Allelopathy that damages fruit trees

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.

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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.

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It is not a grafted tree. And it is very much a black walnut tree.

I agree that Allelopathy is not well understood. What I am objecting to is blanket statements like “Black walnut trees are toxic to anything growing under them”. These are invalidated by my observations.
I think there might be circumstances where black walnuts are toxic. It is important to identify those rather than paint with an excessively broad brush.
A single counter example disproves a false theory. So anecdotal experience is definitive in this regard.

Further my soil could be characterized as clay. Almost all native soil west of cascades in rocky clay.

Not all black walnuts are necessarily equal, according to Richard. One question is whether yours is an Eastern black walnut, not whether it is a black walnut. The EBW range is not west of Texas. Ca. black walnuts are naturally semi-dwarf trees, so obviously have much less aggressive root systems. You say yours is large, however- do you mean 70’ or 20’? 70’ is large.

The sandy soil thing was just an example of variables that are in the mix. A clay soil has its own characteristics that might have an affect also (such as a less extensive root system). No one is saying that every black walnut tree produces enough poison to be damaging to all other plants under it in every situation. Did someone make that statement on this topic?

At any rate, I don’t mean to nit pick and your contribution is helpful. I just would like to know if you are sure that your black walnut is an Eastern one.

How would you like me to proceed? Would some photographs of the tree and leaves and plants thriving under it suffice?

I’m just asking why you don’t think it is a CA black walnut, given you are in its natural range- people don’t plant Eastern black walnuts as ornamental trees, they are generally only available as grafted, selected varieties. They don’t have much difference in appearance besides the relative size.so I don’t see how a photograph would help.

  1. It is a juglans Nigra. I have harvested the nuts and showed it to experienced people. (Who run nurseries)

  2. Tree is around 80 feet tall and had a massive spread before I had it pruned. It is bigger than most city size yards around here.

  3. Lots of people grow them as shade trees around here. Do you only grow native trees? People have been growing non natives for centuries. Nearly all fruit trees (apples, pears, kakis) are non native to US. You might even be growing a few non natives.

It appears you have made up your mind on the allelpathic effects of black walnut and don’t want to see evidence to the contrary.

I don’t know the details of Juglans nigra. But I have read a lot about Allelopathy over the decades and contrary to what has been stated here, I believe it is a well-researched subject. All Allelopathic plants drop one or more toxins to which they are immune. Some culprits (e.g., California pepper tree) put out a general purpose root toxin. Others – I believe eastern Maple put out a more specific toxin that is detrimental to their historical competitors.

What we know and love as “fall colors” is allelopathy in the large. It is warfare!

I was only trying to determine if you were actually growing an eastern black walnut, which I wasn’t clear on until now. I thank you for making that clear, but I wasn’t trying to engage in combat. If you mentioned how tall it was previously, I must have missed the fine print.

While I disagree with Richard’s statement that allelopathy has been widely studied, there have been some studies(widely is a subjective word but a quick search reveals that there are qualified folks who disagree with Richard’s use of it), A lot of the claims in the literature about specific plants being vulnerable to juglans ARE based only on hearsay, from what I’m reading, but here is a legitimate study. Black Walnut Allelopathy in a 28-Year-Old Loblolly Pine Stand and … Can’t link the download to this page, but search with those words and you will find it. There are more, but maybe none that pertain to the species in your own yard. Blueberries are apparently on the list of studied plants, and I’ve had blueberries die with unnatural speed when planted under them.

Apparently, apple trees have been determined to be adversely affected- at least in some conditions. My own limited experience suggests that in some soils many common orchard species are adversely affected but in some soils they are not. Of course, my observations are only circumstantial evidence, not proof and could be a coincidence. I’ve never cut down a BW tree in such an instance and studied the results.

At any rate, black walnut trees are not what I intended to be the primary focus of this topic- I don’t mind the diversion but I find the battle between prairie species and forest species to be more interesting. I’m not managing any fruit trees near black walnuts these days.

Thanks for the clarification of your intent.
I am including some photos for completeness so that we have some actual data on the thread rather than oft repeated quotes about allelopathy associated with black walnut. In fact simply searching for allelopathy on google brings up several links on Juglans Nigra.

The first photo is that of the tree itself, the next two are of the leaf structure and of the fruit. The final one show a Rhododendron growing vigorously around 3 feet from the tree trunk.
Rhodies are supposedly very sensitive to black walnut.
I also have a large fig tree growing in the trees dripline.

Grass grows fine under the tree.

To me, the western red cedar is far more toxic than Juglans Nigra. Nothing will grow under it.

In all fairness, I didn’t really think you had an eastern black walnut either, but my thought had little to do with the correct ID of your tree.

I just didn’t think someone would say to themselves, “Oh yes, I want to get my hands on one of those non-native eastern black walnuts. I love their weedy growth habit, their shells that rival steel for hardness, and their inferior nuts that stain my hands black. Oh boy, where can I get one?”

Then again, where I grew up, black walnuts were essentially trash trees, so I may be biased. :wink:

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I think you might be biased :slight_smile:

I see them around here quite a bit. I wouldn’t have planted it. But it came with the house and is a beautiful (if messy) tree.

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As you likely know, there are selected varieties of the species with the same growth habit, but with thinner shells and much better nut to shell ratio. Here. where they are native, I believe that they are rarely sold by nurseries as seedling trees for landscapes. They are invasive in that they send up saplings from root suckers. Their natural habitat is near stream beds or marshes- at least, in the northeast. Our native hickories grow larger here and tend to be much more attractive trees to my tastes. They don’t compete with BW.s and tend to grow in better drained conditions.

BWs probably became popular there because their poison is not toxic to turf and they allow adequate sun below to keep the grass healthy.

It is interesting that they are not lethal to rhodies, because they are in the same family as blueberries, but not too surprising. However the myth that they are poisonous to them may have come from that association. When you’be been in the dirt and literate for as long as I have been, false horticultural information passed off as established fact never comes as too great a surprise. Dr. Carl Whitcomb achieved his reputation by researching established horticultural myths and proving them false. I wish there were more in academia like him. .

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