In late May, Errald Turner of Newport sent the following question:

“Last summer my [Rosa] rugosa leaves turned yellow and fell off and some of my lilac plants died off a bit. I think I have found the reason – you be the judge. I had sunflowers near both plants for several years. Could the toxin from the sunflower seeds have caused this? They self seed each year.”

Sunflower seed hulls do contain a toxin that inhibits the growth of grasses and other plants. This phenomenon is known as “allelopathy,” a term properly applied when one plant interferes with the growth of another through production of a chemical (called an “allelochemical”) into the environment. Not much is known, however, about sunflower allelopathy, including which plant species are sensitive to the toxin and how much toxin is required for interference. And so Errald’s question is difficult to answer with certainty.

Appointed as judge, I decided to research the literature on this subject. I have learned that grasses and other herbaceous plants seem to be most sensitive to the sunflower allelochemical(s). I could find no reports of sensitivity among woody plants, such as Rosa rugosa or lilacs. Also, growing sunflowers in the garden and allowing them to self-seed each year may not produce a sufficient amount of toxin to cause interference in nearby plants. On the other hand, gardeners who feed sunflower seeds to the birds should not be surprised that nothing grows beneath the feeder where larger quantities of hulls accumulate over time, slowly releasing the allelochemical into the soil as they decompose.

And so, Errald, having reviewed the relevant literature, I doubt that allelopathy is at the root of your rose and lilac problems, but neither can I dismiss the possibility. How’s that for a definitive answer!

Errald’s question brought to mind another tale of allelopathy. Two years ago, I watched one of Littlefield Garden’s smaller magnolia shrubs wither and die over the course of a single summer. A squirrel-planted nut from the Garden’s black walnut tree (Juglans nigra) had germinated the previous spring at the base of the shrub and the resulting seedling had quickly grown into the heart of the magnolia. Walnuts are well known for their allelopathic effect on nearby plants. Their roots produce large quantities of juglone, an allelochemical toxin to many plant species (see sidebar), and so it was easy to diagnose the sudden demise of the small magnolia. I then realized that the slower decline of a much larger magnolia several yards away from the mature walnut tree was also because of increasing concentrations of juglone in the soil around the magnolia’s roots.

Decades earlier, when both trees were planted, the intervening space between root systems was large enough for both trees to grow happily. With time, however, the walnut tree’s sphere of allelopathic influence grew wider and wider.

All of our garden plants grow wild somewhere, a place where they survive through successful competition for essential resources and often by using allelopathy to stake out a space. They behave the same way in our gardens. We are just beginning to understand allelopathy, to learn which plants produce allelochemicals and which species are sensitive to them. As we learn more, we will no doubt have answers to some of our perplexing garden problems.

Allelopathy in black walnut

Since at least the first century A.D., black walnut (Juglans nigra) has been known as a species that interferes with the growth of neighboring plants. Effects of juglone, the allelochemical produced by walnut roots, buds and nut shells, include wilting and eventual death of sensitive species. But not all plant species are sensitive to juglone. The following lists attempt to sort out our current limited knowledge.

Walnut-sensitive plants

. Tomato

. Potato

. Pea

. Pear

. Apple

. Many ericaceous species, including rhododendrons and azaleas

. Magnolias

Walnut-resistant plants

. Most grasses

. Daylilies

. Hostas

. Begonias

. Violets

. Pansies

If you are attempting to garden in the vicinity of a walnut tree, be mindful that your experiences may well add to either of these lists. Take care to keep garden beds free of walnut leaf litter and nuts. And keep an eye on where the squirrels do their planting.

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