Walnut warfare

Recently, a question about using black walnut chips for mulch was posted on our Garden Professors Facebook group page. As gardeners know, black walnut has a reputation as a chemical warfare species that will kill anything growing underneath it – a phenomenon called allelopathy. So it’s logical to wonder about the lethality of walnut chip mulches.

To get a good feel for the science behind black walnut’s allelopathic abilities, I was fortunate to find a relatively recent review on the topic (Willis, R.J. 2000. Juglans spp., juglone and allelopathy. Allelopathy Journal 7(1):1-55.). This well-written review includes a fascinating section on the historical background of walnut allelopathy, which was first mentioned in 36 BC by the Roman author Varro. But the science of allelopathy really started less than 100 years ago, when a Virginia researcher noticed the injury caused to tomato plants growing near black walnut (Juglans nigra) in his home garden. Subsequent experiments by him and others suggested that the orangish hydroquinone juglone leaching primarily from leaf litter and hulls.

SONY DSCSource: Wikipedia.

The research results on walnut, juglone, and allelopathy have been nothing if not inconsistent. For every report of toxicity in an exposed species, another report found no effect. In fact, much of the supposed allelopathy might instead be due to walnut’s highly competitive root systems, which could suck up available water and nutrients over a vast expanse of soil.

Black walnut tree Source: Flickr user davidburn

There are a number of other factors that help account for ambiguous results:

1) Juglone is not the only secondary metabolite produced by walnut species. They are loaded with a number of untested phenolics, flavonoids, alkaloids, terpenes and other quinones which could have allelopathic activity.
2) Juglone concentrations vary greatly among walnut species. They also have seasonal variability in the same individual.
3) Light conditions, rainfall, soil chemistry, and many other abiotic factors can influence juglone levels.
4) Organic matter and clay particles in soils can bind juglone, reducing its movement within the soil.
5) Microbial activity breaks down juglone.

Carefully controlled laboratory experiments can demonstrate juglone allelopathy to a number of plant species, especially at the seedling stage. However, there is little evidence from landscape level research to suggest that allelopathy is the reason that plants are damaged by being in proximity to walnut trees. In fact, the author of the review study concludes that even though Juglans species provides the best known and most widely accepted example of allelopathy, there is “still is no unambiguous demonstration of its effect” as “no one has as yet demonstrated that juglone is actually taken up by plant roots.”

walnut tree Source: Wikipedia

Where does this lead us in our discussion of walnut mulch toxicity? Fresh hulls and leaves appear to be the primary source of allelopathy, but not the wood. And even these sources may be quickly neutralized by soil conditions. Therefore, a walnut chip wood mulch should pose no danger at all to landscape plantings.

One tree’s leaves… over 400 kinds of bacteria!

Okay… this bit of research just blew my mind.

Researchers took leaf samples from just ONE tree in Panama, and identified over 400 different kinds of bacteria making their home there. Sampling 57 different tree species, the total number of bacteria types ballooned to over 7,000. You can read more about the study here.

A few trees. A nearly inconceivable number of microorgansims


That’s a lot. I love this kind of research because it just reinforces how LITTLE we know about this world we live in. Our world is filled with a massively diverse microbiome that we know virtually nothing about. Research is ongoing, and hopefully in the coming years we’ll begin to understand more about how these unseen organisms influence the world we live in. I’ll be fascinated to learn more.

In the mean time, any mention of microorganisms in a gardening context instantly raises questions of the efficacy of products containing (supposedly) beneficial fungi and/or bacteria for our soil. The huge, barely understood diversity of bacteria living in every aspect of our world is a good indication of why the research on adding specific microorganisms to soil generally show no impact, or only an impact in certain specific circumstances. This stuff is complex, and we’re just barely beginning to learn about it. Hopefully in the future we’ll begin to learn how to manipulate the microorganisms that live with our plants, but I wouldn’t expect it to happen over night. Right now, I’m just following the basic rule of adding organic matter to my soil to make a good home for the organisms that live there, and following the research as it opens a window to this unseen world all around us.


How NOT to do an experiment

Over on Facebook I follow some groups who find provocative topics, and today’s “science fair” post was so over the top that I had to share it here.

science fair

Here’s the original post. Now the accompanying text about microwaves is whacky enough on its own (and well worth reading), but my primary interest is with the experiment. This exemplifies why there are basic rules for doing science.

This starts out okay – identical pots, the same type of media (I assume), similar sized plants – but then things go downhill:

1) Replicates are important. There is one treatment and one control, meaning that it’s impossible to run any kind of statistical analysis. Ideally between 10-20 replicates of the control and the experimental treatment are used in this kind of experiment. That’s 20-40 plants total.

2) Variable control is important. Plants in a windowsill are subject to light and temperature gradients. That makes analysis more complicated unless one has an extremely long windowsill so that all plants are treated uniformly. And then our researcher prunes the tops of the plants – yet another variable.

3) Consistency between treatments is important. It appears that the pot on the left is wetter than the one on the right – the media is darker. If it’s not draining well – for whatever reason – then you’ll have a hypoxic root environment. Plants don’t like that.

4) Objectivity is important. It’s difficult (impossible, really) for any researcher to be completely objective. Ideally, the pots would have been watered by another person and then labelled in such a way that the person recording the data would have no clue which was which.

I think it’s really important to get kids excited about science. But it’s just as important giving them guidelines about doing science in a way that advances their own understanding about how the world works. Otherwise, it’s just more fodder for the aluminum hat crowd.

Phosphorus and Big Macs

Minnesota, and I were cruising through old pictures and files and getting all sentimental about the cool stuff we used to do.   A lot of it was never published just because after we were done with one thing we were just too damn excited to move on to the next.  Anyway, one of the neatest experiments that we never wrote up was a phosphorus experiment.  Here’s what it looked like to the casual observer.

Now let me explain the neat part to you a little.  Inside those boxes, underneath three of the six plants in each container, are vials set up like this – three vials per plant (the black tubes provide air to the vials).

Each plant had one root placed into each of the three vials – one vial contained 1 ppm phosphorus, one vial contained 10 ppm phosphorus, and one vial contained 30 ppm phosphorus.  The tub itself was also filled with one of these three solutions (1, 10, or 30 ppm phosphorus) as seen below.

At the end of the experiment we weighed the roots filling each vial, as well as weighing all of the roots from each plant.  Here’s what we found for the individual vials.

As you can see, more phosphorus in a vial meant that the plant would devote more energy to growing roots there – but also notice that the 10 ppm solution has the greatest mass of roots overall.  Here’s what we saw when we looked at the total size of all of the roots from plants for the different solutions.

As you can see, the roots from the plants in the 10 ppm solution are the largest (shoots showed the same trend).  So here’s the way I see it (this is the Big Mac part).  I love Big Macs.  If I see a McDonald’s I want to go in there – I gravitate towards McDonald’s to get Big Macs.  But too many Big Macs aren’t good for me.  They might even stunt my growth!  It’s the same for phosphorus.  Roots do grow towards phosphorus (this isn’t technically correct, but it works for my analogy so I’m sticking with it!), but that doesn’t mean that a tremendous amount of phosphorus is actually good for them.  In fact, it might even stunt their growth!  This could be for a variety of reasons, but most likely because the phosphorus would interfere with the uptake of other elements.


Mosses are soft, green, and tough as nails, as shown in a recent article in the Proceedings of the National Academy of Science (prestigious, high impact journal with a rather unfortunate acronym).

Dr. Catherine La Farge and associates, from the University of Alberta, visited a remote glacier on Ellesmere Island, Nunavut while studying the wild, wide world of arctic bryophyte systematics. Bryophytes are ancient, non-vascular, non-flowering plants – mosses and liverworts, mostly.

Long story short, they harvested bits of moss that had been trapped in ice for about 400 years and were now exposed. Several species were collected, taken back to the lab, ground up, placed on growing media in a growth chamber, and they soon had mosses galore. 

This is fascinating on several levels, as pointed out by the authors.  One is the power of totipotency – the ability of a cell to “de-differentiate into a meristematic state that can then reprogram the cell for development of the organism”  a la stem cells.  Another is the mosses’ ability to “shut down” when dry and “revive when conditions are favorable” (like not frozen in ice for 400 years?!)

The article also graphs the disturbingly accelerating rate of retreat of the Teardrop Glacier, where the mosses were collected. Aargh. The window of favorable conditions may not be open long for these little wonders.

The Canary in the Coal Mine

Three weekends ago marked a milestone of sorts as mean daily CO2 levels at the National Oceanic and Atmospheric Administration observatory at Mauna Loa, Hawaii topped 400 ppm for the first time ever.  Rising levels of CO2 and other greenhouse gases could result in significant increases in temperature in the Upper Midwest over the next century.  When we think about trees in cities the scenario is even worse since not only will urban trees have to deal with overall temperature increases but they must also contend with urban heat island effects, which can add another 8 deg. C or more of heat load.  Because of this ‘one-two punch’ of global climate change and urban heat island effect, I often refer to urban trees as the proverbial ‘canary in the coal mine’ with respect to climate change since they will likely be impacted sooner and more dramatically than trees in woodlands.



In general, organisms have three options to deal with a change in their environment: They can migrate, they can adapt, or they can acclimate.  Since trees are sessile organisms, they can’t pick up and move so migration is out.  Current predictions are that climate will change faster than trees can evolve so natural selection and adaptation will be limited.  Which leaves us with acclimation, or the ability of a tree to adjust its physiology and morphology to its environment.  A common example of an acclimation response is the development of sun and shade leaves on the same tree.  Another example of an acclimation response is an increase in the optimum temperature for photosynthesis in response to exposure to increasing temperatures.  In theory, trees that have a greater capacity to adjust their physiology to increasing temperature will be better suited for future, presumably warmer climates.

We are currently testing this idea in a two-part study.  In part one we are growing trees from several shade tree cultivars in greenhouses under three temperature regimes; ambient temperature, ambient + 5 deg. C, and ambient +10 deg. C.  In part two of the project we planted trees of the same cultivars in two sites in Detroit in cooperation with the Greening of Detroit.  The Greening of Detroit is community based non-profit organization that assists neighborhood groups, churches and schools in their efforts to improve the ecosystem in Detroit through tree planting projects, environmental education, urban agriculture, open space reclamation, vacant land management, and workforce development programs.

Many hands make light work.  Greening of Detroit volunteers plant trees along a street median.

With the help of Greening staff and about 90 Greening volunteers, we recently planted 160 shade trees in downtown Detroit.   One site of the study is in a park, representing a relatively mild micro-climate; while the other site is along a street median surrounding by asphalt with a much higher reflected heat load.  Both sites with be instrumented with environmental sensors and we will compare growth over time as well as physiological responses such as the response of photosynthesis to temperature.  The long-term goal is to identify traits that will be most important to guide future selections of trees of urban and community forestry under changing climatic conditions.

Research Technician Dana Ellison (left) and Research Aide Aiman Shahpurwala finish planting a park tree.

A pick ax as a planting tool?  Dana shows how it’s done in Detroit.

Should back-fill be amended?  My usual answer is ‘no’, but then again it depends what your back-fill looks like…

Sizing things up.  Aiman and Dana collect initial data on trees after planting.

Scientists Put the Dog in Dogwood

(special guest post by/with permission of good friend Mr. John Friel, marketing manager for Emerald Coast Growers – Holly Scoggins)

How do you recognize a dogwood? By its bark.

That old joke might not be a joke anymore, if the innovative folks at Metamorphic Agriculture Developers (MAD) get USDA approval for a new line of ornamental and functional shrubs that blur the line between the animal and vegetable kingdoms.

MAD scientists claim to have successfully introduced genes from Canis familiaris into a cultivar of Cornus canadensis. In other words, they’ve crossed a creeping groundcover dogwood with … a dog. Specifically, a dachshund.

“Imagine a guard dog that you never have to feed, license, or walk,” enthused Dr. Horace Sass. “Imagine a shrub that not only beautifies your home but guards it when you’re away,” adds his colleague, Dr. Ariel Sturgeon. The two bring a unique perspective to their work: Dr. Sturgeon is a mermaid, while Dr. Sass is a centaur.

After considerable trial and error, the team believes its Canis /Cornus combination is the best of both kingdoms. The first hurdle was finding the right plant.

“We tried Physocarpus first,” said Dr. Sturgeon, “but every one that grew to maturity would bark nine times when approached. Our focus group said that was too many.”

Crosses involving Cornus alba succumbed to a fungal disorder that afflicts that species. Said Sturgeon, “The blight was worse than their bark.”

Once they’d settled on Cornus canadensis, the next step was to find the right canine breed. “The pit bull shrubs were tough and sturdy, but their bite was worse than their bark,” said Sass, gingerly rubbing his right foreleg.

While they hope for widespread acceptance of their remarkable new hybrids, the team admits there are challenges, In winter, the plant/pet eventually goes dormant, but not before trying stubbornly to get into the house.

“The whining may be a turn-off for some homeowners,” Dr. Sturgeon admits. “In those cases, we recommend large containers, overwintered in the garage.”