Q: In a garden catalog I received it states that sunflower roots, not the seeds, inhibit the germination of other plants. If so, how far apart should I plant my sunflowers this spring?

A: The ability of one plant to inhibit the growth of another is called allelopathy. Black walnut trees are widely known to inhibit the growth of many other plants which try to grow beneath it. It has been found that a chemical produced by a walnut’s roots is the active component.

Bird lovers have noticed that nothing seems to grow under their bird feeders when they use sunflower seeds. The hulls that fall to the ground exhibit allelopathy toward other plants. Allelopathy exhibited by the seed hulls under a feeder should be gone in two months.

In addition, sunflower farmers know that they can’t plant sunflowers in the same field year after year because the crop residues diminish their yields. I could find no information on whether sunflower roots are allelopathic to other sunflowers. I think that if you space your plants sixteen inches apart and pull out the stems and roots each fall when they die you’ll have few problems.