Allelopathy… Bullies in the Garden?

al·le·lop·a·thy: Chemical warfare by plants. A process by which a plant releases chemicals that inhibit the growth of other plants.

Unlike animals, plants are unable to run, hide, stand their ground or bare their teeth in order to out-compete their neighbors for food, water, nutrients and space. Instead, like siblings who learn to duke it out under their parents’ radar, plants have more subtle ways of keeping other plants at arm’s length. One of these ways is through allelopathy, a process by which plants release chemicals that inhibit the growth of other plants if they get too close. It’s as simple as that — chemical warfare in the plant world!
Since many flowers, vegetables, herbs, grains and weeds give off allelopathic toxins, it helps the gardener to know which plants grow well next to each other and which ones act more like two sisters in the back seat of a car. To learn more about companion plants (which thrive next to each other) and antagonistic plants (which are hostile to one another) in the garden, see the classic book Carrots Love Tomatoes or just check out the many links online.
Possibly the strongest allelopathic plant in WNC, the black walnut (Juglans nigra), is not found in the vegetable garden, but that doesn’t keep it from stunting the growth of your crops. Juglone is the chemical that all parts of any species of walnut exudes. I have personally grown crops within 30 – 40 feet of mature black walnuts only to witness juglone’s stair-step effects. Plants closest to the walnuts were stunted while those farther away grew normally.  If you have black walnut trees within range of your garden, there are still many plants that tolerate juglone’s effects; among the edible thrivers are black raspberry, crabapple, corn, mint, wild grape, paw paw, persimmon, beets and carrots. What doesn’t do well near walnuts? Blueberries, sour cherries, potatoes, tomatoes, cabbage, asparagus and apple.
Our goal as gardeners is to look at the plants in our care and ask, “What can I do to help these plants flourish?” Like us, plants do not live in isolation, so we begin by noticing the world they inhabit – the water, soil, air, microorganisms, insects, birds, weather and the other plants. Then we ask ourselves how each of these makes it harder or easier for our plants to thrive. Understanding allelopathy is simply one more tool in our toolbox.

Vegetables that DO NOT
get along well near each other:

Beans, bush: onions and fennel
Beans, pole: onions, beets, kohlrabi and sunflower
Cabbage: strawberries, tomatoes, pole beans and dill
Carrots: dill
Corn: tomatoes
Cucumbers: potatoes, sage and aromatic herbs
Onion family: peas and beans
Peas: onions, garlic, leek, shallots & gladiolus
Peppers: kohlrabi and fennel
Potatoes: pumpkins, squash, tomatoes,cucumbers & sunflowers
Pumpkins: potatoes
Radishes: cabbage, cauliflower, Brussels sprouts, broccoli, kohlrabi, turnips & hyssop
Squash: potatoes
Tomatoes: cabbage, cauliflower, fennel and potatoes
Turnips: potatoes

While some vegetables appear to have quite a few potential bad companions, other popular staples have none. Among them are asparagus, celery, lettuce and parsley.