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Black Walnuts: Threat or Menace?

Q. Dan in Upper Black Eddy in Bucks County PA writes: "I have two questions regarding walnut trees.

  1. What is the timeframe for the soil around a walnut tree to still be poisoned by the juglone after I cut down the tree?

  2. Should I avoid using shredded leaves on my beds if they have some walnut tree leaves in them? What about if the beds are near or under a walnut tree anyway?"

A. Thanks for asking, Dan! It made me recall an exhaustive Washington State University Extension article I read over the summer written by WSU Extension agent Susan Chalker-Walker, a previous guest on our show. She explains that the belief that certain plants, especially walnuts, are deadly to nearby plants goes back to Roman times. The process is called allelopathy {quote: "death to others"}.

And of course, the most feared plant compound is juglone, the famed "Tomato Kryptonite". Susan reports that the actual research on this subject is both skimpy, inconsistent, sometimes downright contradictory, or just plain made up. But one thing seems to be certain--living and dead black walnut trees contain no actual juglone. Instead, they contain a precursor called hydro juglone that turns into juglone when exposed to soil, which explains the long-held belief that 'juglone is most highly concentrated in the roots'.

Now comes the really weird part. Susan reports that back in 1948, the USDA launched a campaign to convince gardeners that black walnut trees were harmless, which, apparently nobody believed. She adds that the few actual studies performed are no longer available; one because its woefully out of date and the other because it appears to be fraudulent and may have never existed.

Before we get back to Susan's fine investigative reporting, we should answer Dan's questions.

Cutting down the tree will not help, because the roots will still be in the soil. If you need that area for growing (especially tomatoes) you'll have to have the stump pulled and clean out as many stray left-over roots as possible. Most research indicates that the juglone in those roots stays active for ten to twenty years, although one of the articles cited by Susan notes that "juglone does not persist in soils with high microbial activity." More on this in a bit.

The good news: Yes, you can incorporate a percentage of shredded black walnut leaves into your compost pile, as they contain little of the juglone precursor. My advice has always been to limit black walnut leaves to twenty percent or less of your shredded leaf total. It would seem foolish to press your luck with more than that. Composting in a hot pile would also be helpful as the microbes generated in a hot pile would help to denature the chemical.

Shredded leaf mulch: Since the precursor becomes juglone when it comes in contact with soil, it would seem a foolish risk to use the leaves directly. FINISHED compost (no raw material showing) should be fine.

Garden beds directly underneath a black walnut tree? There are so many things wrong with this picture it's making me dizzy. No, no, no! Sounds like another call for stump pulling. Don't forget; it isn't just the juglone--the shade and root competition from the tree also make it a terrible location for other plants.

Now back to Susan's excellent article. Towards the end, she provides some things that gardeners can do to limit the damage.

{quote}: "Provide adequate irrigation for landscape plants during drier, warmer weather. All plant roots compete for water, nutrients, and oxygen. The denser your plantings, the more intense [that] competition will be."

"Mulch well with arborist wood chips to retain soil moisture and to nourish beneficial soil life including mycorrhizae." I'll step in here to suggest that a mulch of compost would be an even faster way to bring more life into your soil. And I'll take a moment to remind everyone that wood of any kind should never be mixed INTO the soil, where it will interfere with nitrogen uptake.

The mention of mycorrhizae is especially interesting, as Susan notes that research has shown that soils rich in organic matter have the ability to 'bind up' juglone and make it less available to nearby plants. It's like I always say, "I don't care what the question is, the answer is compost!" This might also be an excellent case for adding beneficial microbes to the soil--especially in the planting hole of tomatoes.

Of course, the opposite is true. Chemical fertilizers limit or destroy soil life. If you feed your plants repurposed explosives like Miracle-Gro, Osmocote and the like, you're depriving those plants of the positive symbiosis they need to survive.

And finally, a controversial final comment from Susan: "Enjoy your walnut trees! Not only are they a robust landscape plant, [but] they provide food and habitat for wildlife. However, they should not be planted in areas with thousand-canker disease."

In my little Garden of Eating, Evil Squirrels are the main harvesters of black walnuts, which they eat for awhile, then get bored and plant the extras in my tomato beds.


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