The Spoiler remarked to me that other day that no matter now much water he was giving one of our liriope (lily turf grass) it wasn’t growing as well as the others. I asked him if it was the one by the bird feeder and he said that it was and I said that was because sunflower seeds were allelopathic. “Sounds like something for the blog,” he said once I explained the concept.
These are what the liriope that are growing in a row along a border look like–except for the one that’s been affected by the sunflower seed shells.
This is the liriope affected by the sunflower seed shells. Interestingly, the petasites leaves are not affected by sunflower seed shells. It is has long been noted that different plants exhibit different degrees of affect from allelopathic plants–and sunflowers and their shells are by no means the only ones that are allelopathic.
Perhaps the most well-known allelopathic plant is the black walnut tree (juglans nigra). Almost nothing will grow around or near one of those, and black walnut shells have allelopathic affects as well.
Other plants with allelopathic affects include black cherry, sugar maple and forsythia.
However, as you can see by the petastites above, some plants are not as affected by allelopathy as other. Sugar maples are known to inhibit evergreen growth, but only needled evergreens, not broad-leafed evergreens, which is why ground covers like ivy and the broad leafed evergreens like rhododenrons can grow under sugar maples.
So if you are having trouble growing a particular type of plant in a certain spot and your soil tests are coming back fine, check to see if the plants in the area are allelopathic!