When Sunflowers Kill!

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Pretty sunflowers can be fatal to neighboring plants.

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.

Bird Feeders

The most obvious place where sunflower toxicity is visible is under bird feeders.

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Cardinal at bird feeder. Photo: torindkfit, Wikimedia Commons

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:

  1. Black-eyed Susan (Rudbeckia spp).
  2. Boxwood (Buxus spp.)
  3. Clematis (Clematis spp.)
  4. Coreopsis, tickseed (Coreopsis spp.)
  5. Cotoneaster (Cotoneaster spp.)
  6. Dahlia (Dahlia spp.)
  7. Daylily (Hemerocallis spp.)
  8. Dead nettle, yellow archangel (Lamium spp.)
  9. Echinacea, purple coneflower (Echinacea spp.)
  10. Heuchera, coral bells (Heuchera spp.)
  11. Iris (Iris spp.)
  12. Lantana (Lantana spp.)
  13. Lemon balm (Melissa officinalis)
  14. Lupines (Lupinus spp.)
  15. Mint (Mentha spp.)
  16. Periwinkle (Vinca spp.)
  17. Pink, carnation (Dianthus spp.)
  18. Rose (Rosa spp.)
  19. Sunflowers (Helianthus spp.)
  20. Thyme (Thymus spp.)

If you know of other plants resistant to sunflower allelopathy, let me know and I’ll add them to the list.20170508A

25 thoughts on “When Sunflowers Kill!

  1. Anne Janssen

    Hello laid back gardener. Would you know of any vegetables that will grow after a year of sunflowers? What do you think about always planting sunflowers in the same garden and/or perhaps inter sowing with a clover or vetch to avoid depleting the soul? I’d love your thoughts and advice. Thank you.

    • Ideally, you’d clean up all sunflower residues in the fall, from roots to seed cases. By spring, most of the allelopathy effect should be gone. I know of no vegetable that is specifically resistant, but then, this is a recent discovery and it will take time to test plants. You could keep sunflowers on their own (and fertilizing too would be a way of enriching the soil) or not plant them too densely.

      • Thank you so much laidback gardener. You certainly aren’t laidback in replying to all these questions you get. I have now been sidetracked by so many of your excellent posts! Thanks for sharing your knowledge like this.

  2. Gayle

    Hi…I notice that a lot of people recommend planting sunflowers with cucumbers…?? do you have any ideas with that. Thanks.

  3. Spencer James

    Hello! I stumbled across your blog when doing some research. I started growing mammoth sunflowers and since they grow so tall, I was thinking of planting them with my full grown palm trees (roughly 15-25 feet tall). Do you know by chance if these sunflowers can kill full grown trees? I’d rather not risk them falling on houses.

  4. Bonnie Punt

    The tulips we planted near where a sunflower seed bird feeder had been moved to did not come up this year (a few with stunted growth appeared around the edge). Last spring they were beautiful and we added to them last fall. When digging in the soil, there was not a trade of the bulbs left. Could this have been due to the bulls from the black oil sunflower seeds? We have no squirrels in our area.

    • Sunflower seed hulls are allelopathic: toxic to many other plants. It’s quite possible they’ve caused the tulip bulbs to fail to regrow. Also, if you cut the tulip leaves back before they turned yellow, that could easily be a reason. Without green leaves to capture the sun’s energy, bulbs can’t form.

  5. Amy

    How far away from sunflowers do you have to plant veggies in order for them to survive? I just planted sun flowers next to my corn!

      • Katina

        I learned this about sunflowers the hard way. I planted snap peas near sunflowers two years in a row. The first year I got some snap peas but not as many as I should have and the ones near the sunflowers turned yellow. I thought at the time that it was just the planting placement since this was a more shaded part of my garden. The next year I planted some in the same area with no sunflowers and then planted some in a tire with a mammoth sunflower. The peas With the sunflower grew for awhile and then turned yellow and died. This year I got smart. Lol. My sunflowers will all be in their own pots. I did grow a couple with my cucumbers but they are in their own pots buried 3/4 so that the soil is not mixed with the cucumbers soil. Here’s to hoping this works out. I love my sunflowers despite the trouble they cause! I also have a pot on my porch I use for growing sunflowers and zinnias together, so far I have had good luck with that combination. Here’s to hoping it stays that way!

  6. Chuckers

    You have to be careful will all the gardening related info you read on the internet. Most of it is blown out of proportion and even “studies” are very limited in how they are conducted to arrive at a conclusion. I grow sunflowers everywhere, even in my vegetable gardens, but I have never noticed any problems with any close-by, neighboring plants, except for the one time I grew a row if single flower type of sunflowers. But even then, it’s hard to say if the sunflowers stunted the growth and health of those surrounding plants as there were other factors that could have contributed to their poor health and poor harvest. However, I have since stopped growing single-flower sunflowers as I prefer to have the type that that put out multiple flowers all season long because I don’t grow sunflowers for food, rather to attract wildlife, and for their beauty. So it could be that only certain types of sunflowers are toxic enough to other vegetation. And it is my belief (not a fact, just a feeling) that the sunflowers you have to watch out for are the types that are grown for sunflower oil production. True or not, the point of my post is that you have to try things for yourself and see what the outcome is. You can’t rely on information, even information from universities.

    • Your comments might well be right… and of course, different conditions give different results. I had long noticed that that there was a lot of die-back from many plants near my sunflowers, but had no idea why until a botanist friend told me about this. But then, possibly the allelopathic particles cling more to the soil under my conditions.

  7. Amy

    I am an avid composter and this year I planted sunflower garden and each side of my atrium. Unbeknownst to me a tomato plant (most likely from seeds mixed in the compost) started growing right next to one of the sunflowers and is thriving. I left it there because I read your blog and said oh it will just die! But that has not been the case I am even getting tomatoes on it! So perhaps tomatoes are OK to plant with sunflowers? What are your thoughts?

    • Allelopathy has different levels of effects on different plants. Weedy plants (and what is a tomato but a weed with improvements?) often tend to be able to thrive in allelopathic situations.

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