The Problem With Black Walnut Trees

Ever heard of Black Walnut Blight? Or Black Walnut Toxicity? Did you think the term originated because the falling of black walnut fruits can concuss an adult? Ah, were it only so! The first term is sometimes mistakenly used to indicate the condition of plants trying to grow under a black walnut tree, which can look very sickly, indeed. [The accurate use of the term would be to indicate a disease of the tree itself, caused by the bacterium

Xanthomonas juglandis

. It is most common in rainy regions like the Pacific Northwest.] 

A very large (or it feels that way, since it kind of leans toward the house)

black walnut

anchors the very back edge of our yard. Another smaller walnut is in the corner just past our property line. Walnuts can wreak havoc on your planting plans. You can plant some marvelous new choices from your local nursery, amend the soil with delicious compost--and within a year the plant is obviously sick or unhappy. It may be your Killer Tree.

People who are gifted with a black walnut tree have both an asset and a curse. The asset is a protein source that both you and your squirrels (and other animals) can enjoy. The curse is two-fold: like any nut or fruit tree, they can be messy. But their messiness is far outweighed by their ability to inhibit or kill other plants around them. Isn't that weird that plants can do that to each other? Walnuts are not alone in this skill set, they are just better at it than anybody else in the plant kingdom. [first photo:

Monarda fistulosa

, native pollinator favorite, walnut resistant.]

Joe Pye Weed--another pollinator favorite. Ignores walnut trees.

All other trees in the genus 

Juglans

 have the same ability, but to a lesser extent. Essentially, these trees give off a compound called juglone through their leaves and roots that has an impact on other plants. Anything 

in the drip line can be hit, and anything within reach of its roots can be hit. Cutting down the tree will get rid of the drip line impact, but the roots have to completely rot before their impact will be gone. So you may as well keep the tree and choose plants that can take it! That's my reasoning, anyway. 

There are, however, Things You Can Do.

Here I quote from a publication from 

Virginia Tech

: [t]he accumulation and depletion of toxins in the soil is affected by factors such as soil type, drainage, aeration, temperature and microbial action. Soil microorganisms ingest allelochemicals as energy sources, and metabolic decomposition can render the chemicals non-toxic to plants. When soils are well drained and aerated, a healthy population of aerobic microorganisms can accelerate this process."

Lobelia, hummer favorite, walnut tolerant.

In short, the most important thing you can do to encourage healthy plants around a black walnut tree is create healthy soil. So once again, you will do best to get rid of any grass and create mulched beds that will support the insects and microorganisms that eat walnut "juice." Under a walnut tree would be a great place for a bird feeder, since all the scratching and bird poo will increase microbial activity. You might even welcome moles--they're great at aeration, right? OK, just kidding....

Some plants will never do well around a walnut since they are just too sensitive to the juglone. Other plants are strongly armed against it.  The rest fall into that gray area that is simply undocumented. By researching garden forums for your area you may find a consensus of opinion on certain plants that will do well. The key to some of these is simply making sure the plant is having all its other needs met (sun, water, soil preference)--reading some of these forums leads me to believe that it was owner error rather than the walnut that "did in" a particular plant.

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