myths – The Garden Professors™

Maddening mulch myths

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.

*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.

*The wound on the tree on the leftwas covered with wood chips; it sealed over faster than the one left bare on the right.*
From an earlier Garden Professors post

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: reviewin g 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

A DIY Debunking Guide

Debunking myths is at the heart of the Garden Professors blog. The impetus for initiating the blog is rooted in Linda’s ‘Horticultural Myths’ Series and Jeff’s “The Truth About…” books. Unfortunately, I’ve never been especially good at myth-busting or debating. When confronted with someone with deeply held beliefs that are based on misinformation, it usually doesn’t take long for me to lose my cool and my arguments devolve into, “Pull your head out of your a— and face the facts…”

At a holiday dinner not too long ago, a relative suggested “You know, there may be something to the anti-immunization thing…” The words were barely out before I could feel my wife’s hand on my thigh in a futile effort to keep me calm. “Are you fricking nuts?” I shot back automatically. “The only reason Jenny McCarthy or anyone else can even THINK about not vaccinating their kid is because the rest of the herd already took care of business.” Fortunately cooler heads at the table changed the subject before the debate escalated to violence.

Now for the myth-busting challenged among us, Australians John Cook and Stephen Lowandowsky have developed the Debunking handbook. The guide looks at some of the psychology of myth-busting (A simple myth is cognitively more appealing than an over-complicated correction) and suggests debate strategies (Adhere to the KISS principle). The guide is linked at the SkepticalScience website and is largely geared toward dealing with climate change deniers, but the principles and tips are useful for dealing with all manner of scientific misinformation

Container planting: intuition vs. reality

I’m just starting to think about getting my containers planted for the summer and happened to get an email on the topic from a blog reader. John was frustrated with a local columnist’s advice on using gravel in the bottom of the containers for drainage. When challenged, the columnist refuted John’s accurate comments with “logical thinking.” (You can find the posting and comments here.)

Here’s part of the post: “I like to cover the hole with a layer of gravel to improve drainage. Plants need to have their roots exposed to air in the soil to survive and thrive. If the container has no holes for drainage, it will fill with water and drown the plants very quickly. It is better to keep your plants on the drier side than to keep them constantly moist or wet. The big danger in using pots is drowning plants.” Later, he goes on to explain “The potting soil plugs up the drain hole and the water is trapped behind the plug. The layer of gravel creates an area for the water to drain through to escape. The creation of drainage commonly involves a layer of gravel.” This reasoning is part of what he calls “Logical thinking 101.”

As my husband pointed out, this isn’t logical thinking: it’s intuitive. It’s what we think is going to happen in the absence of any evidence. And in this case, it’s wildly inaccurate.

Jeff and I have both discussed the phenomenon of perched water tables in containers as well as the landscape in previous posts and on our Facebook page. The fact is, when water moving through a soil reaches a horizontal or vertical interface between different soil types, it stops moving. Here’s a photo from a very old research paper on the topic:

A layer of silt loam sits above a layer of sand, and water from an Erlenmeyer flask drips in. Intuition says that when the water reaches the sand, it will move more quickly through the sand because the pore spaces are larger than those in the silt loam. But intuition is wrong, as this series of photographs clearly demonstrate. Water is finally forced into the sand layer by gravitational pressure, after, of course, saturating the silt loam.

Intuition has its uses (I am quite proud of my own intuitive powers), but it doesn’t trump reality.

**This is an older post, so I’ve added this link to a peer-reviewed publication on the topic by Dr. Jim Downer and myself.**

The End (hopefully) of Molasses Malarkey

I’ve been discussing the purported insecticidal properties of molasses in my last couple of posts. I’m hoping this will be the final nail in the coffin (or stopper in the bottle):

Here’s the end of the original blog piece linked above:

“Microbial bloom and Fire Ants
“These two things seem unrelated. Microbes and specifically bacteria consume simple sugars (which is why your momma made you brush your teeth). When soil born microbes are exposed to simple sugars, their numbers can double in just 30 minutes. As microbes go through their life cycle, they add organic matter and micro nutrients to the soil, improving the soil and making nutrients more available to your plants. Regularly applying molasses to your soil and plants greatly improves the quality of the soil over time. Soils with high microbial activity are easier to dig in and stay moist longer.”

I’m actually going to leave this paragraph alone, since it’s relatively accurate (except for the sentence about applying molasses to your plants, which I dealt with in my first post). Let’s move on:

“So, about the Fire Ants…since it seems that the big universities can’t make money studying the effects of molasses on Fire Ants…they don’t do any research on the subject. But, it has been proven that molasses makes Fire Ants pack up their mound and migrate to your neighbor’s yard. It may be that the bloom of microbes, irritates the little stinkers. It could be that they are running from a specific microbe. It could be that they just hate sugar (they eat mostly protein which is why you can turn a greasy over baked pan upside down over a Fire Ant mound and they will clean it for you). What ever the reason, applying molasses to your yard makes them leave.”

This entire paragraph is nonsense, beginning with equating grease with protein (it’s a fat) and ending with the supposed lack of research on fire ants. There’s a LOT of research on fire ants; pest studies are very well funded. Out of the 1500+ articles I pulled up on fire ants in the Agricola database, only one includes molasses. And that’s in a 1986 study comparing different kinds of baits (“Comparison of baits for monitoring foraging activity of the red imported fire ant (Hymenoptera: Formicidae)”), where molasses was found to be more attractive to fire ants than peanut oil. How this translates to “molasses makes Fire Ants pack up their mound and migrate to your neighbor’s yard” I’m not quite sure.

“If you’re crunched for time and money, molasses is the answer to a lot of your gardening problems. The benefits are undeniable, your yard will smell great and you get to feel good about letting your kids and pets play in the yard. Whether you choose dry molasses (applied to soy chaff) or the liquid (which is cheaper to use), molasses is the single best thing you can do for your soil and plants.”

The typical snake-oil pitch! (For a completely unrelated but accurate and amusing example of an old-time snake-oil pitch, check this link. You’ll see the similarities).

“It was brought to my attention that I forgot to add this info. (It is hard to remember everything when you are trying to rule the world!) During moquito weather mix:

3 tbsp molasses
1 tbsp Liquid Garlic (a deterent and has some fungicidal properties)
1 tbsp liquid organic fertilizer of your choice (seaweed, fish emulsion, etc) into 1 gallon of water

Spray with abandon, every week if necessary but it may last up to 2 weeks if we don’t get much rain. This also works like a charm on lace bugs on azaleas and lantana.”

Spray with abandon???? This has to be one of the most reckless pieces of advice I’ve ever read. Whether it’s a fertilizer or (more importantly) a pesticide, it should *never* be applied lavishly. (Though this is such a dilute solution that it probably isn’t much different than water.)

This topic has made me crave the molasses popcorn balls my grandmother used to make. Anyone have a recipe for those?

The Return of Molasses Malarkey

Last time I posted I began discussing this link about horticultural molasses. Let’s continue with the dissection:

“When molasses is sprayed directly on plants, it is absorbed straight into the plant. Once absorbed, the sugar content of the plant goes up. If you need proof, go pour a Coke on a spot in your lawn, in a week you will see exactly what I mean. Simple sugars are how plants store energy for rainy days and winter hibernation. So, why is this important to you as a gardener? Aside from basically giving your plants a power boost, you are stopping bugs. “What?” you ask. Yes, it stops bugs. Insects are very simple creatures. They can only feed within a narrow window of sugar content. When the sugar content of plants is raised, insects can’t feed on them. They take one bite and move on.

“The second way molasses controls insects, is by being directly ingested by the insect. What most people don’t know is that only Sugar Ants and bees can process the simplest sugars. Insects have no way of expelling the gas that builds up from fermenting sugar and the vegetation in their gut (draw your own mental pictures please). Plus, they have exoskeletons and can’t get bloated. Their delicate internal organs are crushed from the inside out. All a bug needs to do, is walk through or try to feed on a molasses covered plant. Insects are constantly cleaning themselves. They will try to lick the molasses off their feet and swallow it. If they take a bite of a molasses coated plant, they will swallow it.”

Some specific observations and comments:

1) “If you need proof, go pour a Coke on a spot in your lawn, in a week you will see exactly what I mean.” I just don’t think I can do this comment justice, so I’ll leave it to your imagination as to what you might see and how it relates to a 1% molasses solution sprayed onto leaves.

2) “Simple sugars are how plants store energy for rainy days and winter hibernation.” Actually, no. Simple sugars are difficult to store as they contain a lot of water and they can be quite reactive. Plants transform simple sugars (monosaccharides and disaccharides) into polysaccharides for storage.

3) “Insects are very simple creatures. They can only feed within a narrow window of sugar content. When the sugar content of plants is raised, insects can’t feed on them. They take one bite and move on.” Obviously the author has never seen Men In Black.

Give me sugar…in water

4) “What most people don’t know is that only Sugar Ants and bees can process the simplest sugars.” Please explain this to the cockroach I once saw in a sugar bowl.

5) “Insects have no way of expelling the gas that builds up from fermenting sugar and the vegetation in their gut. Their delicate internal organs are crushed from the inside out.” Did you know that termites are significant producers of methane gas – a byproduct of fermentation? And they release it the old-fashioned way.

From NASA’s website on methane production

More next week!

Molasses malarkey

Yesterday I received this link from a Facebook friend who said “when I read this I thought of you.” More likely she was thinking of (enjoying?) the mental agony I suffered as I waded through this morass of misinformation. (By the way – those of you who are educators of some sort – this would make a great “how many things are wrong?” question for your students.)

There’s SO much to discuss in this post that I think I’ll split it up into separate posts. Here’s the first paragraph:

“Cheap, easy and does it all!

“Not your kitchen molasses! That has had the sulfur removed and you need it in there. Horticulture Molasses does things for your plants like nothing else can and it’s the cheapest gardening product per square foot…a gallon can cover a half-acre! Put it in a sprayer, turn some music on and start spraying every inch of your yard, no need to be careful. You simply can’t over do it. Molasses raises the sugar content of plants and kills insects,causes a massive bloom of microbes in the soil and drives out Fire Ants, what more do you need?”

I’d not heard of “horticulture molasses” before, but there are so many new products sneaking into garden centers that I’m not too surprised. Let’s look at some specifics here.

“Kitchen molasses has had the sulfur removed.” This isn’t quite accurate. Molasses doesn’t contain sulfur naturally; sulfur dioxide is sometimes added as a preservative during the processing of sugar beets or sugar cane and ends up in molasses.
“Put it in a sprayer…and start spraying every inch of your yard, no need to be careful. You simply can’t overdo it.” This is some of the most irresponsible advice I’ve ever seen. If this is such a powerful insecticide (as you’ll see later in the post), then OF COURSE you can overdo it.
“Molasses raises the sugar content of plants.” This bold statement has no basis in reality. Exactly how it is supposed to get inside the plants? Not through the protective cuticle. Through the stomata? Possibly. But how much sugar could be taken up this way? There are 256 tablespoons in a gallon. Three tablespoons means that molasses is about 1% of the total volume in a gallon of this mixture (you’ll have to look at the bottom of the linked post to see the recipe). And since molasses is only about 50% sugar, then a gallon of mixture is about 0.5% sugar. We’re talking about homeopathic levels of sugar here.
“Molasses…kills insects, causes a massive bloom of microbes in the soil and drives out Fire Ants.” The microbe information is more or less correct (maybe not “massive” given the concentration of molasses used). Microbes love carbohydrates. The insecticidal claims are nonsense. And since the next paragraph of the original post addresses this in more detail, I’ll hold off my dissection until my next post.

Why did the worm cross the road?

…To get to the other side. Of course.

All the rain we’re having is causing the earthworms to crowd the sidewalks and driveways. They fling themselves out of the ground and onto the pavement because they’re drowning, right? Nope. Urban myth (by the way, why are there no Rural Myths?). The punchline is not too far off: they can only move about above ground while it’s raining. They use rain events to safely relocate, and can allegedly live for a while in a puddle. But we all know what happens when the sun comes out…crispy Ramen time.

I had just come in from flinging a bunch of worms off the pavement in front of our building – they were going to get mashed or eaten by robins otherwise. I probably put them right back from whence they came, against their earthwormy wishes.

Portrait of Two Worms
– by H.L. Scoggins

Then I read Bert’s post…it’s like reporting that the Easter Bunny has rabies or something. I’ve always been delighted to see earthworms, under any conditions. To the point of saving their squirmy little lives whenever possible. Not sure what to think about this new bit of information.

Invasives! Natives! No, wait, biodynamics

Just had to get your attention there. We’ve had a great discussion over native and nonnative plants over the last few weeks. I’m going to completely switch gears and move on to another topic – biodynamics.

If you’re not familiar with this term, let me refer you to my online column here. Biodynamics is a set of agricultural practices based on a belief system, not science, but is an increasingly popular approach, especially in the wine industry. (You can read a discussion of biodynamics in the vineyard in The Skeptical Inquirer here. This article is engaging as well as accurate – my column is pretty dry by comparison.)

Biodynamics is steeped in mysticism and includes special preparations that are used to treat soils and plants. Preparation 500, for example, is created by mixing water with manure that has been packed into a cow’s horn and buried for a set amount of time. Other preparations are more gruesome, requiring a stag’s bladder or cow’s intestine. A whole certification process has emerged in support of these practices.

While it may be easy to dismiss these practices, it turns out that biodynamic farms or vineyards are generally healthier than conventional systems. Does this prove a mystical force at work? Not at all. Biodynamic systems are also organic – using all of those good practices (low or no till, reduced pesticides, reduced fertilizers, polyculture, etc.) that have been demonstrated to be effective over decades of research. When comparisons are made between biodynamic and conventional systems, the impact of organic practices are hidden.

The few scientific studies that have compared biodynamic to organic systems – in other words, specifically testing the effectiveness of special preparations – have found no repeatable, significant differences.

Why do I even care about this? Well, it’s because it’s pseudoscience. It’s a practice that takes on the mantle of science, but doesn’t stand up to repeated scienific testing. Belief systems can’t be tested – even the inventor of biodynamics asserted that his methods were “true and correct unto themselves” and didn’t need to be tested.

Apparently simply being organic isn’t sexy enough anymore.