Annuals Gardening

When Sunflowers Kill!

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.

20170508B Torindkfit, WC.jpg
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

56 comments on “When Sunflowers Kill!

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  4. Marjory

    Add potatoes to the list of plants they dont kill. I have struggled to grow potatoes because of the gophers in my feild. One year volunteer sunflowers came up in my potato row, so i just let them grow. That year i had amazing potatoes, and i think because the sunflowes sent a message to the burrowing critters to get lost. Now i grow sunflowers with my potatoes!

    • Very possible! Thanks for the info!

      • Joan Chantler

        Wow- nothing like personal experience. I just read they don’t get along, so I’m glad you shared this. Please keep us posted.

      • Irises, bachelor buttons, asparagus, carrots, green onions, lilies, 4 oclocks, zinnias, Marigolds, a few others I have a YouTube garden channel I grow alot of sunflowers every kind among other things.

      • My Lamberfeeties on YouTube. I do live in the outback of Oregon very rural area silver lake to be exact. No silver no lake just good people. Very dry high desert very hot sandy soil. Lots of sage and tumble weeds so I guess add sage to that list ?

    • Debra Dunaway

      What kind of potatoes? Yams ,sweet, red skinned , etc.?

  5. Anteginni Leblanc

    I’m interested to hear your thoughts on 4 sisters gardening, which adds sunflowers to the traditional 3 sisters guild for corn pole beans and squash. From what I have read, it is planted in the center of a corn mound for its deep roots. It is said to be beneficial considering corn has a shallow root system. Last year I successfully grew squash and zucchini with sunflowers, though it didn’t trellis like I was hoping. I wonder if it will truly inhibit the growth of my corn and beans?

    • I’m no longer able to answer individual questions. However, you can use the search tool on the blog page to see if a past post has answered your question.
      Good gardening!
      Larry Hodgson

  6. Shel grows sunflowers

    Question?… Will planting sunflowers hurt people especially little people? I planted some by a children’s playground, because they love picking them when going on walks!

  7. This year in my gardens I planted 4 heirloom tomato plants and beside 3 of them I planted sunflowers. When the tomatoes were about 5 foot and looked very healthy, one by one within about a week or two, three of them started to wilt quickly and I know it was not a water issue. The only one to make it was the one with out a sunflower planted beside it. BTW the survivor is loaded with tomatoes now =) * Note: the sunflowers were planted approximately 2 + feet from the tomatoes.

    • You’ll never find an answer to this kind of question. Sometimes in the gardening world, things go wrong and there is just no clear explanation. I’d try planting tomatoes elsewhere, though, in case there’s a soil disease.

      • Oh yeah I know that lol, have been at this for over 50 years. Always got to try though and another clue is that the 4 tomatoes were planted in the 4 corners of that garden bed (approx 20′ X 30′). Funny after all the years of gardening I never even heard of this and absolutely not curious enough to try it again! Happy growing =)

  8. Linda Stiber

    Spiderwort is also thriving under my bird feeders and have done so for the last 3 years, and as you stated the daylilies are doing well as are the Dutch Iris and bee balm.

  9. I have a sunflower question. I have a sunflower seedling that somehow was broken off. I put the stem in water by my kitchen window and it rooted at the very bottom of the stem. Should I plant it with just the roots under the soil or can I cover part of the stem above the roots as well, since it is a spindly stem? I am concerned that burying the stem might cause it to rot off from watering the soil.

    • I suggest planting it deeper; after all, if it can’t stand up on its own, it won’t thrive. If you cover only 2 inches or so, the roots should be near enough to the air above for oxygenation (lack of oxygen to the roots is the real threat, the stem won’t rot).

  10. I have Lupin on my property that I would like to eradicate. Any suggestions? I used Round-up for years and my field was looking good, then it came back with a vengeance. What will kill it besides digging it up?

    • Digging won’t stop it (and besides, what a job!), as it’s probably mostly coming back from seed. Can you mow this area? Frequent mowing (as per a lawn) will kill the sprouting seedlings and seriously hinder the existing plants, eventually leading to their death. (Lupines are not long-living). Other than that, you don’t have much choice but to use a herbicide repeatedly.

  11. I came across your blog while searching the internet about sunflower kernels(that we use in our bird feeder) and whether they kill grass. All of a sudden I have a major infestation in my grass causing a lot of it to die. I can tell it’s traveling toward the other side of the yard. I’m pretty sure it’s caused by army worms, as I have seen 2-3 in my house in the past week (I’m guessing they were on my dog). How in the world would I suddenly get a major infestation of army worms? It made me wonder – the only thing I’ve done differently is buy a bird feeder and put sunflower kernels in it. The birds absolutely love it but a lot of the kernels fall. Could that have attracted the worms? Or could we have voles and/or moles?!? Ugh! Any advice is appreciated.

    • I see no link between armyworms, that are pretty much specific to cereals and other grasses, and sunflowers. And since the mother moths lays her eggs on grass leaves, they wouldn’t have been carried on the sunflower kernels. I don’t know where you live, but the fall generation of the moth has to fly south to mild climates to overwinter. It won’t survive frost. Thus, the whole population is wiped out in the fall in cold climates (areas where the ground freezes). There is therefore no reason to believe they will show up in your lawn the future.

  12. Kathleen Quarzenski

    I have grown sunflowers for a few years in the same garden as my rhubarb. Last year I noticed the rhubarb faltering. Who kills rhubarb? So this year I reset the rhubarb after amending the soil and did not plant any rhubarb but did plant potatoes, tomatoes, onions, cabbage, green peppers, broccoli, brussel sprouts and baby bear pumpkins. The only crop to flourish was the red cabbage. So I figured “what the heck”. I started to fertilize regularly and that seemed to help the rhubarb, broccoli, brussel sprouts and baby bear pumpkin. All the other crops are a bust! How can potatoes not flourish? The garden is in full sun with an underground soaker hose system.
    Any suggestions as to what I should amend the soil with this fall? I was thinking lime and fertilizer.
    Worst garden I have ever had. I definitely think the sunflowers were toxic.

    • Don’t add lime unless you know the soil is too acid. Fertilizer, yes, but just not growing sunflowers should bring it out of its slump.

  13. 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.

      • Pungh0Li0

        I would love to know. Out of several tomato plants only one was looking terrible… and it was the one with a couple sunflowers growing with it. I’ve pulled them so hopefully it helps.

  14. 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.

      • Interesting. My husband planted some Armenian cucumbers in the same large pot as a couple of the single bloom large-flower sunflowers this year. The cucumbers did not do well. We got a few, but most of them turned yellow and died and the plants themselves failed to thrive. We certainly learned a lesson!

  15. 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!

    • Even 1 foot would be enough. The effect is very localized.

      • 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!

      • Best of luck!

      • Hi, does this apply to climbing and runner beans too?

      • You’ve have to try it and see. Both near and well away so you can really tell the difference.

  16. 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.

  17. 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.

    • Not a problem. Their allelopathy is mostly going to affect other annuals and fast-growing plants. Well-established plants won’t have a problem.

  18. Lindsay Howell

    Do they kill catle ????

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

  20. 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.

    • JoanChantler

      Hello – you asked about other sunflower resistant plants Here’s a list I’ve complied from several sources – some are the same as yours. Which Plants Will Grow Under a Bird Feeder?
      Plants that seem to be unaffected by the allelopathic properties in sunflower seeds include:
      • Fairy roses (they have thorns that discourage four-footed predators)
      • Day lilies
      • dianthus
      • liriope
      • creeping rosemary
      • purple coneflowers
      • tickseed coreopsis
      • mint (also a rodent deterrent)
      • cotoneaster
      • lemon balm
      • heuchera (coral bells)
      • iris
      • lupines
      • Mother-of-Thyme
      • dahlias
      • lantana
      • climbing roses can be trained around the bird feeder pole
      • black-eyed Susan
      • clematis
      • vinca
      Remember, if anything seems to be misshapen or stunted, it’s the toxin that’s doing it. If the plant isn’t thriving, try another plant on the list.
      Before You Plant Anything…
      • Clean up old bird seed and hulls from under bird feeders to prevent the growth of mold and disease-carrying organisms.
      • Start with a good soil mix that includes organic compost. Geranium “Roxanne’
      Geranium ‘Tiny Monster’
      Lupinus ‘Blue Shades’
      Iris pallida variegata
      Heuchera ‘Plum Pudding’
      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

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