Monday, September 11, 2023

Juggling the juglone

Don’t do it! It’s the worst place to grow plants. The soil is toxic and they’ll all die. If you have ever read or heard stories about walnut trees, especially black walnut which is common around here, that’s the gist of it. But is it true? This popular belief has been widely reported and quoted over the years, often from reliable sources.Consequently, gardeners abhorred the thought of a walnut tree growing anywhere near their gardens. Because the black walnut is prized and much sought after for its beautiful wood, there’re many who would say the trees are worth more dead than alive.

The theory is walnut trees produce a chemical that inhibits the growth of other plants — the term used is allelopathy. The chemical is juglone and is believed to be exuded by the roots of the tree making the soil toxic to many plants. Other parts of the tree, including the leaves, are also said to contain juglone and it’s long been the recommendation that wood chips from a walnut tree should not be used as mulch and the leaves should not be composted.

However, it’s since been revealed that juglone is not initially present in the living tissue of the tree, but a non-toxic precursor called hydrojuglone is and it’s only converted to juglone when in the soil. I’ve composted my share of leaves from black walnut and never had an issue using the compost in my garden.

Supposedly, juglone doesn’t affect all plants. There are long lists available of those that can be grown beneath walnut trees and others that can’t. Tomatoes, it seems, can’t be grown beneath the tree, but onions can. The problem with these surprisingly long lists is they’re apparently based mainly on observation — what’s been seen to be growing well or not, but not necessarily taking into account other factors that can affect growth.

There have been few if any scientific studies that give solid evidence supporting all these plant recommendations and the same goes for the belief that growing anything beneath walnut trees is an actual problem.

The question is, where did this belief arise? We are fortunate in that someone has done the work to find out. Linda Chalker Scott, Associate Professor and Extension Horticulturist at Washington State University has only recently completed a review of all possible material related to the issue and published a fact sheet on it.

Chalker-Scott reveals that back in the 1920s, despite farmers growing crops successfully near walnut trees, the belief took hold. By 1948, the United States Department of Agriculture felt the need to issue a press release assuring the public that there was no need for concern and that the trees were harmless to other crops, but still the belief persisted. Then, in 1951, experimental field testing began and guess what — little to no adverse effects were evident.

Some laboratory testing was done using juglone directly on seeds and seedlings and there appeared to be negative effects, yet the results were inconsistent. As with any laboratory testing, it doesn’t necessarily relate to what occurs in nature. Chalker-Scott concludes that all the scientific evidence for black walnut allelopathy can be traced to two publications, one of which has been withdrawn and another that is non-existent.

So, what is going on under black walnut trees? Ignoring the myth, the answer is probably what goes on under all large trees. The roots of most trees mainly occupy the top couple of feet of the soil and spread far beyond the dripline. Light rain never reaches the ground beneath the canopy and consequently, the tree grabs all the moisture from the soil along with the nutrients. It’s always difficult to grow much under any large tree, although many gardeners have been successful.

As for tomatoes, it’s the one plant that’s said to be a good indicator of the allelopathic effect. They often fail to grow when planted beneath a black walnut tree, but then why would anyone try to grow tomatoes there? It’s about the worst place. Tomatoes need lots of water — none there. They also need good, rich soil — hardly likely. And what tomatoes really need is plenty of sunshine.

Good luck with that in the shade of any tree. Don’t do it.

Read the original work of Linda Chalker-Scott here.

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