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” – definitely 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 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 it is quite possible that one of the black chemicals is affecting plants – it may not be juglone.
Also interesting is the fact that walnut trees contain almost no juglone. This makes since juglone is quite toxic to plants including walnuts. The walnut produces the 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. It is quite possible that one or more of these other chemicals is 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 wlanuts, 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 gardeners perspective, the fruit and leaves pose the greatest source for juglone.
The amount of juglone potential also changes during the season. It is highest in 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 soil and held quite tightly. It is held more tightly by clay and organic matter than by sand. As a result of this, the affects of juglone on other plants is 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.
Clearly 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 does damage seedlings of some plants. But this can’t be extrapolated to the field. It could just as well be due to some other chemical.
There is clearly allelopathic mechanisms going on under a walnut tree. They definitely 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 microbe 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 alleleopathic 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 affects 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 away from known sensitive plants.
Death By Juglone
You might wonder why, if juglone is toxic to plants, would 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.
- 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