In every garden there are those plants that just don’t seem to do well in close proximity to others. No matter what you do – give extra water, fertiliser, compost – they just don’t thrive and some just down right die back despite your greatest efforts. If this is happening in your garden – you might just have an allelopathic problem causing it.
Allelopathy refers to the usually harmful effects of one plant on another plant. Pronounced AL- LEL-O-PATHY this effect can result from both crop and weed species, from the release of biochemicals, known as allelochemicals, from plant parts by leaching, root exudation, volatilization, residue decomposition, and other processes in both natural and agricultural systems. Allelopathy is characteristic of certain plants, algae, bacteria, coral, and even fungi. But how do we avoid it or even better, can we use it to our advantage?
Allelopathy occurs when one plant species releases chemical compounds, either directly or indirectly through the microbial decomposition of residues, that affect another plant species. It can have your hard work undone for you in as little as one season. As people become more and more concerned (rightly so) about the use of chemicals like glycerophosphate, the research into and use of allelopathy is becoming more popular. If you haven’t heard this phenomenon don’t feel like you are missing something – you have probably been using it in some form in your garden already without knowing the scientific term for it.
In fact. gardens the world over have been using these techniques for centuries. Long before the term allelopathy was used, people observed the negative effects that one plant could have on another. Theophrastus, who lived around 300 BC noticed the inhibitory effects of pigweed, a type of Amaranth, on alfalfa better known as Lucerne. In China around the first century CE, the author of Shennong Ben Cao Jing, a book on agriculture and medicinal plants, described 267 plants that had pesticidal abilities, including those with allelopathic effects. In 1832, the Swiss botanist De Candolle suggested that crop plant exudates were responsible for an agriculture problem called soil sickness.
You will have heard of companion planting – another way of saying this or that plant grow really well together for example strawberries and borage. We also know that we don’t plant garlic and with our beans or onions. This is partly due to allelopathy of particular plant groupings. We also now that crop rotation with certain plants particularly annual vegetables is important for reducing disease and pest but did you know that some of those vegetables also leave residual allelopathic chemicals in the soil preventing your next crop from growing as it should? Some commonly cited effects of allelopathy include reduced seed germination and seedling growth.
Like synthetic herbicides, there is no common mode of action or physiological target site for all allelochemicals. However, known sites of action for some allelochemicals include cell division, pollen germination, nutrient uptake, photosynthesis, and specific enzyme function. For example, one study that examined the effect of an allelochemical known in velvet bean, indicated that the inhibition by this compound is due to adverse effects on amino acid metabolism and iron concentration equilibrium. Black walnut (Juglans nigra) and butternut pumpkins (Juglans cinerea) are the landscape plants most recognized by gardeners as being problems for their other plants.
Garlic mustard is an invasive plant species in North American temperate forests. Its success may be partly due to its excretion of an unidentified allelochemical that interferes with mutualisms between native tree roots and their mycorrhizal fungi.
Common Allelopathic Plants
Wheat
If you’ve mulched with straw or planted a wheat cover crop, you know the power of allelopathy in your garden. Wheat is useful for reducing the number of broadleaf and grass weeds in a plot as it grows.
Also, the dried plant matter (otherwise known as straw) may release some general allelopathic chemicals early on in decomposition. Using wheat in between garden rows to control summer weeds can be more effective than most other mulches in hot weather.
Marigolds
The roots of some types of marigolds, namely French-type marigold (Tagetes patula), release a chemical in the soil that prevents root-knot nematode eggs from hatching. A lot of gardeners will plant marigolds intermittently throughout their gardens as a general preventative to help control root-knot populations.
In addition to this some marigolds will prevent the incursion of kikuyu grass into garden beds as well. They also repel unwanted pests such as ants, tomato worm, cabbage moth and caterpillars, fruit fly, and white fly. A most handy plant and pretty as well.
Sunflower, Walnut, and Sorghum
Some allelopathic plants, such as sunflower, walnut, and sorghum, are able to suppress the growth of a long list of other plants with their chemical powers. These three plants release allelopathic chemicals through their root systems and while their plant parts decay.
Walnut leaves are an excellent mulch if you want to suppress most weeds for a while. They work particularly well on nightshade family plants.
Similarly, sunflower stalks and roots make fantastic weed mat for many months until they fully decay.
Buckwheat
Some allelopathic plants are especially good at suppressing specific plants rather than all sorts of plants. Buckwheat, for example, is great for reducing amaranth seed germination.
Learned by chance a few years ago because of a beetle problem in mid-summer scorching heat, buckwheat was the only thing that grews well in those conditions. So I grew it as an interim cover crop. Incredibly, almost no more amaranth seeds sprouted that year or the next!
After that experience, I learned that researchers have even been exploring ways to use the allelopathic benefits of buckwheat to reduce populations of herbicide-resistant pigweed in farm fields. Pigweed is a distant cousin of grain amaranth.
Brassicas
All brassicas, such as cabbage, mustard, kale, rapeseed, radish, and more have some allelopathic properties. Mustard, for example, has the power to suppress many fungal pathogens in the soil if tilled into the soil.
Some kinds of radish have a particular knack for suppressing johnsongrass. Extracts of black mustard can limit the germination of some legumes like alfalfa, lentils, and oats (which have some legume-like properties).
Broccoli can be allelopathic to later-planted broccoli or any other crops in the brassica family. So, it’s a good idea to rotate your broccoli bed every time you plant. Also, do not plant other brassicas where broccoli has recently been grown.
On the other hand, some brassicas may also contain a growth-enhancing hormone called brassinolide. Rapeseed plants, in particular, have been found to have high levels of this. This hormone may have a beneficial effect on other plants if the crop residues are left in the ground after harvesting the seeds.
This is not quite the same as other examples of allelopathy. But if rapeseed residues end up benefiting faster-growing plants and slower growing plants lose to those steroidal driven, aggressive competitors, then even growth hormones can be understood to have allelopathic outcomes.
Now that you know some examples, and may have already used some of them in your garden, let’s look a little further a field!
There are many scientific studies on plant allelopathy. However, the plants studied tend to be limited to those with industrial agricultural significance such as wheat, corn, sorghum, sunflowers, etc. With respect to weed suppression, Putman et al. (1983) found that compared to unplanted control treatments, residues of several fall-planted cereal and grass cover crops significantly reduced growth and dry matter production by several weed species during the following summer.
So, unfortunately, there’s no exhaustive list of allelopathic plants we can refer to find out which plants might harm other plants or how to use them. Much of what we know about allelopathy comes through observation and trial and experience. More and more research is coming out each year about the uses of allelopathic plants both in the garden and in commercial agriculture. Scientists are constantly looking at how mother nature can help us in the quest to grow more and to build better ecosystems.
As alluded to above the companion planting chart available below. Gives us an insight into more allelopathic plants. It is those plants that don’t grow well together for one reason or another. Below are some further plants that are known to be allelopathic but it is not exhaustive. Remember our priniciple of observation – so grab your journal and head outside in your garden. Record what you see. Is the tomato plant suffering? What’s growing nearby? Could there be an allelopathic issue you haven’t thought of?
Some other Allelopathic Plants to be aware of.
Some are weeds that might be causing allelopathic issues in your garden. Others are plants you may include in your landscape on purpose.
Allelopathic Plant List
| Ailanthus (Tree-Of-Heaven) | Aster |
| Barley | Brassicas (All to varying degrees) |
| Bearberry | Blackberry |
| Buckwheat | Thistles |
| Cedar | Corn (specifically corn gluten) |
| Cucumber | Fennel |
| Elderberry | Eucalyptus |
| English Laurel | Camphor Laurel |
| Ferns | Foxtail (Yellow and Giant) |
| Forsythia | French-type Marigold |
| Garlic mustard weed | Goldenrod |
| Johnsongrass | Juniper |
| Kentucky Bluegrass | Kikuyu |
| Nutsedge | Oats |
| Oregano | Rye |
| Rosemary | Quackgrass |
| Ragweed | Rhododendron |
| Sorghum | Sugar Maple |
| Sumac | Sunflower |
| Tall Fescue | Walnut |
| Wheat |
If you plan to be planting any of these items in your garden, do a quick online search of your plant name plus the word allelopathic to get more details on the current research. It is also wise to do a search both online and in our companion guide to see if there are any specific details of what not to plant nearby or what to plant to assist in growth. Some of the plants listed above also emit a substance called Juglone- Walnuts particularly. So below is a list of plants that will tolerate or work as buffers between Juglone emitters and other plants.
Plant Tolerance to Juglone
The following lists of plants tolerant to juglone were compiled from published sources. They are based on observation under various settings, but few plants have been experimentally tested for sensitivity to juglone. Many factors affect sensitivity, including level of contact, health of the plant, soil environment, and the overall site conditions. The lists provided here are strictly guides and cannot be considered complete or definitive.
Trees, Shrubs and, Vegetables Tolerant to Juglone
| Trees Tolerant to Juglone | Shrubs Tolerant to Juglone |
| Eastern Red Cedar (Juniperus virginiana) | Barberry (Berberis spp) |
| Most maples except silver maple (Acer spp) | Hazelnut (Corylus americana) |
| Ohio Buckeye (Aesculus glabra) | Daphne (Daphne spp) |
| Goldenrain Tree (Koelreuteria paniculata) | Forsythia (Forsythia spp) |
| Serviceberry, Shadblow (Amelanchier) | Witchhazel (Hamamellis spp) |
| Sweetgum (Liquidambar styraciflua) | Rose of Sharon (Hibiscus syriacus) |
| Pawpaw (Asimina triloba) | Snowball Hydrangea (Hydrangea arborescens) |
| Yellow Poplar (Liriodendron tulipifera) | St. Johnswort (Hypericum prolificum) |
| River Birch (Betula nigra) | American Holly (Ilex opaca) |
| Black Gum (Nyssa sylvatica) | Juniper (Juniperus spp) |
| Hickory (Carya spp) | Spicebush (Lindera benzoin) |
| Virginia Pine (Pinus virginiana) | Mockorange (Philadelphus spp) |
| Catalpa (Catalpa bignoniodes) | Exbury Hybrid Azalea “Gibraltar” & “Balzac” |
| Sycamore (Platanus occidentalis) | Pinxterbloom Azalea (Rhododendron periclymenoides) |
| Eastern Redbud (Cercis canadensis) | Sumac (Rhus copallina) |
| Black Cherry (Prunus serotina) | Smooth Sumac (Rhus glabra) |
| Fringetree (Chionanthus spp.) | Current (Ribes spp) |
| Callery Pear (Pyrus calleryana) | Black Raspberry (Rubus occidentalis) |
| Flowering Dogwood (Cornus florida) & Dogwood (Cornus alternifolia) | Elderberry (Sambucus Canadensis) |
| Oak species (Quercus spp) | Maple-leaved Viburnum (Viburnum acerifolia) |
| Staghorn Sumac (Rhus typhina) | Koreanspice Viburnum (Viburnum carlesii) |
| Hawthorne (Crataegus spp) | Vegetables Tolerant to Juglone |
| Black Locust (Robinia pseudoacacia) | Onion |
| Persimmon (Diosypros virginiana) | Beets |
| Sassafras (Sassafras albidum) | Squash and Melons |
| American Beech (Fagus grandifolia) | Carrot |
| Arborvitae (Thuja occidentalis) | Parsnips |
| White Ash (Fraxinus americana) | Beans |
| Canada Hemlock (Tsuga Canadensis) | Corn |
| Honeylocust (Gleditsia triacanthos) | Fruit Trees Tolerant to Juglone |
| American Elm (Ulmus americana) | Cherry, Nectarine, Plum, Peach (Prunus spp) |
| Carolina Silverbell (Halesia caroliniana) | Quince (Cydonia oblongata) |
| Blackhaw Viburnum (Viburnum prunifolium) |
Vines and Flowers Tolerant to Juglone
| Vines Tolerant to Juglone | Herbaceous Flowers Tolerant to Juglone |
| Clematis (Clematis spp) | Yarrow (Achillea spp) |
| Virginia Creeper (Parthenocissus quinquefolia) | Hosta (Hosta spp) |
| Wild Grape (Vitis) | Bugleweed (Ajuga reptans) |
| Wisteria (Wisteria spp) | Spanish Bluebell (Hyacinthoides hispanicus) |
| Herbaceous Flowers Tolerant to Juglone | Hollyhock (Alcea rosea) |
| Shasta Daisy (Leucanthemum x superbum) | St. John’s Wort (Hypericum spp) |
| Aster (Aster spp) | Anemone (Anemone spp) |
| Liriope or Lilyturf (Lirope spp) | Morning Glory (Ipomoea spp) |
| Astilbe (Astilbe spp) | Jack-in-the-Pulpit (Arisaema triphyllum) |
| Lobelia (Lobelia spp) | Iris (Iris spp) |
| Fibrous and Tuberous Begonia (Begonia) | European Wild Ginger (Asarum europaeum) |
| Virginia Bluebell (Mertensia pulmonariodes) | Pot Marigold (Calendula officinalis) |
| Bee Balm (Monarda spp) | Bellflower (Campanula latifolia) |
| Grape Hyacinth (Muscari botryoides) | Glory of the Snow (Chionodoxa lucilae) |
| Daffodil (Narcissus spp) | Chrsyanthemum (Chrysanthemum spp) |
| Primrose and Sundrops (Oenothera spp) | Spring Beauty (Claytonia virginica) |
| Cinnamon Fern (Osmunda cinnamomea) | Crocus (Crocus spp) |
| Phlox (Phlox paniculata) | Dutchman’s Breeches (Dicentra cucullaria) |
| Mayapple (Podophyllum peltatum) | Bleeding Heart (Dicentra spp) |
| Jacob’s Ladder (Polemonium reptans) | Jacob’s Ladder (Polemonium reptans) |
| Solomon’s Seal (Polygonatum commutatum) | Crested Wood Fern (Dryopteris cristata) |
| Christmas Fern (Polystichum acrostichoides) | Purple Coneflower (Echinacea purpurea) |
| Primrose (Primula spp) | Epimedium (Epimedium spp) |
| Lungwort (Pulmonaria spp) | Winter Aconite (Eranthis hyemalis) |
| Bloodroot (Sanguinaria canadensis) | Dog’s Tooth Violet (Erythronium spp) |
| Siberian or Blue Squill (Scilla siberica) | Snowdrop (Galanthus nivalis) |
| Stonecrop (Sedum acre, Sedum spectabile) | Sweet Woodruff (Galium odoratum) |
| Lamb’s Ear (Stachys byzantia) | Gentian (Gentian spp) |
| Meadowrue (Thalictrum spp) | Cranesbill Geranium (Geranium sanguineum) |
| Spiderwort (Tradescantia virginiana) | Sunflower and Jerusalem Artichoke (Helianthus spp) |
| Trillium (Trillium spp) | Christmas Rose (Helleborus spp) |
| Globeflower (Trollius spp) | Common Daylily (Hemerocallis spp) |
| Tulips (Tulipa spp) | Coral Bells (Heuchera spp) |
| Pansy and Violet (Viola spp) | Orange Hawkweed (Hieracium auranticum) |
| Zinnia (Zinnia spp) |
It is worth pointing out that before planning to use any of the above species you should check with your local authority for weed listings and obviously know your climate. Not all the species listed can be planted in all areas.
Tips on Using and Preventing Allelopathy in the Garden
Now that you have some good background on what allelopathy is; how you might already be using it in the garden; and know some plants to look out for; let’s get to some helpful tips.
1: Be Careful with Crop Residues
I’m a permaculture gardener. So, I use a lot of cover crops and leave a lot of crop residues on my garden beds as green manure or mulch. However, I’ve learned to be careful with crop residues.
As a habit, I remove all the plant parts from sunflower, sorghum, corn, and cucumbers from my garden beds. I either compost them or use them as mulch under some of my fruit trees to suppress weed growth around the root zone. I don’t leave them in beds though because they tend to take a while to decompose and can leach allelopathic chemicals into the ground until they are fully composted.
2: Control the Allelopathic Weeds
Weeds in general can be used in a variety of ways. See this article for why I love weeds. Weeds with allelopathic properties can deprive your preferred plants of nutrients and make it difficult to germinate seeds. It’s best to completely remove and burn those weeds to make sure they don’t negatively impact your yields.
3: Know Your Cover Crops
Many cover crops are beneficial specifically because they have allelopathic properties. Mustard, tillage radish, wheat, buckwheat, and rye are good examples.
– Wheat and Rye
Wheat and rye are good general weed suppressants and make good winter cover crops. However, because of their allelopathic properties, it’s a good idea to remove or till them in at least a few weeks before you plan to plant new crops in those beds.
– Tillage Radish
Tillage radish, which makes a great soil breaker for heavy clay soils, is often espoused to be an allelopathic weed preventative. However, in studies, no allelopathic weed suppression has been found. Instead, the dense canopy from the fast-growing radish simply out-competed other plant growth.
Research does suggest that concentrated extracts of tillage radish might possibly delay lettuce seed germination. So, as a precaution, I don’t plant tillage radish directly before lettuce in my crop rotations.
– Mustard and Buckwheat
I’ve already detailed the biofumigant benefit of mustard and the amaranth seed germination reduction benefit from buckwheat. But new research on the allelopathy of cover crops is emerging fairly often. So, it’s worth doing research each year to stay current on allelopathic uses for these cover crops.
4: Use Crop Rotation
There are a lot of great reasons to use crop rotation, which makes it good general practice. However, allelopathic chemicals can build up in the soil from season to season. That, in turn, can lead to a general decline in garden health.
For example, planting sunflowers in the same bed for a year or two might be fine. But, after enough allelopathic chemicals build up, even sunflowers may not be able to germinate in that location for a few years.
Tip 5: Maintain Living Soil
If you are in the habit of applying compost and supporting the biological life in your soil, the long-term risks of allelopathic plant damage to your garden are naturally minimized.
Although allelopathic chemicals can linger for a while even after plant residues are removed, they will eventually decompose and become inactive in the soil. By maintaining living soil, and adding compost annually, decomposition of allelopathic residues tends to happen a lot faster.
Also, by top-dressing beds with a few inches of compost, you create a barrier between seeds and allelopathic chemicals lingering in the soil below. Seeds can germinate. Then, a few weeks later, when roots reach the soil level, those allelopathic compounds will have lost some or all of their power through speedy decomposition.
Conclusion to Allelopathic Plants
There is a lot we don’t know and more to research about allelopathic plants. But hopefully, this primer will help you use the benefits effectively and avoid the pitfalls that come from inadvertently planting allelopathic plants in the wrong places.
