Not many people know about the dark side of sunflowers (Helianthus annuus). However, the beautiful bright blooms do hide a nasty secret: sunflowers are allelopathic, that is, they give off toxins (terpenes and various phenolic compounds) from all their parts (roots, leaves, stems, flowers, seeds, etc.) that impede the growth of other plants or even kill them. This is a protective system for the plant: they kill their neighbors, but not their own seedlings, so this gives the plant, an annual that only reproduces by seeds, a head start, making sure it can come back the following year without too much competition.
That said, if sunflowers are grown year after year in the same spot, even their own seedlings will eventually start to suffer.
The efficacy of sunflower toxin is such that the sunflower extracts are being considered as potential organic herbicides. Studies show that certain sunflower cultivars are much more phytotoxic than others, which suggests it might be possible to breed sunflowers specifically for their herbicidal effect.
Reducing Sunflower Toxicity
To reduce the effect of sunflower toxicity, cut back, chop up and compost the plants, including their roots, in the fall (yes, the sunflower’s toxic parts decompose readily in compost bins) and rain and natural decomposition will eliminate most of the toxins left in the soil before spring. Or continue to grow sunflowers on that spot.
The most obvious place where sunflower toxicity is visible is under bird feeders.
Sunflower seeds are favorites with birds, but the hulls fall to the ground over the winter, weakening or killing the plants below, notably lawn grasses. Then sunflower seedlings, originating from seeds the birds dropped without eating, germinate and grow: not necessarily what you had planned.
To prevent or reduce this effect, cover the ground under your bird feeders in the fall with a tarp or cloth and remove it, along with the hulls and seeds, in the spring. Or place your feeder over a surface free of plant growth: perhaps a patio or deck. Or grow sunflower resistant plants underneath.
You could also use hulled sunflower seeds (sunflower “hearts”) as bird feed, although they are more expensive.
One would hope that hybridizers could develop a toxin-free sunflower to be grown specifically for use in bird food, but this is not, as far as I know, being done.
Plants Resistant to Sunflowers
There has been little study of plants that are resistant to sunflower allelopathy, although I did find the following list on the site of Toronto Master Gardeners:
- Black-eyed Susan (Rudbeckia spp).
- Boxwood (Buxus spp.)
- Clematis (Clematis spp.)
- Coreopsis, tickseed (Coreopsis spp.)
- Cotoneaster (Cotoneaster spp.)
- Dahlia (Dahlia spp.)
- Daylily (Hemerocallis spp.)
- Dead nettle, yellow archangel (Lamium spp.)
- Echinacea, purple coneflower (Echinacea spp.)
- Heuchera, coral bells (Heuchera spp.)
- Iris (Iris spp.)
- Lantana (Lantana spp.)
- Lemon balm (Melissa officinalis)
- Lupines (Lupinus spp.)
- Mint (Mentha spp.)
- Periwinkle (Vinca spp.)
- Pink, carnation (Dianthus spp.)
- Rose (Rosa spp.)
- Sunflowers (Helianthus spp.)
- Thyme (Thymus spp.)
If you know of other plants resistant to sunflower allelopathy, let me know and I’ll add them to the list.