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

Q. Mike: HELP! This is the first year we'reattempting a veggie/herb garden at our present home. We picked a spotwith good sun and drainage—and a bit of afternoon shade from the "OldWalnut Tree" that we thought the plants might like. I put a 4x8 footframe around the garden, replaced the sod with several bags (440 lbs.)of garden soil, and then planted tomatoes, cucumbers, herbs andflowers—using Miracle-Gro potting soil around and below each plant. Ialso spread some lime around the garden, as the site was the formerhome of some huge rhododendrons.
All looked well for a couple ofweeks. Then the tomatoes and basil began to look yellow and stoppedgrowing. We put in five different varieties of tomatoes, and all seemsick. The (huge!) walnut tree is dropping immature nuts, leaves, andtwigs 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, beautifulblack walnut cut down; it seemed to be becoming increasingly toxic,with a large part of the nearby garden turning into a desert. Thequestion is: How long after a black walnut tree has been cut down willit still affect vegetation?  We've done away with our compostbecause it included leaves and branches from the walnut.  
---Ken Lane, Wayne, PA

A.    Thanks guys—those aregreat questions about a fascinating garden problem—the plant-killingproperties of black walnut trees! (And because you asked thosequestions, 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 yourgarden, 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 therebefore. 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 fewrhododendrons 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 killplants REAL good. And it doesn't sound like you've fed the poor puppiesanything yet. Plants need a naturally rich, compost-amended soil orregular feedings with a gentle organic fertilizer. No chemicals!(You're lucky there isn't much of their toxic chemical fertilizer inMiracle-Gro potting soil or you'd have hurt the poor plants more.)

OK—Now the Black walnut basics. Thetrees contain a natural substance called juglone that inhibits thegrowth of many plants (or just plain kills them). It's contained inevery part of the tree—bark, wood, leaves—but it is strongest in theroots. In fact, those roots are SO 'full of it' that Dr. Paul Roth,Professor Emeritus, Department of Forestry, Southern IllinoisUniversity, once warned me that a walnut's toxic effects on otherplants will continue for several years if the roots are left in theground. Sorry Ken. Oh, the trees are also "good sprouters" and willattempt to regrow as well. So if you do decide to cut a walnut downwithout having the stump pulled, cut it as low to the ground aspossible.

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

The area over which walnuts affectsensitive plants generally extends 50 to 80 feet from the trunk, orabout 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 mostpopular home-grown veggies (tomatoes, peppers, eggplant, and potatoes) and ornamental favorites like petunias,azalea, viburnum, hydrangea and rhododendron. Blueberries too.  

However, grasses—especially Kentuckybluegrass—are felt to THRIVE near the trees (as long as they get enoughsun, of course). And a surprising number of other plants can alsoapparently coexist with walnuts. The Ohio State University Extensionoffice has compiled a two-page list of plants that don't seem to beaffected (most of this information is based on observation, not hardresearch), including squash, melons, beans, carrots and corn; clematis,forsythia, marigolds, begonias, violets, zinnia and pansies (yay! Ilove panies!); most Spring bulbs (double yay!), some daylilies, peoniesand hostas; and some fruit trees and arborvitae. (For the complete listgo to:  You'll also find lots of similarlists on the web.)

But Dr. Root wanted to emphasize thatthere's more than just juglone at work when plants fail to thrive neara walnut tree. There's also the serious shade they throw and the fiercecompetition for food and water from their BIG root systems. Let's faceit—no thirsty, sun-loving plant is going to thrive underneath any bigtree!

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

You Bet Your Garden  ©2004 Mike McGrath

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