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Keeping to Their Own Kind

Black walnut trees don’t mind bulbs and ground cover, but they kill competing broadleaf species

The black walnut is a unique tree. It selects its neighbors and wipes out its competition. The roots, bark, wood, leaves and husks of the black walnut contain an enzyme called juglanace. This enzyme remains in the tissues until they are decomposed beyond recognition. The horticultural term used to describe the competition-controlling properties of black walnut is allelopathic response.

Juglanace acts like a selective weed killer, allowing only certain noncompetitive species to grow in close proximity. The juglanace enzyme does not affect the growth of grasses, including ornamental grasses, or bulb species such as tulip, narcissus, hyacinth, crocus, gladiola, daylilies and allium. Groundcover plants such as liriope and mondo grass, (not a true grass) seem to thrive when planted under the canopy of a black walnut tree.

But juglanace is deadly to broadleaf plants such as azaleas, rhododendrons, dogwoods, forsythia, viburnum, spirea, privet and holly to name a few.

Even in areas where black walnut trees have been lumbered, it takes years before broadleaf plants become established, with a gradual progression starting at the outer reaches of the branches and moving toward the trunk. For decades, the areas near the trunk will only allow grasses to grow. Juglanace appears to remain active in the wood until it is totally decomposed.

There are exceptions: I have seen hydrangeas and yews growing along the outer periphery but not in close proximity to the trunk.

I had a friend whose garden was near a large black walnut tree. In close proximity to its drip line, he could grow only sweet corn and onions. Only in areas of his garden yards away from the drip line was he able to grow lettuce, tomatoes, beans, peppers and root crops such as carrots and beets.

So watch what you plant around black walnuts. And watch what you compost. If you put leaves, nut shells, husks or twigs of black walnut in your compost pile, make certain that they are thoroughly decomposed before you spread the compost in your garden.


 

Plow vs. No-Till Farming

Q Doesn’t no-till farming, with its extensive use of herbicides, contribute to water pollution and soil contamination? Wouldn’t we be better off just going back to plowing and cultivating the fields, even though gas and diesel fuels are so expensive?

–Carl Levitt, Lusby

 

A Plowing destroys the soil structure, destroys organic matter, creates a plow pan, causes rapid release of nutrients, exposes the soil to surface erosion and increases leaching of nutrients into the groundwater. Furthermore, because it exposes weed seeds to germinating conditions, it requires more cultivation, more hand labor or greater use of pre-emergent herbicides to control weeds. During periods of drought, plowing results in lower water retention in the soil, resulting in more drought losses.

No-till does not disturb surface soil, results in accumulation of organic matter in soil and reduces chances of erosion and loss of nutrients by leaching. With each succeeding year, fewer herbicides are needed because soil is not disturbed to expose weed seeds to germinating conditions. Less fuel is needed to grow crops, lighter equipment can be used, thus reducing soil compaction and problems associated with plow pans. Rhe herbicides used in the initial treatment are contact types with short residuals. And with each succeeding year less and less is needed.


Ask Dr. Gouin your questions at frgouin@erols.com. All questions will appear in Bay Weekly. Please include your name and address.