be see by comparing the two spotted gum trees that were on site before this project began. Corymbia maculata is a threatened native species in Australia and the continued health of these trees should have been paramount before any site work was initiated.

Unfortunately, these sorts of projects, conducted by teams with no soil or plant scientists and published in journals that are not specific to urban plant and soil sciences, are neither well-designed nor useful. The mindset of many researchers outside the applied plant and soil sciences is that there’s no real science to preparing soil, installing plants, and maintaining the site. This current paper does not even meet the standard of being experimental: it is merely a report on what happens to insect populations when a landscape is altered. There is no basis for comparisons. Any conclusions drawn are anecdotal.

It’s bad science.

This past week I was out of town at a conference, and since the week was supposed to be a scorcher I made sure my husband was going to water the container plants daily. And indeed, temperatures were in the 90s, dropping to the mid-60s at night. But the container plants looked great when I got home and I didn’t think much more about it until the next day. My husband called me into the living room, pointing at our massive mophead hydrangea which looked like it had been torched. Leaves and blossoms were wilted and browning. Every single stem was affected. Since our landscape is on an automated sprinkler system, what the heck happened?

This is when caution and objectivity are important. I wasn’t going to go cut the whole thing down, even though it looked terrible. Instead, I made observations of the site (not just the plant):

• The site is on the north side of the house, where plant only receive direct sunlight in the morning and late afternoon during the summer.
• No other plants were affected – not even the smaller hydrangea to the west of the damaged plant.
• The irrigation system had been working normally.

When diagnosing plant problems, it’s also important to consider the history of the plant and the landscape:

• Hydrangea is at least 55 years old.
• No soil disruption or other site disturbance
• No pesticide or fertilizer use
• Mulched with arborist wood chips

Given that no other plants were affected, the problem was with the hydrangea itself. Hydrangeas use a lot of water to support their large, thin leaves and massive flower heads. When the weather suddenly turned hotter and temperatures stayed abnormally high in the evenings, the plant could not recover its water loss overnight. Many flowers and leaves experienced terminal wilt – that means they lost too much water and tissues turned brown. Other flowers and leaves were able to recover as day and night temperatures returned to normal.

What could we have done to prevent this? Had we seen the wilt occurring during the day, we could have turned on the sprinklers manually in that part of the landscape. Hydrangeas are a good indicator of low soil water. In future summers, as we continue to experience hotter and drier conditions, we will keep an eye on our hydrangea and use additional irrigation if necessary.

One of the most annoying weeds in garden and landscape beds is horsetail (Equisetum spp.), a genus native throughout North America and most of the rest of the world. They have survived since prehistoric times because they are highly adaptable to their environments and are almost impossible to eradicate. There is great debate among gardeners on whether to pull or cut horsetail. Online you can find statements such as this:  “…each time you break the stem, little portions under the soil regenerate new plants. Essentially, you will be creating more horsetail.” This and many other websites recommend cutting instead.

Unfortunately, this is bad advice. The trick to eradicating any perennial weed without chemicals (or at least bringing them to manageable levels) is to starve them to death. Plants depend on their roots (and rhizomes in the case of horsetail) to survive, so anything that reduces root resources is going to eventually kill the plant. Obviously the more above-ground material you can remove, the less photosynthesis occurs and fewer resources are transported to the roots. Pulling weeds, especially if done with a forked weeder (also used in this post), is going to remove far more material than simply cutting weeds off at the surface.

Once you start a weed removal project, you have to keep after it: once is not enough. There will be rhizomes or roots left underground to support new stem growth, and once they reach the soil surface they will start producing resources to send to the roots. “Constant vigilance” is needed to keep these shoots in check. You can significantly reduce the repeated pulling by adding a thick layer of arborist wood chips to the newly weeded site. This forces the roots to put even more resources into stem growth to reach sunlight, meaning fewer weeds and more successful, desirable plants.

There is one caveat for controlling any weed that spreads underground. If you can’t control the spread from adjacent properties, you will not be able to eradicate the problem. In such cases, you may want to install a root barrier along the edges of your gardens. You simply dig a trench and install the barrier of your choice, making sure there are no gaps between the sections. Treated timbers, concrete pavers, and other materials that are slow to degrade can be used. The depth is going to depend on your soil conditions and the weeds of interest; some preliminary digging to determine the depth where you find weedy rhizomes and roots will help. Keep in mind that root barriers will also interfere with the root spread of your desirable plants.

If root barriers are not an option, the other method you can try is to densely plant low shrubs and perennials along the property line to create a competitive line of defense. The roots will compete for space, water, nutrients, and oxygen; the crowns will create a shaded environment where invading stems struggle for space and sunlight. You will still have to watch for invaders, but the amount of weeding needed will be far less than it was before. And don’t forget the mulch, both for the benefit of your barrier plants and to force invaders to use more resources to get their stems to the surface.

This method works for ALL plants – not just horsetail. (Plant physiology is funny that way.) Bindweed, English ivy, Himalayan blackberry, and Canada thistle are all weeds that I have personally controlled through physical removal and deep mulching with arborist wood chips. If you’ve had success with this method on another aggressive weedy plant, be sure to post a comment!

I just returned from one of my self-imposed retreats where I have no cell phone service nor internet. This means I can focus on writing without interruption. One of my projects this year is to publish a scientific critique of permaculture (stay tuned for that late 2023). Part of my process is to read popular permaculture publications and I am focusing on Gaia’s Garden.  Earlier I’ve posted some general critiques of the book (you can find them here, here, here, and here), but until yesterday I had missed a big, fat problem: a section labeled “The Power of Sheet Mulch” (pages 71-75 in the first edition).

First of all, let me state that part of permaculture philosophy is to follow nature in our gardening practices. There are few examples of sheet mulching in nature. The only one I can think of occurs in deciduous forests, where fallen leaves can create a sheetlike cover several inches deep. Combined with microbial activity and wetter soils, these sheets create low soil oxygen conditions. During the winter the leaves are broken down and by spring have degraded to the point that air and water movement have resumed enough to support plants as they break dormancy.

In contrast, here’s what permaculture recommends. At the outset, let’s acknowledge that none of the practices discussed remotely resembles any natural process. Furthermore, there are no science-based citations to support the practices or the claims of success. It’s completely anecdotal.

A sidebar provides the methods and materials for constructing the “ultimate, bomb-proof sheet mulch.” Here’s a condensed step-by-step breakdown:

1. “Soil amendments, depending on your soil’s needs” are added to the top of the soil before the sheet mulch is applied. How you determine your soil’s needs involves either “us(ing) a soil test or your own understanding of your soil’s fertility to guide the type and quantity of soil amendments.” These include “lime, rock phosphate, bonemeal, rock dust, kelp meal, or blood meal.”
2. A “thin layer of high-nitrogen material” is placed on top of the amendments.
3. Next we “…apply a layer of weed-suppressing newspaper or cardboard (or even cloth or wool carpet).”
4. On top of the sheet mulch another thin layer of high-nitrogen materials is applied.
5. Next we are to add “about 8 to 12 inches of loose straw, hay, leaves…” or any other bulk mulch materials recommended.
6. Then we should add “an inch or two of compost” or “you can substitute manure or several inches of easily compostable material.”
7. Finally “2 inches of…straw, fine bark, wood shavings…” or other listed “weed- and seed-free organic matter” adds the finishing touch.

So. Much. Stuff.

So let’s add this up: a half-inch or more of sheet material, an inch or more of high nitrogen material (from those two additions), 8-12 inches of bulk mulch, another 1-2 inches of compost (or several inches of the substitute), and 2 inches of “the finished look” materials. That’s at least 12 inches of wet (oh, each layer is sprayed down with water as it’s applied), poorly drained material on top of the soil.

I’m all for following nature in how we manage our gardens and landscapes. But deep sheet mulching isn’t natural. It’s bat-sheet crazy.

“Dirty Dozen” is one of those short, alliterative phrases that’s easy to remember and fun to use. In today’s post, I’m applying it to garden products whose production or use can be damaging to the health of ecosystems, environments, and even humans. How many of these products are in your garden shed, or appear in ingredient lists of other products? Each short description below has one or more links to additional information. After you count them up, see how you rank on the Charlotte Scott Meme-O-Meter^TM

Product manufacture damages ecosystems

Peat moss. Peat moss bogs are slow-growing ecosystems that store vast amounts of carbon. Removing peat moss destroys these ecosystems (which can take centuries to regrow) and releases C0₂ into the atmosphere. Do a little experimentation with sustainably sourced or locally available crop residues and see if you can find a more environmentally friendly substitute.

Kelp. Kelp, or macroalgae, are the basis of intertidal and subtidal food webs. Removal of these plants creates underwater deserts where little life can be found. Restoration can be done, but it’s a slow and expensive process. Ask yourself why you think you need kelp and compare this to the facts.

Bat guano. Seems like a reasonable use of a waste product, right? Nope – and Dr. Jeff Gillman explained why in a post more than a decade ago. If you want a sustainably produced, manure-based fertilizer, you can find many options at garden centers.

Products whose use damages garden and landscape soils

Rubber mulch.  Recycling used tires is a good idea; grinding them up and putting them on top of your living soil, not so much. There are much better mulch choices out there.

Landscape fabric. Landscape fabric does not control weeds, nor is it permeable once installed. It’s a sheet mulch that restricts water and air movement between the soil and the atmosphere. Weeds will quite happily colonize the surface while the roots of desirable plant struggle for water and oxygen below the barrier.

Plastic mulch. There is nothing worse (see chart in this link) you can put on a living soil unless your intention is to kill everything under it. You may see it used in agricultural production, but that doesn’t make it a good choice for your gardens and landscapes. Again, there it a much better alternative to this and all other sheet mulches.

Epsom salt. Would you be so excited about buying this stuff if it was correctly labeled as magnesium sulfate? That’s all it is – an inorganic chemical. Despite its soothing name, Epsom salt is not a cure all for anything except a magnesium deficiency in the soil. Overuse can create nutrient imbalances in soils.

Products whose use damages plants and plant-associated microbes

Phosphate fertilizer. The most overused nutrient in home gardens and landscapes, and one that can cause iron deficiency in plants. The only time you should add anything containing phosphate – including compost or other rich organic material – is if you have a soil test indicating a deficiency.

DIY garden remedies. Self-proclaimed gardening experts come up with all kinds of home-made potions as safer alternatives to conventional fertilizers or pesticides. There are good reasons that both fertilizers and pesticides are regulated at state and/or national levels: it’s the only way you can know exactly what the active ingredients and when, where, and how to apply these chemicals. To follow some “chemical-free” recipe from the internet is playing Russian roulette in terms of collateral damage to soils and non-target organisms.

Wound dressings. Unfortunately, many gardeners don’t understand that plants and people respond differently to injury. While antibiotic dressing and bandages are good for healing our nicks and cuts, trees have a completely different response. Slathering black goo or paint over tree wounds is the last thing trees need to seal damage naturally.

Product whose use can be harmful to human health

PAM hydrogels. PAM (polyacrylamide) hydrogels have limited usefulness in home gardens and landscape – worse, they have to potential to injure people and pets. (There is also a list of references used in the linked article.) After these same materials are used in labs for gel electrophoresis (used for DNA analysis, for example), their disposal is generally regulated. No such regulations exist for using them in the landscape. Hydrogels based on starch or other natural polymers are fine – but avoid anything that contains “acrylamide” or “acrylate” on the label. Better yet, use a well-chosen mulch to absorb and release water to the soil.

Time to take the quiz!

All jokes aside, sustainability and gardening require constant adjustment and learning. You came here, you read through this list, and you are thinking critically about your practices.

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.

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.

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.