Are sunflower shells toxic to plants?

Sunflower-seed shells can stop or stunt the growth of other plants.


Is it harmful to use the empty shells of sunflowers as mulch or to till it into the soil? I have so many of them from my bird feeders, but I know some of the seeds killed the grass a few years ago and probably stunted the growth of some plants. I've tried shell-less mixes, but they're very expensive.

A: Sunflowers have natural plant chemicals (sesquiterpene lactones) that are fairly effective at helping this species fend off competitors trying to weasel into its growing territory.

The chemicals are in the shells as well as the leaves and roots of sunflowers. This natural plant-to-plant defense (called "allelopathy") works well enough that scientists have investigated sunflowers as a possible source for weed prevention.

Not all plants are inhibited by sunflower parts, though. Some species are very sensitive and some don't seem to mind sunflower competition at all.

If you rake up your shells and use them as mulch, you'll probably run into stunted growth, depending on what you try to grow there. If you're not planting anything, then no problem. The shells probably will even prevent at least some weeds from growing. (I've heard gardeners claim that sunflower residue stops poison ivy, for example.)

I wouldn't till the shells into the soil or use them in the compost pile either. Both will dilute the chemicals, and they'll also break down over time (especially in contact with soil or compost microbes), but even diluted amounts will be enough to affect the most sensitive plants in the short term.

I haven't found a good list of sunflower-sensitive species, but in my own garden, I've grown coralbells, liriope and 'Knock Out' roses around bird feeders without trouble.