What are Allelopathic Plants?
Many of permaculture’s Fortress Plants are allelopathic, to some degree, meaning they excrete mild toxins preventing the germination and/or root growth of other plants. There is a full range of compounds. Some are quite subtle and only toxic to one or two species, like buckwheat, which, very specifically reduces amaranth seed germination. Others, like Black Walnut, are quite intense. They don’t let anything grow around them. This isn’t as bad as it sounds. In fact, with a bit of understanding, it can be quite good. Obviously, it’s never totally straight-forward. There are always mitigating combinations, always opportunities to observe and learn and improve.
What Makes a Plant Allelopathic?
Coined in 1937, the word “Allelopathy” means to cause suffering or harm to another. The word was derived from the Greek compounds, allilon + pathy, meaning “mutual harm” or “suffering.” It is a biological process whereby one organism produces biochemicals that cause a noticeable effect on the germination and/or growth and/or reproduction of other species. Generally considered detrimental, allelochemicals can actually have either beneficial or detrimental effects on surrounding plants. These are known, respectively, as positive allelopathy and negative allelopathy. Another way to think of it is as chemical competition. It plays an important role in the distribution and abundance of plants within a community.
Allelopathic chemistry is complex. There are many allelopathic compounds, and they all work in different ways. With some the influential chemical substances come from the living plant. With others, the release of toxins happens when the plant dies and/or decays.
Black Walnut
The juglone emitted by Black Walnut is famous for stunting the growth of nearly everything under and around her canopy. But Hackberries will thrive under Black Walnut. Hackberries are allelopathic, too, but in a different way. They repel the grasses and other shallow rooted species un-effected by Black Walnut’s juglone. The really interesting thing is how they compliment each other. The underground alchemy of their toxins co-mingle to create an environment hospitable to currants, wolfberry and chiltepine. Chiletpine is the wild parent of chile pepper, and a member of the Nightshade family, which is normally especially sensitive to juglone. But the Alchemy. It changes everything. If chiltepine can grow under Hackberry, then her domesticated cousins tomato, bell pepper and eggplant can too. Plants in the Nightshade family are narcissistic, meaning they love the leaf litter of their own kind. Imagine a lovely patch of Nightshades along the sunny edge of a Black Walnut canopy, backed up by the protective embrace of a couple Hackberry tree. That’s a lot of food!
But that’s not all.
Non-leguminous nitrogen fixers, like Russian Olive, Goumi and others in the Elaeagnus family are immune to juglone. And other juglone tolerant species, like Mulberry, Elderberry, Black Locusts and Acacia can be planted around the outside of Black Walnut’s canopy to act as a buffer for other, non-juglone tolerant species. Even more food. And all of it surrounding a tree that doesn’t let anything else grow there.
See? Just a little bit of understanding can create a whole new world.
Brassicas
Brassicas actually repel themselves, making them unable to keep growing in the same location. So don’t plant your broccoli, cabbage, kale, rapeseed, radish and mustard in the same place every year! But, on the other hand, some brassicas, rapeseed in particular, contain a growth-enhancing hormone called brassinolide. This is beneficial to other plants, assuming the plants are chopped and dropped after harvesting the seeds. And when tilled into the soil, mustard will suppress fungal pathogens.
You don’t have to become an expert on it in order to start, you just want to know it’s something to know about and start book-learning and observing in real life, a little bit at a time. It’s quite fascinating.
Stinging Nettles
Stinging nettles, for example, are detrimentally allelopathic to the germination and early seedling growth of wheat, barley and oat, lettuce, spinach and radish. That may sound like a lot, but there are also plenty of other places to plant those things. And Sister Stinging Nettle is so wonderful in so many ways, that it’s not much of a compromise.
A Quick & Nasty Overview
- Bearberry has multiple phytochemicals with allelopathic properties
- Buckwheat, very specifically reduces amaranth seed germination.
- Elderberry roots prevent Douglas Fir from growing.
- Fennell repels nearly all garden plants, except Dill. Fennel & Dill get along great.
- Forsythia has detrimental effects on Black Cherry Trees and Kentucky bluegrass.
- Forsythia and Goldenrod cancel each other out, so don’t plant them together.
- French Marigolds prevent root-knot nematode eggs from hatching.
- Goldenrod inhibits the germination of competing plant seeds.
- Sunflower roots generally suppress seed germination around their root zone.
- Wheat reduces broadleaf and grass weeds.
A Quick List of Allelopathic Plants
- Ailanthus (Tree-Of-Heaven)
- Aster
- Barley
- Bearberry
- Brassicas, to different degrees
- Buckwheat
- Canada Thistle
- Cedar
- Corn
- Cucumber
- Dog Fennel
- Elderberry
- Eucalyptus
- English Laurel
- Ferns
- Foxtail, Yellow and Giant
- Johnsongrass
- Juniper
- Kentucky Bluegrass
- Nutsedge
- Oats
- Oregano
- Rye
- Rosemary
- Sumac
- Tall Fescue
link to Fortress post for bearberry n elderberry https://artfulhomesteading.com/fortress-plants/
Homeschooling Experiment
Is This Tree Allelopathic?
For this experiment you will need
- glass jars
- filtered or distilled water
- radish seeds
- plain paper towels, with no color or design
- a spoon
- masking tape for labels
- plastic sandwich bags
Collect two or three leaves from a few different trees you suspect might be allelopathic. Tear the leaves into small pieces, putting the pieces from each tree into its own glass jar. Label the jar with the name of the tree. Add just enough water to cover the shredded up leaves. Follow the same procedure for each. Let the mixtures steep for 2 or 3 days, stirring them several times a day.
The Experiment Begins
Fold 5 or 10 radish seeds into a paper towel, with leaf bits and water from one jar. (Use the same number of seeds across the whole experiment, to get a balanced germination rate.) Put this in a plastic baggie. Fold it shut and label it with the tree name.
Repeat this for each jar you have.
Make up an extra baggie with just plain water, as a control.
Put them in a dark room.
Check them after 7 to 10 days.
Analyze the Seed Germination
Which ones sprouted? Which ones didn’t? Make a note of how many did/didn’t for each baggie. Compare the parts of the seedlings to each other. What, if any, differences are there between the seeds in the control baggie? Do different trees interfere with different aspects of germination? Make a table to document your results.
Make a Conclusion
Did the trees you suspected of being allelopathic have any detrimental influence on the radish seeds? And if they did, how would you describe it?
How else can you investigate this? Might you want to try it again with different plant seeds?
Are there other trees you’d like to test?
Happy Experimenting!
Have fun, learn a lot about the wonders of Life, and let me know about your results in the comments section!
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There is always something new to learn when it comes to the wonders of nature. Thank you for this list of Allelopathic plants. I have always thought that the toxins of Allelopathic plants come out after they die and decay. I never knew some are toxic in their lifetime. I see them as a way of nature balancing the growth of wide plants.
Some come out after they die and decay, and some while they’re still alive. Nature is very diverse. They are an aspect of balance, yes. And also an aspect of competition. Just like with animals, some of us are more aggressive, and in different ways, for different reasons. It’s all quite subtle, fascinating and wonderful, isn’t it?!
😀
This article on allelopathic plants highlights the fascinating world of plants and their interactions. It discusses how certain plants produce biochemicals that can affect the germination, growth, and reproduction of other species. While some allelopathic plants, like Black Walnut, are known for inhibiting the growth of surrounding plants, there are also cases where allelopathy can be beneficial. For example, Hackberries thrive under Black Walnut and create an environment suitable for other plants like currants and chiltepine. Additionally, the article mentions other allelopathic plants and their effects, such as Brassicas repelling themselves but also containing growth-enhancing hormones, and Stinging Nettles inhibiting the growth of certain crops. Understanding allelopathy can offer new possibilities for food production and gardening.
Thanks, Jason, for your close and introspective reading.
What a delightful website, I started out on this page about Allelopathic plants and explored around some more to find your content is so interesting. How did you get in to homesteading and learning about plants? I can’t seem to keep even aloe vera alive so this is very impressive.
Thank you, Shelby
I guess I’m just endlessly fascinated by life. It never ceases to amaze.
🙂