Liz Douville

Isn’t it ironic how the old gardening concepts become new with more knowledge and the switching of outdated terms in preference to more technical scientific terminology?

The concept I am thinking about is that of companion planting. No, that’s not planting with your spouse, children, grandchildren or special friends. Companion planting was always explained as a gardening technique based on who benefits who down the garden path.

Pliny the Elder wrote that “rue is very friendly with figs,” and ever since gardeners have exchanged countless suggestions for compatible plantings to produce a more successful garden. Does it work?

The problem with using the phrase “companion planting” is that it is broadly used to describe plant interaction in the realms of science, pseudoscience and the occult, according to Linda Chalker-Scott, a Washington State University horticulturist and associate professor, and author of “The Myth of Companion Plantings.”

A Google search of the phrase turns up nearly 10,000 hits on every website imaginable. However, only slightly more than 200 sites were .edu sites. On these sites, scientific literature and studies are credible.

On the less credible sites, and in some popular books, you will find pseudoscience claims that companion plants can be determined by “sensitive crystallization” of their extracts (to discover which plants love each other) or through study of plant’s “rhythm, its vibration, its music and its note.”

According to Chalker-Scott, “When the science gets lost in the supernatural, then it’s time for academicians and professionals to consider using different terminology.”

There are several alternate phrases or terms that can be used to replace companion planting. Intercropping, polyculture, and plant associations terminology make it easier to reflect their function more accurately. These groups include plants that attract beneficial insects, plants that fix nitrogen or have the ability to draw pests away from desirable plants, and others that have the ability to secret chemicals that are toxic to other plants.

One of the misconceptions from the earliest days of companion planting is that marigolds deter garden pests. Science has proved that is correct, but not in the way gardeners traditionally thought.

Marigolds contain a chemical called thiopene in their roots and flowers.

Thiopene is toxic to certain types of soil-dwelling nematodes — worm-like microorganisms — that can plague tomatoes, melons, strawberries and other vegetables.

The marigolds would have to be planted for several years to be effective. In some cases, it is an in-ground problem, not a flying insect problem.

Members of the onion family have shown some promise in deterring pests. In one study, rows of spring onions planted between rows of broccoli were found to help protect broccoli from certain pests. In farming, sweet alyssum is interplanted with lettuces to attract predators of aphids that damage the lettuce.

The bottom line according to Chalker-Scott is that the phrase “companion plant” is too vague to be useful to plant scientists and professionals; intercropping and plant associations are more definable and credible. There is no scientific basis, for any of several lists that exist describing “traditional” companion plants.

Like horoscopes, these lists are fun to use, but they should not be perceived or promoted as scientifically valid.

Mark your calendar for Saturday, Aug. 19, 9 a.m.-noon, to attend the OSU Extension Open Garden Day at Hollinshead Water Wise Demo Garden and Community Garden in Bend. Information tables with brochures plus Master Gardeners to answer questions, insects display, tour of the gardens, and a sales table for soil thermometers and row cover.