Allelopathic plants: nature’s weedkillers

by Caroline Brown

Allelopathy is a process by which a plant releases chemicals that can either inhibit or benefit other plants. Since most allelopathic plants cause harm to other plants, that’s the what I’ll be discussing here.

Species competition ensures the biodiversity of ecosystems. All plants and animals have developed techniques for out-competing other species for nutrients, water, territory, and other resources. For example, certain plants have extremely dense root systems.

Allelopaths are plants that have an advanced weapon in their arsenal. The allelopathic plant competes with other species through “chemical warfare” by releasing chemicals that inhibit the growth of its competitors.

Allelopathic substances work like herbicides, preventing the germination and growth of the seedlings of competing species. Plants that are under stress, such as those with pests, diseases, or less than optimum access to nutrients, sun, or moisture, are at an even higher risk for being eliminated by allelopaths.

Depending on the plant, allelopathic substances can be released from a plant’s flowers, leaves, leaf debris and leaf mulch, stems, bark, roots, or soil surrounding the roots. Some of the chemicals biodegrade over time while others can be persistent in the soil.

Probably the most well-known allelopathic plant is the black walnut (Juglans nigra) tree. All parts of the tree–roots, bark, leaves, nuts, and even rainwater that falls off a leaf–release an allelopathic substance called juglone. Some species are affected by it and others aren’t bothered at all.

My great-aunt used to have a huge black walnut tree in her yard. I always thought it was bare underneath because we kids were always playing and horsing around underneath it! Maybe that was part of it, but the juglone must have been at work as well.

If you’re wondering what to grow near your black walnut tree, try serviceberry/juneberry/shadbush (Amelanchier sp.), tulip Shasta daisy (Leucanthemum x superbum), speedwell (Veronica sp.), or American arborvitae (Thuja occidentalis), or one of many others. More information about plants that are affected by juglone, and those that are not, is available here.

Other common trees with allelopathic properties include eucalyptus, sugar maple, tree-of-heaven, hackberry, southern waxmyrtle, American sycamore, cottonwood, black cherry, red oak, black locust, sassafrass, and American elm.

Allelopathic study is in its infancy, but early research suggests that allelopaths can be used as effective herbicides for organic weed control. For example, an allelopathic crop might be used to control weeds by planting it in rotation with other crops.

I wonder if organic herbicides based on allelopaths are commercially available, or will be soon. It’s interesting to think about the possibilities of allelopathic chemicals for the organic gardener.

Photo courtesy of Wikipedia

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67 Responses to “Allelopathic plants: nature’s weedkillers”

  1. How about Neem oil and pyrethrins? Do they qualify?

  2. I learned about the Black walnuts when I first moved to PA – pretty amazing what plants learn to do to ensure their future generations!

    I really appreciate the suggestions for things that can be grown under the Black walnuts… for now, whenever I plant a new tree, I try to give the walnuts a wide berth! ;)

  3. This is great, Caroline. I’ve been interested in companion gardening since reading “Roses Love Garlic” some years ago.

    Nature provides everything we need…we just need now to rediscover secrets like this one.

  4. Interesting question Ginger. I can’t tell for sure but it seems that neem and pyrethrins don’t qualify as allelopaths because the substances they produce kill insects not plants. But still good examples of natural ways of “pest control.” Now if they could only discover a “deer-lelopath.” ;)

    JLB, I’m reading about “plant guilds” in a book on permaculture. Plant guilds are groups of plants that work and grow together synergistically. The walnut/hackberry guild is the most interesting because of the presence of juglone. It also includes currants, chiltepine, wolfberry, and even peppers & tomatoes!

    Hi Michelle, I haven’t read Roses love Garlic yet though it’s on my (long) list. There are so many natural synergies, some tested and others untested, but there is a lot that our ancestors knew about growing that we don’t because we have come to rely too much on easy solutions like chemical preparations.

  5. This is really interesting. I don’t know much on this topic, but am wondering a few things.

    1) If allelopathic herbicides kill plants, then how will the plants you want to grow not die?

    2) Will this be the new development with genetically modified plants? Could the next generation of corn have allelopaths in the genetic makeup so that farmers will not dump a gagillion pounds of herbicides? Oh wait, that would just be efficient.

    I really need to get my gardening game on again soon. Since I’ve left the farm over the summer, I’ve forgot about this world. Hopefully, since I’ll be moving out of my house I can hopefully have a yard and some time to experiment. Thanks for the post!

  6. Hi Matt, good to “see” you again. Good questions all. 1) Researchers don’t know exactly why but there are some plants that are able to resist the allelopathic substance. It must have something to do with competition & evolution but no one knows exactly why.

    2) I think there is a Monsanto corn in the works that will have an allelopath that kills non-Monsanto corn. KIDDING! But doesn’t it seem like something that could be true? I have not come across any research on this but allelopaths do seem like something that someone would try to squeeze into a GMO species.

    By the way, I downloaded your research paper on sustainable agriculture and I’m looking forward to reading it. Thanks for making it available. And good luck with getting a new place and getting a garden going.

  7. Garden Green Garbanzo Beans never require the use of insecticides because of a natural plant acidity. Insects will not bother the plant. They are allelopathic by nature with germination inhibitors keeping the pure garbanzo lineage dating back 7,500 BC.

  8. Green Garbanzo beans are a natural allelopathic plant, never requiring the use of insecticides, having a highly acidic composition, they are one of the healthiest, safest foods on the plantet. Annually renewable and a nitrogen fixing legume plant, never depleting the soil such as grains, corn, potatoes. etc.

  9. I happened upon your interesting site and was enjoying the reading and info. I then read the entry by this doug moser character. Unfortunately I visited his site. This is arrogance and misinformation at its worst. Chickpeas or garbonzos as some call them are somewhat allelopathic but are usually attacked by armyworms which can destroy up to 25% of a crop. It’s sad that he is using this site as a way of advertising his own product especially when misleading people. He also claims that insecticides will never need to be applied to a growing crop. What about seed treatments, grass herbicides, and fungicide treatments. I know people who grow garbonzo beans and they need to apply these chemicals so that the crop can survive. doug mosers claim on his web site of his product being organically grown is an out and out lie. I appologize for the negativity but this guy is leading people to believe untrue statements.

  10. Hi Jay. Thanks for your comment. When I got those comments from Doug, I was honestly torn between taking them down because I didn’t appreciate that he was using my site as advertising, and leaving them up because of supporting a small farmer. I don’t know anything about chickpea farming so I can’t validate his claims. I will say that it is possible to organically grow chickpeas–there are several brands available–and I don’t believe that chemicals must be used for crop survival.

    But regardless, I think that I should go with my instinct about not allowing people to post advertisements on my site. That’s really not the purpose of my blog. I decided to leave Doug’s comments posted but I took down the links to his site.

  11. I want to known that, is the plants of Cucurbitaceae family Allelopathy in nature ?

  12. Hi Ahire, I have not heard or read anything that indicates Cucurbitaceae (includes gourds, cukes, pumpkins, squash, watermelons, melons, etc.) have allelopathic properties. It’s probably possible but it certainly doesn’t seem to be common in the family. I’d be curious to hear your experiences.

  13. Hey caroline, i heard that hair works to fend off deer, im not sure where this came about, but its worth a try on a small section- just scatter the hair all the way around the perimeter- hope for the best, plan for the worse, and remember your tool box when heading to the field. Soil science -painfully seeking out the amazingly obvious

    Cheers!!!!

  14. Hi! I’m doing a lab on the allelopathic affects of alfalfa on rye, clover, and radish. I was just wondering if when the allelopathic chemicals from a plant that is edible get on to another plant that is edible, will this second plant still be edible?
    Thanks.

  15. Read my book on Homoeopathy for Farm and Garden, where these issues are discussed.

  16. to be found at considera.org or narayana-verlag.de

  17. I knew about back walnut for many years. I suspected black cherry for a very long time. Now I have confirmation of what I expected. No garden vegetables will grow under their canopy, nor in the region 10 to 15 feet beyond.
    Thank you Caroline.

    Nik

  18. I am having trouble getting any type of flowers to grow under my Yoshino cherry tree is it an alleopathic tree or any suggestion. thank you

  19. Hi Carolyn

    I’m not finding much info about red oaks, every time I search I come back to the same sites about walnut. I just moved my potatoes from my regular garden, as they would appear to be incompatible with everything I wish to plant into that garden, just under the drip line of my red oak tree! Does this mean I will have to move them again???

    I found in several places that blueberries love to be under oaks, what about strawberries?

    I’m having a hard time finding a tree compatibility list to go with fruits and vegetables, my yard is almost completely shaded by the neigboring trees. I have a crabapple, two cherries and a red oak to contend with in my lawn.

    Thank you for taking time to anwser my questions, concerns.

    • blueberries would love being under oaks b/c oak leaves are super acidic and blueberries thrive in acidic soil. The acid soil could explain why some plants don’t work under the oaks too. Strawberries like acid soil, so do potatoes. What state do you live in? Try this–google search on “plant communities” and the name of your state. you might be able to find a list of plants that work well together in your area.

      Hope this helped a little! Let me know what you find.

  20. I’m in Iowa, central Iowa actually. I’ll do the plant search and see what I come up with thank you for your help.

  21. Hello Carolyn,
    I live in Victoria, Australia and have recently been affected by the Black Saturday fires in February. I want to try and protect my home and farm by planting fire resistant/retardant plants and trees. I’d like to use exotics (and some natives)that feed the soil rather than some of the natives that seem to attract fire and are allelopathic. There are natives that are legumous, that can be used for fodder as well as wind and shelter breaks, which I have collected seed to propagate, but I don’t know enough about allelopathy, and need to find out more. I want to create shelterbelts which have as many species(native and exotic) as possible, compatively. Of some of the burnt trees in front of our home, one is an old oak tree, that seemed to catch the embers and slowed the fire down, and has since regenerated, as did the peach tree, cherry plums, elm and a boobiallah. That is why I think mixing the types of plants and trees are important. Do you have advice on the above issues? Terrible wild fires are something that, it seems, we can’t stop, and other factors like water issues, artificial chemical fertilizers, pesticides, herbicides, have to have
    a big connection. It is that knowledge that I need to feel a little safer in a future fire prone area.
    Thankyou, regards, Karen.
    p.s. I did’nt know about blueberries and oaks, your site is very informative.

  22. Hi Kathy, General advice that I saw is that deciduous trees are more fire resistant than conifers due to sap in bark, needles, etc. Pine needles as mulch would not be a good idea for ex. I found some good general resources for you. But I don’t know much about native vs. exotic species in Australia so I will leave it to you to judge what is what.

    Australian resources: This is a great bibliography on fires and vegetation from the Australian Nat’l Botanic Gardens here: http://www.anbg.gov.au/bibliography/fire-plants.html

    And a very informative brochure with list of plants/trees from the ROyal Tasmanian botanical Gardens:
    http://www.fire.tas.gov.au/mysite/publications/1709%20Brochure.pdf

    Some good general advice is here, but keep in mind it’s targeted towards the US Western states: http://www.ext.colostate.edu/PUBS/natres/06303.html

    http://www.edcfiresafe.org/fire_safe_vegetation.htm

    http://smslrwma.org/nativeplanting/pdfs/NativePlantsForFirescapingBySDCNPS.pdf

    http://www.fountaingrove2.org/OSMA/Fire_Safety/Fire_Resistant_Trees___Shrubs/fire_resistant_trees___shrubs.html

    As for allelopathy specifically, you might want to check out Toby Hemenway’s book on permaculture called Gaia’s Garden.

    good luck Karen! You have a big project ahead of you. Keep me posted on how it works out.

  23. So now that I’ve read all of this, I will stop feeding black-oil sunflower seeds in the feeder right over a perennial planting plot. I’ve cleaned up as many of the hulls as I can, but it’s not possible to get all of them on a rough soil surface. How long must I wait before I plant perennials there, or do you have another suggestions to make it safe to plant perennials there?

  24. can i just ask if iris cristata can be a possible candidate as allelopathic plants?

  25. I have the same question–how long do the toxins stay in the soil? And if I amend it with clean soil, will that dilute it enough to let things grow under the birdfeeder? (switching to hulled seeds!)

  26. AWESOME!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

  27. WE RECENTLY WORKED OUT THE ALLELOPATHIC EFECT OF MANGIFERA INDICA ON COMMON BEANS .IT REDUCE THE GERMINATION PERCENTAGE AND SEEDLING GROWTH AND FRESH WEIGHT AND DRY WEIGHTS OF SEEDLINGS

  28. so, hello..i was wondering if moss could have something to do with allelopathy..i have a project in micropropagation involving moss in some indoor plant design and i was asked to look for allelopathy..and so i did, but so far, i didn’t find anything about it regarding moss..so..if you could give me some direction, i’d be thankful..:)

  29. My Yoshino Flowering Cherry tree will not flower. It goes directly to leaf. The tree was planted last year about 4-5 ft. This year the tree is about 6-7 ft.

    Help,

    Thanks

  30. Allelopaths tend to be medicinal as well. Juglone is a very effective anti parasitic and anti fungal medicine. It can be extracted fresh from the whole hulls of the black walnut in 100 % ethanol over two months or so.

    My entire yard and thus garden space is completely covered in black walnut (which is supposed to contain the most juglone). I still have managed to grow enough food to keep my household in fresh vegetables all year around. The trick seems to be thickly mulched, no till, garden beds, and soil innoculation with a mycorrhizal fungi.

    Plants with thujone tend to do remarkably well underneath them: oregano, wormwood, sage, tansy. All allium species do well also as they also have their own exudate.

    Furthermore black walnut trees are valuable. The wood of a few thirty year old trees would probably pay your mortgage if you sold them to the right buyer.

  31. Moss needs a constant and very acidic Ph. Fish emulsion, molasses, and humic acid will do wonders.

  32. Is Wisteria and allelopathic plant/vine?

    As you know the vine is very invasive and “takes over”,
    and I am having some problems establishing flowers
    growing near the vine.

    Any suggestions?
    Thanks in advance
    Bob Garrison

  33. I had never given a thought that cottonwoods might be allelopathic until, once again this spring, all of the understory redbuds and dogwoods I planted last spring failed to emerge – again. I’ve killed a few plants in my day, but this is every one every spring since I started trying to reclaim the water’s edge of my yard where the 100 ft. cottonwoods persist.

    Any ideas what I can plant as an understory?

  34. Are the wild sweetpeas (perennials) that grow in the northwest, in particular northern California, allelopaths? They have a tap root like no other, so they must me mining minerals/nutrients.

    I have a field that’s full of them where I’d like to farm veggies and wonder how they’ll do.

  35. We have recently cut down a juniper that was infecting nearby apples with cedar apple rust. Now we are wondering what garden plants (not vegetable) will grow in the soil that is left in a large circle around a stump. We are not interested in another tree, but perennials and possible a shrub.

    • I’d suggest you dig 10cm (4 inches or so) all the way out to the drip line where the tree used to be and then fill it with compost from somewhere else and use that soil to kill off some other painful annoying weeds you need to get rid of.

      we had a similar problem with the black walnut we had growing but strangle enough it used to allow clover to grow (which was very odd considering one of its pet hates is grasses / clover). But somehow it managed to survive.

  36. It is very educational.More than that it would be very useful for my science project.I’m very thankful to this information. it will also help for my education and vegetational knowledge too.

  37. m yeah thanks this is very useful to my home work..
    jah bless

  38. After many pounds of sunflower seed hulls have gone into our 25′ x 20 ‘garden, I’m worried. Are leaves and worms and kitchen-made compost enough to amend the soil ?
    Thank you for your helpful site!
    EJ

    • I’d suggest you mix your compost with some fresh manure and give it another 3 months and then plant some nutrient & mineral fixing plants amongst your current plantings to try help as a booster. do a search on google, lots of stuff like chamomile & legumes (peas/beans) family will help to bring the good up and hopefully that will be enough to void the bad toxic effects. just don’t overdo it haha

  39. hi…do shorea robusta also comes under the category of allelopathic plants? and how?

  40. Does anybody know if the hulls of black oil sunflower seeds have an allelopathic effect on sugar maples?

  41. i love your info it helped me a lot with my science project. Carolyn have come to save the science project!!!! HURRA!!!!

  42. I have a Mugo pine that is dying because I mulched it with shredded pine bark. I suspect the pine bark has a chemical that is acting as an allelopath. I am busily trying to remove the pine bark as well as pruning out the dead limbs. The pine bark has had no effect on any of the other plants in my beds. Is there any indication that this is a problem with other pines being mulched in this way? The Mugo pine was a very healthy little plant before I mulched it. It had been mulched previously with shredded hardwood bark. I’ve searched the literature and I’ve found no indication that pine substances can be selectively allelopathic to other pines but there is no other explanation for the severe reaction that’s taken place. Any other ideas?

  43. when was this published??

  44. There was mention of hair to deter deer. I was given some fleeces from sheep and am putting wool thinly on leaves, and rather more generously to protect the buds, on my little Walnut trees. I lost quite a few last winter through all the buds being eaten off. Will now have much harder shooting in oct/nov, and have gone in for plastic protection tubes. THe wool might be relegated to being put on transplants of Douglas fir in the hope that it will be unpleasant enough in the mouth to deter the deer from eating the lead shoots of Douglas transplants.

    Allopathics for Phytopthera Ramorum urgently required. Ditto Bootlace fungus.

    That Phytopthera has got into Larch, and something is needed that will kill all the regenerating Larch when the big ones are felled to stop further infection.

    I blame Monsanto for usurping God by messing about making Genetic Modifications in plants.
    Ilyan in Wales

  45. Help! I mulched my vegie beds with pecan leaves for 5 years thinking I was doing something great! Now, nothing thrives there. Is there any way I can bring the beds back or should I just start over in another location?

  46. we have a plot in front of our house and used for kitchen gardening ,we divided it in small plots and sown garlic raddish,carrot, spanich .there was a plant of Acacia Arabica. when we sown these vegetables acacia arabica was having flowers and almost there was no germination of vegetables except the Spinach ,and taking into consideration of nutrient and fertility status almost there was luxury growth of Spanich.although the shading effect of plant was not so much.

  47. I just purchased maximillian sunflower plants but didn’t know they were allelopathic. Can I plant them near arborvitae? Are they harmful to annual flowers?

  48. Reblogged this on soilsamplesplus and commented:
    Careful of allelopathy! Oak and Tree-of-Heaven leaves may hinder growth in other plants. Various opinions exist about whether the toxins break down in composting, but this is a good start.

  49. Hi,

    My neighbor just cut down a bunch of cottonwood trees and mulched the branches. I’m wondering if it’s safe to use the cottonwood mulch for my little trees in our front yard? The trees are Sitka spruces, cedars, sequoia, and other conifers, which range in height from 3 feet to 12 feet.

  50. Useful …..to foresters

  51. Does anyone know if there is any relation between pH and how allelopathic a plant is?
    I am considering doing a science project on this, but I can’t find enough information!

  52. Does anyone know if I can use my California pepper tree leaves to provide any value anywhere in the yard?

  53. how can we know that the plant is allopathic or not

  54. Can anyone tell me a outlasting effects of Eucalyptus? I have a large stand of them that are about 10 years old. I want to cut them down and replant with something smaller that will let in the light better and allow me to grow a small hedge down the side. Up until now everything I plant has died under them. So if I chop them down will I be able to use the soil? Is the chemical imbalance irreversible? Thanks.

    • Hi Teresa, I can’t say for sure but think that the effects of the Eucalyptus will be temporary. Many people suggest that the reason Eucs are allelopathic is because their root systems are very pervasive and they suck up all water and other nutrients, leaving nothing for any other plants. So if you get the roots out, or most of them, it makes sense to me you’ll be able to grow other plants there. You should check with a certified arborist and get their opinion on this.

  55. I wonder whether pine trees which shed needles on the ground are an example of allelopathic plants. I have observed that nothing grows under the evergreen plants.

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