I am consulting for a rather large food forest (20 acres) and needed to check out juglone effects because they want to include Walnuts with Persimmons and Pecans. I hope to share more info about the project soon. Here are my notes I thought I’d share.
Plants effect one another: by shade, by root competition, by mulching each other with leaves. These effects are basic and easily seen, but plants also effect each other in far more intrinsic ways, one of the most significant called allelopathy.
Allelopathy is the science of the effects plant chemicals have on each other; how one plant, if planted near another, will thrive or languish because of the chemicals its neighbor emits.
The methods of emissions can become quite advanced: from root exudates of nitrogen and the simple release of chemicals as a plant rots down over winter, to release of chemicals into the air, via the leaf stomata.
The chemicals are fairly specific: killing some plants but benefitting others, depending on species. In this way, allelopathy is more a situation of plants choosing the partners they want nearby than an all out homicide (herbicide?) of any plant nearby.
One of the most famous allelopaths is Juglone. It’s produced by plants in the genus Juglans, which as you might huess gave the allelopath its name. Black walnut (Juglans nigra) produces the highest amounts of this allelopath in the genus Juglans.
Juglone works by slowing the metabolism of plants, turning their leaves yellow when taking serious effect. Despite the strength of juglone, it is not very soluble in water, and hence, doesn’t spread very far, even in porous, well drained soil. The effects are often weaker where the soil is better draining, strongest where the soil is hard and non-porous.
The potential dose of juglone is strongest under the tree’s drip line. Nevertheless, it is recommended that sensitive plants be kept at a distance of even sixty feet from the tree to ensure root exudates containing juglone aren’t a problem. Obviously this is only a worry with larger walnut trees, but because they grow over time, keep this in mind.
See this post and its pdf. for more info on juglone, and lists of plants likely susceptible to its effects.