Sunday, February 3, 2013

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.

If you haven't already noticed it, check out the page tab above titled "Black Walnut Lists" of some plants known for black walnut tolerance (and those that aren't!). 


Anonymous said...

Hello- Great article on Black Walnuts - but the toxicity of juglone to other plants is called just that 'Juglone toxicity' not Black Walnut Blight which your first sentence would suggest. Black Walnut Blight, caused by the bacterium Xanthomomas juglandis, is a disease of black walnut trees.
P.S LOVE your articles on birds and HATE tree mulch volcanoes. If I ever go to jail it would be from going on someones property to tear apart tree volcanoes and save the trees.

R K Young said...

Alli, you are absolutely correct--I will see how I can rephrase that to indicate the difference! The term is sometimes used to indicate the "blight" happening to the plants under the tree--but I should have made that clear.

Yes, tree mulch volcanoes are one of the WORST "blights" out there, and the are far too common!

R K Young said...

There, that's better. Thanks, Alli!

Jason said...

We had a big old black walnut in the neighbors' yard that included a good chunk of our yard under its drip line. At first we were oblivious, even tried growing tomatoes on the edge of the drip line. Then we found there were in fact many perennials that could do well near the black walnut. But I'm glad we don't have one at our current house.

Cindy said...

Luckily we have no Black Walnuts to deal with..but what a stunning Lobelia picture! I am going to try to grow them again this year!

David said...

I have a few black walnuts, and have consulted lists to see which of my natives (trees, shrubs, and forbs) will do well under it. I'm ready to consult a list again as I plan what to grow in my woodland project.

It is nice to see that cardinal flower is tolerant of it, especially since the one tree is located near a wet area which might be perfect for it. :) Thanks.

R K Young said...

I honestly think that's part of the confusion on some of the lists. Some plants that might do fine by a black walnut don't--because they are in a spot that's too dry or shady or whatever. But, as more people ask that question (whatever the question is), others step up to get the answer. Part of what I love about the social economy!

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