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Black Walnuts? No Worries!
Some Plants Thrive Near Those 'Terrible' Trees

Q. Mike: HELP! This is the first year we're attempting a veggie/herb garden at our present home. We picked a spot with good sun and drainage—and a bit of afternoon shade from the "Old Walnut Tree" that we thought the plants might like. I put a 4x8 foot frame around the garden, replaced the sod with several bags (440 lbs.)of garden soil, and then planted tomatoes, cucumbers, herbs and flowers—using Miracle-Gro potting soil around and below each plant. I also spread some lime around the garden, as the site was the former home of some huge rhododendrons.
   
All looked well for a couple of weeks. Then the tomatoes and basil began to look yellow and stopped growing. We put in five different varieties of tomatoes, and all seem sick. The (huge!) walnut tree is dropping immature nuts, leaves, and twigs in the garden area every time we get a strong wind or rainstorm.Is this tree the culprit here? Or is some other factor to blame?
                       ---Bill Kellaris; Langhorne, PA  

Q. We recently had to have a big, beautiful black walnut cut down; it seemed to be becoming increasingly toxic,with a large part of the nearby garden turning into a desert. The question is: How long after a black walnut tree has been cut down will it still affect vegetation?  We've done away with our compost because it included leaves and branches from the walnut.  
---Ken Lane, Wayne, PA

A.    Thanks guys—those are great questions about a fascinating garden problem—the plant-killing properties of black walnut trees! (And because you asked those questions, many of our listeners will now walk out to their garden,look up and say, "Oh, so that's why I grow such awful tomatoes!")

Anyway—you with the new garden first.With your description of the black walnut debris falling into your garden, I'd say the tree is definitely at least part of the problem.The funny thing is you say that there were big old rhododendrons there before. Those plants aren't supposed to do well under walnuts either.But I have heard that there are a few (VERY few) varieties that can,however—and so I can only guess that you cut down one of the few rhododendrons that can grow near a walnut to plant a garden that can't.Solution: Get in a time machine and leave that rhodo right where it is!Eh, was. Eh…

Now, you also say you spread lime,which you should never do without testing the soil; lime can kill plants REAL good. And it doesn't sound like you've fed the poor puppies anything yet. Plants need a naturally rich, compost-amended soil or regular feedings with a gentle organic fertilizer. No chemicals!(You're lucky there isn't much of their toxic chemical fertilizer in Miracle-Gro potting soil or you'd have hurt the poor plants more.)

OK—Now the Black walnut basics. The trees contain a natural substance called juglone that inhibits the growth of many plants (or just plain kills them). It's contained in every part of the tree—bark, wood, leaves—but it is strongest in the roots. In fact, those roots are SO 'full of it' that Dr. Paul Roth,Professor Emeritus, Department of Forestry, Southern Illinois University, once warned me that a walnut's toxic effects on other plants will continue for several years if the roots are left in the ground. Sorry Ken. Oh, the trees are also "good sprouters" and will attempt to regrow as well. So if you do decide to cut a walnut down without having the stump pulled, cut it as low to the ground as possible.

What can you do with those toxic roots in the ground? Dr. Roth explained that constant tilling helps break down juglone in soil, as does raising the pH (hey—we're back to lime again!). And other sources have told me that large amounts of compost can help. Bet hey, you know me—I think large amounts of compost can help ANYTHING!

The area over which walnuts affect sensitive plants generally extends 50 to 80 feet from the trunk, or about twice as far as the crown of the tree when it's fully leafed out.Plants noted for dying quickly within this range include the most popular home-grown veggies (tomatoes, peppers, eggplant, and potatoes) and ornamental favorites like petunias,azalea, viburnum, hydrangea and rhododendron. Blueberries too.  

However, grasses—especially Kentucky bluegrass—are felt to THRIVE near the trees (as long as they get enough sun, of course). And a surprising number of other plants can also apparently coexist with walnuts. The Ohio State University Extension office has compiled a two-page list of plants that don't seem to be affected (most of this information is based on observation, not hard research), including squash, melons, beans, carrots and corn; clematis,forsythia, marigolds, begonias, violets, zinnia and pansies (yay! I love panies!); most Spring bulbs (double yay!), some daylilies, peonies and hostas; and some fruit trees and arborvitae. (For the complete listgo to: http://ohioline.osu.edu/hyg-fact/1000/1148.html.  You'll also find lots of similar lists on the web.)

But Dr. Root wanted to emphasize that there's more than just juglone at work when plants fail to thrive near a walnut tree. There's also the serious shade they throw and the fierce competition for food and water from their BIG root systems. Let's face it—no thirsty, sun-loving plant is going to thrive underneath any big tree!

And what about making compost with the leaves? Dr. Roth told me he never puts ANY black walnut leaves in his compost pile; he saves them to use as a 'killing mulch' to get rid of unwanted plants! (VERY clever! And I'll add that the chipped uproots would be even more effective!) The experts at Ohio State say that well-shredded walnut leaves lose their plant-harming capability after a month of hot composting. But if you have a LOT of black walnut leaves going into your pile, they suggest you test the finished compost by planting some extra tomato seedlings in it before you use it on a larger scale. Juglone, they explain, is tomato Kryptonite!

You Bet Your Garden  ©2004 Mike McGrath