Sunflowers are allelopathic. They produce a phenol which is phytotoxic in all parts of the plant. This phenolic compound is more toxic to some plants than others, and the levels present in the soil can even rise to the point of becoming autotoxic. That is why sunflowers in cropland situationsneed to be rotated frequently.

We have several species of sunflowers here. Most common is the annual sunflower, Helianthus annuus. Although most rangeland situations have enough grass cover to keep sunlight from hitting the seeds and triggering germination, cropland is a different story. After winter wheat is harvested, I have to get my ground covered fairly quickly or the sunflowers will start showing up in force. If the sunflowers are allowed to do much growing, next years wheat crop will show the ill effects. If the sunflowers go unchecked for two years, even the sunflowers will do poorly the third year. The compound does seem to be water-soluble, so with enough rainfall, it will wash away. Here, annual rainfall is just over 20", so the effects seem to linger a little longer than they do to the east.

In the field, you can go out and look underneath the sunflower, and the ground will look like someone put a very thin coat of varnish on the soil surface. It is a sticky substance thatdrips down from the leaves. Some plants it affects negatively, others not so much. I was unfortunate last year in discovering some field bindweed in my best field. I did not plow the area and risk spreading the problem, and soon sunflowers appeared. I had hoped they would discourage the bindweed, but instead the bindweed grew up aroundthe sunflowers throughout the summer.[:@]

About the only plant around here that can hold a candle to sunflowers as far as allelopathy is concerned is the black walnut.