QUESTION: We have been told that we can't grow garden plants under walnut trees because they produce a chemical that is toxic to other plants. Is this true?

ANSWER: To an extent it is true. Some plants produce toxic chemicals that prevent plant growth. The most common occurrence of this characteristic, which botanists call allelopathy, is the black walnut. Since the time of ancient Rome, gardeners have observed that walnut trees are toxic to many other plants.

Although this toxicity is not fully understood, it is known that a toxic substance called juglone is exuded from all parts of black walnut trees. However, not all plants are susceptible to damage. In fact, some seem to have improved vigor when grown near a black walnut tree.

Research has shown that roots of the tree must have contact with, or be very close to susceptible plants to cause injury. This may include a much larger area than most people expect however, since the roots of a mature walnut tree occupy an area at least five or six times that of the tree's crown. There is also the potential for damage from natural rainfall leaching the toxic substance from leaves and branches onto the plants below.

Even after a walnut tree is removed, the toxic substance will remain in the soil for about a year. Although leaves, husks and bark contain juglone, mulches of these tree parts are reported not to cause the toxic response that occurs with actual root contact.

The use of allelopathy for the control of weeds is at the cutting edge of environmentally sound pest management. Researchers at the University of Michigan have found that cereal rye is extremely effective in controlling many common garden weeds. In an experimental plot, rye was planted as a cover crop in the fall. In the following spring, the rye was mowed down or sprayed with a short-term herbicide. Large seeded vegetables such as beans and peas were then planted through the mulch.

Home gardeners can easily implement allelopathic principles in their own gardens by planting a fall cover crop of rye and using the residue as a mulch following the procedure developed at the University of Michigan.

Another option, would be to grow a cover crop of rye in the back area of the garden, mow it monthly, and spread the clippings between the rows of vegetables. Aisles between trees in home orchards might also be planted with rye and kept mowed.

Although a mulch of cereal rye may not control all weeds, it will certainly help to reduce overall weed populations.

QUESTION: We continue to hear about using vinegar for weed control. Does it really work?

ANSWER: In May 2002, the USDA Agricultural Research Service issued a press release describing their research on weed control using vinegar. Green house and field studies indicated that while 5 percent vinegar solutions (commonly found on grocery store shelves) did not produce reliable weed control, solutions of 10, 15 and 20 percent provided 80 to 100 percent control of certain annual weeds (foxtail, lambsquarters, pigweed, and velvetleaf). Perennial weeds (Canada thistle) treated with a 5 percent vinegar showed 100 percent shoot burn down but roots were not affected, therefore shoots always re-grew.

EDITOR'S NOTE: For answers to local gardening questions, contact Master Gardener Rachel Gana at 642-8723 or e-mail her at:

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