Make sure your plants get along with each other

When planting a garden, one thing we should consider is how well different plants interact.

Gardeners learn, sometimes from sad experience, that one plant must not crowd out another, nor shade another, nor take most or all of the soil’s resources. You know not to put a pumpkin plant, with its huge leaves, next to something that would be overwhelmed, like beets.

Many of us are also familiar with the concept that plants have evolved resistance to certain diseases or insect predation.

Other interactions are less recognizable.

There are several gardening books with titles such as “Roses Love Garlic” or “Tomatoes Love Carrots.” These folksy names point out a scientifically proven fact: some plants produce chemicals that have genuine effects, good or ill, on other plants. The name of this phenomenon, for those of us who must know such things, is “allelopathy.”

In the case of the books I mentioned, the allelopathic effects were positive. These benefits can result from different factors. Sometimes the roots of one plant will secrete a chemical, such as a hormone, a sugar, or some different type of compound, which stimulates a neighbor to grow. Some of them can give seeds a push to germinate, and others might promote flowering. That is the background of the practice known as “companion planting.”

Not all of these interactions are positive. When people first started closely examining relationships among plants, they noticed some highly undesirable events. Most plants that have negative allelopathic effects seem to produce something that travels in the soil.

People have been growing plants for about 10,000 years. You might imagine that over the course of millennia, natural scientists would have noticed these kinds of interactions. People did notice them, and described them as early as the first century A.D. The ancient Romans knew and wrote about several examples, including the classic, black walnut. Many plants do not thrive when they are trying to grow close to this tree. All parts of black walnut secrete a chemical that interferes with other plants, such as asparagus, cabbage, tomato family members, even apples and blackberries. To complicate matters, some of these plants are allelopathic to others.

So much in biology is more complicated than we might like. Few cases are as straightforward as the black walnut.

Because of this, scientists were dubious about the concept of allelopathy for a long time, and no wonder. It is often not just a case of one root, one chemical, one effect. Despite these misgivings, allelopathy has been a hot topic in horticulture research.

These characters that affect other plants are surprisingly common. Eucalyptus has some toxicity to yarrow as well as certain members of the grass family.

Then again, some grasses are toxic to certain trees, as when researchers in Oklahoma found that Bermuda grass might limit the growth of pecan trees.

Some investigators reported that fresh needles from Aleppo pine could interfere with the growth of Bermuda grass.

Creosote is one of the more curious allelopaths. For nonrelated species, it serves as a “nurse plant,” sheltering them as they develop; yet it interferes with the germination and growth of its own seeds.

This keeps them from competing with its parent for resources.

Knapweeds are noxious and invasive weeds that are major problems in many parts of Nevada. These plants out-compete our native plants for nutrients and water.

Some of them also produce chemicals that affect root growth of our native plants, especially native grasses. I would say that was adding insult to injury.

Some evidence, on the other hand, suggests that knapweed could behave relative to its own seeds in a way similar to creosote, and inhibit the growth of its own seedlings. There is an effort underway to determine whether this knapweed compound might limit other weeds.

One driving force in studies is to find a way to use allelopathy deliberately. If a plant can keep its neighbor from growing, that might be a terrific alternative to commercial herbicides. It would be poetic if we could get the plants we like to eliminate the plants we dislike.

Not everything is certain in biology. For instance, lantana is a popular landscape plant for Southern Nevada, but a noxious invasive weed in certain parts of the world. Among the reasons why it is listed is its allelopathy to several other plants.

Some plants produce chemicals that interact with other plants. We usually only notice when they interfere with one another.

Before installing an unfamiliar plant in your landscape, it might be a good idea to do some research and learn whether it will have an effect on the species that you already have.

Angela O’Callaghan is the Social Horticulture Specialist for University of Nevada Cooperative Extension. Contact ocallaghana@unce.unr.edu or 702-257-5581.