Allelopathy that damages fruit trees

Most gardeners are aware of the negative affect of planting many plants next to a black walnut tree- its alleopathic properties (the creation and release of chemicals damaging to competing plants) are pretty widely known, but the kind of chemical warfare this tree engages in is far from unique to the family Juglans.

I believe that one reason trees grown in a seldom mowed meadow never seem as healthy as when grown in mowed sod is that many, or at least some, meadow species compete with this type of chemical warfare and slow the growth of most common fruit tree species- it isn’t all about the competition for nutrients, including water. Golden rod, a common “weed”, AKA native flower, in the northeast, has been shown to use allelopathic compounds to slow the growth of our native black cherry trees. I suspect the chemicals have a similar affect on closely related fruit species as well- not just cherries but all Prunus, including peaches, plums and nectarines. Apples and pears are not such distant cousins of these either.

I’m interested in the thoughts and experiences of other growers. The commercial fruit industry is not engaged in research on this issue because they don’t grow fruit trees with this type of competition.

“allelopathy as an inhibition factor in ornamental tree growth”. If you search Google with these words it will lead you to an interesting file. Maybe someone else can construct a link.


Is this what you meant?

(It’s just a matter of copy-and-paste of the url of the google page.)

Spotted knapweed, a very serious invasive here, is alleopathic; no doubt there are others. Makes me wonder if you could make tea from some of these things to use as a natural weed control.


Yup, that’s the file. I doubt more than a fraction of species that have this kind of power going for them has been studied- it may be more common than synergy between species. It’s war out there! Broccoli even wages this war against its own family. Don’t plant collards where you grew broc the year before.


When growing vegetables and finding information,I found charts like this one,but never went deeper to understand the whys,It is kind of interesting though. Brady


mowing meaning you eliminate all tall species and are left with various grasses, clover, dandelion, and other low growing forbs (chickweed), which are less or very little allelopathic. Dandelion is a healthy carrier for a virus that attacks apples, though. Not the same but still a threat.

Everyone talks about black walnut, but how about Carpathian walnut?. I planted four in large cages, but thought they were dead, so lazily planted a September Fuji next to one, to save building a cage. Wouldn’t you know, the walnuts actually survived. I haven’t gotten around to transplanting the apple after more than a year. Just got too busy. Or maybe I should chop the walnut, since we are quite a ways north to really expect any harvest ever.

Mowing, meaning also that you reduce any plants ability to manufacture their poisons. The higher you let grass and other herbaceous plants grow the deeper and more competitive the roots. I think its safe to assume that those deeper roots can produce a higher volume of chemical compounds. Carl Whitcomb performed an experiment that indicated the damage from turf, even mowed turf, towards the establishment of trees includes allelopathy. It is part of the basis for the standard recommendation of maintaining a grass and weed free ring for establishing trees. Of course, commercial fruit growers tend to maintain wide herbicide sprayed strips in commercial fruit production where weeds are allowed to grow back in late summer.

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Yeah, Juglans regia (of many common names) likely also produces juglone, though maybe not quite as much as black walnut. It is a characteristic of the genus Juglans and depending on what you read–the family Juglandaceae which also includes Carya (pecan and hickory).

Rye produces an allelochemical that inhibits germination of other plants. So it doesn’t surprise me that other grasses do the same. There are several USDA and extension publications out there giving recommendations on how to approach killing rye before planting corn/soy into it since the use of cover crops is growing (but not fast enough) in the corn belt.

The negative relationships they are aware of are the ones that are important to commercial agriculture. For most grain crops the plan is to kill everything that competes, allelopathically or not, so this is not an adequately researched issue. It just happens that several of my customers have it in their heads that mixing a meadow with an orchard is a wonderful thing and they are not to be deterred by my pleas. The vole issues and plant bug issues and fungus encouraging moisture issues (more dew) are bad enough.

California Pepper Tree (Schinus molle) is allelopathic: “…the oils in the leaf litter from this tree deter understory growth, making it difficult to grow other plants beneath the canopy.” Unfortunately for me, I know this firsthand as one of my neighbors has a line of large pepper trees right across the fence from me. I had a lot of plants die or be significantly stunted in the area up to 10 yards from the fence line.

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I remember that about those trees. At the house I lived in Topanga in S. CA. and first explored horticultural pursuits beginning over 50 years ago, eucalyptus and pepper trees were a major component of our landscapes and their negative affects on plants under them was well known even then. We just didn’t have the highfalutin word for it. Going to college can make you seem smart- of course now all the info is available on the internet. Here we are.

Carpathian walnuts are usually grafted on the black walnuts, this was the reason I decided not to plant one.

I have a large black walnut in my backyard. There are several Rhododendrons happily growing under its shade. I also grow tomatoes not too far from the dripline. They do quite well.

I also compost the black walnut leaves and use it on my plants. I use the mulch on the Rhododendrons which are supposed to be very sensitive to Juglans poisoning.

I haven’t noticed any ill effects. If it damaged my other plants, I would have no hesitation getting rid of it. But I know it doesn’t really hurt plants as much as I hear on the forums. In fact, I notice no effects.

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Yup. After they’re past the juvenile stage out here, people marvel at how there are no annual grasses or weeds of any sort growing underneath. And yet, they are totally lost as to why nearby plants are under extreme stress or dying.

My neighbor across the street just planted a row of Citrus and Avocado about 10° downhill and 15 foot distance from a row of California Pepper Trees. :thinking:

Which species? I’ve not noticed ill effects from the California native black walnut.

It looks exactly like this.

It is bearing nuts now and the squirrels are having a great time burying the nuts under supposedly sensitive plants.

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OTOH, I have a red cedar in the front that is decidedly allelopathic. Nothing really grows under it. Whatever it produces appears to be extremely toxic to other plants.

There are multiple species of black walnuts.

The Eastern black walnut is claimed to be allelopathic but recent publications say otherwise.

The California black walnut (occurs primarily south of Sacramento) is not.

There are others, native to the eastern hemisphere.


Not in all cases. I definitely have an Eastern Black Walnut that seems benign.(other than attracting every squirrel in the neighborhood)

I have been planting in our woods for a while. It appears that yellow birch grows slower near sugar maples, but white birch is fine.
Golden rod suppresses a lot of things like raspberries, but american chestnut does quite well right in the middle of the clump of the stuff.
I often plant it there on purpose, so the rabbits leave it alone. Hazels are OK near walnuts, even though they are in the birch family and aren’t supposed to like juglone…and carpathian walnuts may make less juglone than the black ones.

Kind of fun to see the differences :slight_smile:

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Try planting blueberries under it.

At any rate, allelopathy is not adequately studied to be fully understood. It is very possible that its effect varies not just with species reaction to the toxins, but also the nature of the soil. Just a logical leap takes me to the possibility that in sandy, well drained soil, the poisons would leach away more and be less destructive. There are countless other variables in soil chemistry that could also play part in the poison’s efficacy. One should never assume a single anecdotal experience is definitive- as hard as it is to resist for all of us. We are wired to respond to what we “see” with our own eyes.

How are you even sure it is an eastern black walnut? Was it planted for its nuts- a grafted tree? Are eastern black walnuts a common feature of the land there? The only difference in appearance to what I remember of the trees native to S. CA is the greater size of what grows here in NY.