The common statement “nothing grows under walnut trees” is not true. “Walnuts produce juglone” is not entirely true either. “You need to compost walnut wood chips before using them in the garden” is false. “The allelopathic properties of walnuts are well understood” – not true.
This is a popular subject that is routinely discussed and written about, but the truth around walnut trees is anything but clear.
The Truth About Walnuts
Much of the material in this post is based on an extensive review article done by R. J. Willis (ref 1). In that review, Willis concludes that “While the genus, Juglans provides what are probably the most widely accepted examples of allelopathic plants, it must be concluded that there is still no unambiguous demonstration of its effect.” That is a strong statement given the fact that this phenomenon seems to be accepted by just about everyone.
Willis goes on to make several arguments for the fact that we still can’t conclude the effect exists, and I will only look at one of them here. Even though people have tried, they have not been able to show that roots absorb juglone, even in the lab. If plant roots do not absorb the chemical, how can it be responsible for damaging plants?
Don’t misunderstand me, there is certainly something going on under a walnut tree that affects plants – we just don’t know enough about it to say that juglone is the culprit.
What Is Allelopathy?
From Wikipedia, Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the germination, growth, survival, and reproduction of other organisms.
The word allelopathy derives from two separate words. They are allelon, which means “of each other”, and pathos, which means “to suffer.” (ref 2).
Do Walnuts Produce Juglone?
The commonly held belief is that walnuts and other trees in the genus Juglans produce a chemical called juglone, and it is this chemical that affects other plants. It turns out that walnuts don’t produce juglone, or at least not very much of it. Instead, they produce a chemical called hydrojuglone. When hydrojuglone is exposed to air or other oxidizing chemicals, it is converted to juglone.
When you cut a fresh green walnut shell, the inside is whitish green. It quickly turns yellow because the hydrojuglone (clear color) is oxidized to form juglone (red). The amount of juglone is quite small, and so it appears yellow instead of red. If you wait a few minutes, the shell then turns black as the juglone is converted to even more chemicals.
Besides being an interesting fact, this color change also illustrates something important about juglone. In the air, it is quickly converted to other chemicals. Remember that ideal soil is 25% air. These other chemicals have not been studied very much, and one of the black chemicals may be affecting plants – it may not be juglone.
Also interesting is the fact that walnut trees contain almost no juglone. This makes sense since juglone is quite toxic to plants, including walnuts. The walnut produces less toxic hydrojuglone and stores it instead.
Juglone Is Not Alone
All the talk in gardening circles is about juglone, but the walnut tree also produces many other chemicals that probably also exhibit allelopathic properties. These include compounds like flavonoids, terpenoids, alkaloids, steroids, carbohydrates, and amino acids, with mixtures of different compounds sometimes having a greater allelopathic effect than individual compounds alone.
The properties of most of these have not been studied as well as juglone. One or more of these other chemicals may be causing the symptoms we see under walnut trees.
Where is Juglone Found?
References frequently say that all parts of the walnut tree contain juglone, but how much do they really contain? As discussed above, there is almost no juglone in walnuts, but they do contain chemicals that can become juglone, so scientists have started measuring the “juglone potential” chemicals. These are the chemicals that can end up producing juglone.
The following list shows the relative amounts of juglone potential in various parts of the plant (ref 3).
- Fruit – 100
- Whole leaf – 57
- Flower bud – 23
- Phloem (outer bark) – 5
From a gardener’s perspective, the fruit and leaves pose the greatest source of juglone.
The amount of juglone potential also changes during the season. It is highest in the growing parts of the tree. Leaves in spring, as they are growing, have more than later in summer. Fruits have more as they reach maturity. One study found that green leaves had 20 times the juglone potential as dry leaves. Growing roots seem to have a more constant amount throughout the season, probably because they are always growing.
Juglone In Soil
Juglone is quickly absorbed by the soil and held quite tightly. It is held more tightly by clay and organic matter than by sand. As a result of this, the effects of juglone on other plants are more dramatic on sandy soil that contains very little organic matter. If you garden under a walnut tree, increasing organic matter will help plants grow.
Juglone is also found in higher concentrations in soil that is wet and does not drain well. A research project looked at a 25-year-old grove of walnuts and pines growing on different kinds of soil. The pines on well-drained soil showed almost no effect, while the pines in poorly draining soil were almost dead.
Juglone And Microbes
Once juglone is in the soil, microbes use it as a food source. When soil samples are removed for lab study, they need to be analyzed immediately since the levels of Juglone can drop to 1% of the original amount in as little as 48 hours.
Happy microbes mean less juglone in the soil, and you get happy microbes by providing organic matter.
Juglone vs The Environment
At the start of this post, I mentioned that there is insufficient proof that juglone causes the problems we see under walnut trees. In the lab, juglone damages seedlings of some plants. But this can’t be extrapolated to the field. It could just as well be due to some other chemical.
Allelopathic mechanisms are going on under a walnut tree. They affect the types of wild plants that grow there. What is not known is how much of this is due to other environmental factors. How do the dense roots under a walnut tree affect the plants growing there? Do walnut tree roots impact the microbial community living there? Probably, since juglone has antimicrobial properties. Are other chemicals involved? Does juglone get converted to something else, which in turn harms the plants growing there?
We do know walnuts produce chemicals that result in juglone in the soil, and we know juglone has allelopathic properties. The rest of the story is mostly an educated guess at this point, with lots of unknowns. For example, we don’t know how and if juglone gets into plants.
Composting Walnut Material
Leaves and branches should be removed from gardens and composted. The toxic effects of juglone are gone in about four weeks. Fruit should be composted longer, and it may even be a good idea to remove it from the garden if you get a lot. In my experience, squirrels do this for you, and then they bury the nuts where you don’t want them.
Walnut Wood Chips
Heartwood will contain very few juglone-producing compounds and should not be a problem. The inner bark contains some, but it is minor compared to other parts of the tree. Any juglone that is produced will be degraded by microbes. Although many sites recommend composting walnut wood chips before using them or letting them sit for 6 months, this practice is not necessary. It is a good idea to keep it away from seedlings of all types and from known sensitive plants.
Death By Juglone
You might wonder why, if juglone is toxic to plants, walnuts produce it? It is a defense mechanism. Juglone is toxic to seedlings and other plants, so it reduces the competition around the tree, making it easier for the mother tree to grow.
Juglone is also toxic to insects and animals, so it reduces damage to leaves and flower buds and may reduce browsing by deer.
It is poisonous to humans if ingested, but skin contact is usually not a problem. Some people are more sensitive, and in extreme cases, black walnut sawdust can cause blistering (ref 3).
Gardening Under Walnut Trees
The idea that most things do not grow under walnut trees is false. If you look at the list of plants that are not affected by walnuts (ref 1), you quickly realize it is much longer than the list of sensitive plants. I’ve discussed how to garden under walnuts in Growing Under Walnut Trees.
References:
- Juglans spp., juglone and allelopathy; http://www.allelopathyjournal.org/Journal_Articles/AJ%207%20(1)%20January,%202000%20(1-55).pdf
- Allelopathy; http://csip.cornell.edu/Projects/CEIRP/AR/Allelopathy.htm
- Black Walnut Allelopathy – Tree Chemical Warfare; http://www.warnelloutreach.org/publications/Walnut%20Allelopathy%2011-10.pdf
- Photo Source: Wikipedia
I didn’t know about this and collected the rich-black top soil from the english walnut tree of mine. I transplanted the germinated seed to that soil and it’s growing painfully slow. I think there really is something with walnuts that messes up with the seedlings.
Thank you for all The remarks. I put some
Of my wood ash into my (very very) large wood pile not even thinking about some of the wood was black walnut and I was worried that I ruined this compost pile. I’m hoping I didn’t as I use it for an extensive vegetable garden. I will let this pile finish up it’s breakdown for the summer and sift and use it in the fall. So/ I just want to be certain- do I need to worry about the bit of walnut ash I used? I am careful about the amount of ash I use- it’s a giant pile that I move and flip with a tractor so it’s not a little pile. I’ll start a second pile in spring while this one finishes its process over the summer. So- please if anyone knows for sure one way or the other- let me know. But I THINK it will be okay if I’m sure it’s composted well.
Thanks for the balanced outlining of this information. Some folks are all behind the juglone ideas, others say it’s bunk. At least I now know that there is in fact SOMETHING to it, but that we don’t know for sure. I’ve definitely had issues growing many plants around my walnuts, but in my case I think it might be more of a water issue than anything.
I’m hearing a lot these days about the benefits of adding wood ashes to compost. Minerals are apparently unlocked and more available to the soil after the wood burning process. I have taken down a dead walnut tree and have lots of walnut firewood. Will the walnut wood ashes, after composting, hurt the plants I will grow in them?
No – provided you don’t add too much. Wood ashes do have a high pH.
I have read that the ashes are ok
I am attempting to determine how long the Juglone will last in water before the water will again be suitable for fish?
Hello
We are 4 French students who work on the juglone
We found your article very interesting
We begin to make our researches but we do not understand how the juglone crosses(spends) leaves up to the roots of the affected plants
Hoping that you answered us
Thank you in advance
Sorry – but I don’t think anyone knows. The movement of Juglone in plants, and soil is still mostly a mystery, especially in non-lab situations. Please send me a copy of anything you publish.
Interesting article. We have (had) a walnut tree partially over our roof. Juglon is coloring our harvested rainwater and we had to cut some branches hanging over the roof. This because we use the water for washing. Would be nice to know if there is a way to filter it out of the water because after every hot summer there comes still a lot of (“old”) juglon with the first rain in our tank. If the harvesting tank is full the rainwater is infiltrated deep in the ground. But after a hot summer its always empty and then we have to waste the water.
Very informative post. Having grown several black walnut trees, I have observed many plants growing unimpeded beneath them, therefore always was skeptical about the commonly held beliefs. As to the nuts: I have attempted, in several different ways, to germinate many, many black walnuts– totally without success– not even one. Yet when squirrels bury them, there is at least a moderate success. I wonder what they do? I have removed outer husk, and left it on; I have nicked both bare shell, and husk; buried in a variety of soils, at varying depths, yet nothing! I read somewhere that when early colonists wanted to increase the number of black walnut trees, they would leave a supply out for squirrels to bury. Any thoughts?
Sorry – I have not tried growing them from seed.
Most nuts need their humidity kept high. Any drying out and they will normally fail. I just stick nuts (hazel, sweet chestnuts, Walnuts etc) in a grow bag (compost bag) with holes cut in them. Plant then as soon as they are picked and keep the gag moist. Leave outdoors on top of something (above ground) and hope the mice don’t get them. Outdoors will give them the chill they also need over winter. I have had a lot of success with this simple method, 50 to 80% germination rates depending on the nut.
Spread them out where you want them planted, then run over them with a vehicle. Actually, I discovered that accidentally, after I de-husked them by driving over them in the lawn. I thought I popped all the nuts out of the ground, but more than 15 germinated the next spring. The germination rate must have been very high.
We have black walnut trees that are surrounded by trees and shrubs and have various “herbs” growing under and around them. It’s a big mess but I am not fussy as long as the area isn’t dominated by invasives. The squirrels are happy with these trees so I plan to plant more. I’d rather have big oaks for squirrels but with the oak death problem and the very slow maturation of oaks I suppose walnuts will have to do. I wish my local university would go back to planting native trees like it used to. Now it’s all useless ornamentals that probably aren’t native.
Non-natives are not useless and in some cases they perform better than native varieties. Oaks are very slow growers.
Every non-native tree planted is the replacement of a native tree. Particularly until human development starts to decrease that’s a bad thing. Those ornamentals are useless for the indigenous squirrel population. I realize oaks are slow growers but the campus also has some very large ones and it’s not going anyplace soon.
Mr. Ayers – Following your reasoning, if you’re not a “Native American,” YOU are a replacement of a Native American, so you should be removed! You’re not useful to the Indigenous population! This idea that ONLY NATIVE PLANTS SHOULD BE PLANTED is ridiculous because there are plants that grow around the globe – who is to rightly say where they originated? Any plant that can be utilized in any way, should be encouraged – as long as it isn’t rampantly and harmfully invasive. I have horribly rampant Virginia creeper and wild grapes – but they’re “native,” so I cannot complain and root them out?!
Saying I cannot grow peaches because they originated in China (yeah, I just read that that’s where they DID come from, and were carried on the Silk Road from China to Persia, hence their name, Prunus persica), is carrying this too far. I’m going to grow food plants because they will feed me and other people, as well as birds and other wildlife. I’m removing my indigenous black walnuts and thorny honeylocust trees because I do not care for the MESSES they make, and the fact that they are springing up EVERYWHERE in my yard. I don’t NEED them – the squirrels have plenty of other such trees nearby, as well as native hickory and oaks. I am going to replace them with (yes, non-native) apples, peaches, pears, plums, cherries, hardy figs, hazelnuts, and many different berry bushes, as well as grapes; I also have seedling pawpaws, persimmons, sassafras and other natives getting ready to plant when I’ve removed the pesky walnuts and honeylocusts. And I’m planting ornamental rhododendrons, azaleas, dogwoods, redbuds, mockorange, forsythia, etc. – some are natives, some bear edible fruit, flowers or leaves, or are medicinal – because I LOVE them. Being forced by the will of others to not grow certain things solely because THEY declare them “non-native” and “useless” is coercion on the order of communism and socialism – two “isms” with which I DO NOT agree. I’m not forcing YOU to grow only certain things – please have the courtesy to allow ME to grow whatever I choose. It’s MY little piece of the planet, not yours!
And by the way, I’ve planted a few oak trees, too. There may be some disease killing oaks, but there’s another killing walnuts, and the EAB killing the ash. Our God-given responsibility, dictated at Creation, is to take care of the planet as much as possible, starting with our own little Garden of Eden. He never told Adam and Eve WHAT to plant – HE planted the earth and told them to tend it, to make it fruitful. So I’m making mine FRUITFUL, the way I choose to.
When the Spanish Jesuit missionaries came to North America, some of them brought peach pits which they planted and introduced to the Indigenous tribes. Those tribes fell in love with the fruit and spread them across the country. I now live in a little village on the shore of Cayuga Lake, in central New York, near which the Cayuga people had an orchard of well over a thousand peach trees in the 1700s. After the American Revolution, a General Sullivan and his army were sent through this region to destroy everything the Haudenosaunee (Five Nations) people had, to punish them for siding with the British and killing American settlers. They destroyed that amazing orchard, as well as all the crop fields and harvested crops, and the homes and granaries, and many thousands of the native people died of starvation, disease, and cold.
My point? The native people embraced a wonderful tree (and probably many others introduced to them), while generously introducing the invading people to their own crops – corn, squash, beans, tomatoes, potatoes, etc. Then the invaders did all they could to destroy the native people and take their lands. I’m not “Indigenous,” except that my own ancestors came here hundreds of years ago and I was born here. I’m growing where I was planted, just as things grow from seeds dropped by birds and animals. We’re not going to root-out all non-native people, so why should we attempt to root-out all non-native plants? I think you would find that your own diet would suffer a lot if you stopped eating all “non-native” foods. Could you exist on a diet of grass and weed seeds, cattail roots, tree bark, grubs, birds, lizards, frogs, turtles, snails, and other native foods eaten by Indigenous people?
There’s nothing wrong with embracing plants, foods, and other facets of other cultures, as long as the memory of the old ways is also encouraged and maintained. Live and let live. LEARN and USE what you learn.
I can honestly say that I’d never heard of this topic before. I’ve grown half a dozen walnuts from seed that will soon be ready to plant into my hedge.
I’ll have to look for some info on weather they harm other nut trees, hawthorn and silver birch before I plant them.
Thanks for raising the topic