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Cody DeBaun
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Location: Denton, TX United States Zone 8a
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Hey all!

So I've been reading a lot about allelopathy lately. It seems much more complex than I had initially thought.

I thought allelopathy meant that a tree just didn't play well with others. The more I look into it, though, the more it seems like allelopathy really just means that a given tree has a strong personality and may clash with some other guests at the party 

I didn't see a thread anywhere dedicated to a sort of overview of allelopathic plants and what they disturb, so I thought I would kick one off!

This article does a great job covering allelopathy, and identifies Leucaena leucocephala, box elder (Acer negundo), mile-a-minute vine (Ipomoea cairica) and sticky snakeroot (Ageratina adenophora) as having allelopathic effects.

The poster child for allelopathy, black walnut trees concentrate a chemical in their leaves, bark, roots and hulls called hydrojuglone, which oxidizes into juglone in the soil when those parts die and/or fall to the forest floor. Juglone inhibits respiration in many plants; it has been reported in apples, pears, tomatoes, pines, birch and many others, but this study shows that its effect is particularly potent to autumn olive (Elaeagnus umebelleta), black alder (Alnus glutinosa), italian clover (Trifoliums incarnatum), lespedeza (Lespedeza cuneata), and amur honeysuckle (Lonicera Maackii).

This study showed strong allelopathic effects from extracts of red maple (Acer rubrum), southern magnolia (Magnolia grandiflora), neem tree (Azadirachta indica), and strongest from swamp chestnut oak
(Quercus michauxii) and red cedar (Juniperus silicicola). All showed greater allelopathic effects than black walnut in the reduction in radicle length and hypocotyl length of lettuce plants. Red cedar as mulch also drastically inhibited growth of Florida beggarweed.

Siberian Elm (Ulmus pumila) has shown a similar inhibitory effect on radical and hypocotyl length of seedlings, specifically among riparian plants.

Studies here and here demonstrated the allelopathic effect of pine needles on grasses and cress, also root and shoot inhibitors but maybe affecting photosynthesis as well.

Vining velvet bean (Mucuna pruriens) has been shown to release a strongly allelopathic gas from its seedlings as they sprout.

Here's a study That shows allelopathic effects from the composted leaves of particular plants: live oak leaf compost seemed to be more allelopathic than black walnut leaf compost, though black walnut and eucalyptus both demonstrated a somewhat mild inhibitory effect on garden crops as well. The paper did note that the extremely high concentration of those leaves probably contributed substantially to the demonstrated effect, and that under normal conditions the effects would probably be much slighter.

What other plants are allelopathic? What else can be considered allelopathic?


 
Bryant RedHawk
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Location: Vilonia, Arkansas - Zone 7B/8A stoney, sandy loam soil pH 6.5
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Indeed allelopathy is complex and several studies you mention point out that it can be part of the effect, not all of the effect.

note that one of those studies (the one on mulches and composts) bring up the fact that the pH of their test compost was well above 7 making it alkaline. This would have as great an effect on seed germination rates as the allelopathic compounds.
Several of those studies mention similar issues with their methodology thus giving me question as to the solidity of their experiments and results.
Even with those issues, these papers indicate that the allelopathic compounds do play important roles in suppression of certain plant growth rates and seed germination.

What would be a really great study would be to isolate the compounds and test them with no other influences going on.

Great post with good information.

Redhawk
 
Todd Parr
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Location: Wisconsin, zone 4
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I planted two black walnut trees 5 or 6 years ago.  They have produced nuts for the last couple years.  I have had several large, well-established white pines die in the area around them.  Is it a coincidence, or can the walnut trees actually kill established trees?  Anyone know?
 
Bryant RedHawk
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Location: Vilonia, Arkansas - Zone 7B/8A stoney, sandy loam soil pH 6.5
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Todd,  How close to those Pines are the Black Walnut trees? Has anything else changed close to the pines?

Usually it is the smaller trees that are effected by Allelopathy not well established trees.
Do keep in mind though that Black Walnuts can alter the pH of the soil nearest them as well as put out allelopathic compounds into the soil. They also attract fungi that might not be helpful to pines too.

Many things to consider that will effect a tree.
 
Todd Parr
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Location: Wisconsin, zone 4
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They are 15-20 feet away.  Here is a picture.  Walnuts are at the far right.  You can see one smaller dead white pine in the foreground.  That one died last year if I remember correctly.  The big one was fine until recently.  There is another that you can't see that died two years ago, also smaller, but at least 7 years old.
walnut.jpeg
[Thumbnail for walnut.jpeg]
 
Bryant RedHawk
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Thank you for that photo Todd.

I doubt that the allelopathy of the Black Walnut trees had anything to do with those pines dying.
Check the bark on the dead trees for pin holes and you might even pull some of the bark off to check, you are looking for evidence of pine borer.

While I don't have any pines on Buzzard's Roost, we do have red cedars (sacred to us) and they too are subject to dying from borer activities.
The few we have had die, that is what killed them, found out by stripping bark and uncovering the tunnels made by the borer larvae.

Redhawk

 
Todd Parr
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Bryant RedHawk wrote:Thank you for that photo Todd.

I doubt that the allelopathy of the Black Walnut trees had anything to do with those pines dying.
Check the bark on the dead trees for pin holes and you might even pull some of the bark off to check, you are looking for evidence of pine borer.

While I don't have any pines on Buzzard's Roost, we do have red cedars (sacred to us) and they too are subject to dying from borer activities.
The few we have had die, that is what killed them, found out by stripping bark and uncovering the tunnels made by the borer larvae.

Redhawk



Thanks Redhawk.  I didn't really think allelopathy could be the culprit, but I'm far from an expert in these matters.  I'll check for borer "tracks".  I hope that isn't it because I spent many, many hours planting the perimeter of my property with white pines, both for windbreak and privacy.  I'm on top of a hill and have strong winds there.  Without good windbreaks in the winter, the heat loss from my house is excessive, and in the summer storms I lose trees to wind breakage.
 
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