If our yard and garden plants were people, we’d probably describe most of them as gentle, good-natured, pleasant, genial and even polite — except for weeds, of course.

Roses, sweet peas, fruit trees and Kentucky bluegrass all appear friendly, but did you know that some of our seemingly best-behaved trees, flowers and lawn grasses are ruthless to one another?

Their unseen animosity toward one another is far deeper than competing for sunshine, space, water and nutrition. Some members of the plant world actively claw their way to the top by injecting other plants with toxic materials. And these aren’t rare jungle oddities — this is happening right in our own backyards.

When plants release chemicals that harm or suppress other plants, it’s called allelopathy, which is a term for complex and subtle chemical warfare between plants. This ability has been realized for thousands of years, but many of the details have yet to be discovered.

Mulching around young trees helps prevent grass from releasing chemicals that slow tree growth. Michael Vosburg / Forum Photo Editor
Mulching around young trees helps prevent grass from releasing chemicals that slow tree growth. Michael Vosburg / Forum Photo Editor

WDAY logo
listen live
watch live

What we do know is that plants can release chemicals through their roots, as vapors through the foliage or as residue in the leaves that drop and linger in the soil. Chemicals released by allelopathic plants can interfere with other plants’ cell division, photosynthesis, nutrient uptake or the ability of seeds to germinate and grow.

This chemical warfare has made many invasive plant species highly aggressive in taking over territory. For example, research has shown that goldenrod, when taken outside its native range and introduced to a new area, exudes more allelopathic chemicals than normal. A Chinese study revealed that two-thirds of noxious weeds tested positive for allelopathic chemicals they exude into the surroundings.

Chemical warfare in plants is a practical issue in our yards and gardens. It not only makes weeds more aggressive, with some weeds producing chemicals that harm surrounding plants, but can also cause our landscape plants to fight amongst themselves.

The following are documented research examples of allelopathy.

  • Black walnut trees are well-known to exude the chemical juglone from their roots, leaves and fruit. Garden plants adversely affected in the vicinity are tomato, potato, cabbage, broccoli, rhubarb and peas.
  • Kentucky bluegrass, a major component of most Upper Midwest lawns, produces chemicals that can inhibit the growth of trees and shrubs.
  • Russian olive trees exude compounds that diminish growth of sycamore trees, yet have no effect on sugar maples or cottonwood.
  • Chemicals released by the roots of creeping Jenny and Canada thistle harm cabbage, tomato and carrots.
  • The grassy weed foxtail depresses poplar tree growth.
  • Goldenrod chemicals reduce growth of sugar maple.
  • Creeping red fescue and perennial ryegrass exude products that inhibit dogwood and forsythia.

ARCHIVE: Read more of Don Kinzler's Growing Together columns

Researchers have only scratched the surface of this complicated science. The plant kingdom has over 300,000 species, each with the ability to produce different chemical combinations of alkaloids, steroids, carbohydrates, phenols and other compounds that can be released into the soil and air. To completely know how each plant affects others, scientists must analyze each species, extract chemicals and then test the reaction on all other plants to see which species are affected, and which aren’t.

Because there are so many unknowns in this science, how can we protect our plants from being adversely affected from the chemicals of other plants? Here are a few basics.

  • Because many weeds exude products into the soil, eliminate weeds as best you can from gardens and landscape.
  • Mulches are our plants’ best friends. Mulches keep other plants at bay, preventing them from releasing chemicals into the soil around or above the roots of our desirable plants.
  • Because lawn grass has been found to exude chemicals that depresses tree growth, add mulch around trees using the 5-5-5 rule. Apply a layer of wood product mulch in a layer 5 inches thick in a 5-foot diameter circle, and kept 5 inches away from the trunk.

Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at kinzlerd@casscountynd.gov or call 701-241-5707.