Companion Planting; Does It Work?

Companion planting is a procedure that is sometimes recommended to naturally reduce pest problems by planting two types of plants close to one another. For example, planting catnip with cabbage is supposed to reduce worm damage on the cabbage. Controlled studies are needed to determine whether such a practice is effective. We now have results from two studies that give some insight into companion planting.

The University of California looked at the effect of planting cabbage with catnip, nasturtium, marigold, summer savory and basil. The cabbage-catnip plots had reduced cabbageworm eggs and larvae but the amount of worm injury was the same. Also, the average weight per cabbage head was reduced probably due to competition from the catnip for sunlight, water and nutrients. The other cabbage companion plantings also failed to show positive results.

The University of Georgia studied companion plantings of beans-marigolds, cucumber-nasturtium, cabbage-thyme, eggplant- catnip, tomato-marigold, and tomato-basil. None of these combinations prevented insect damage from the major garden insect pests.

What about cover crops of marigolds and nematodes? Dutch researchers looked at the effectiveness of over 800 varieties of marigolds on nematode populations. Note that this is not companion planting because two crops are not interplanted. It seems that nematodes are attracted to marigold roots but are killed when they try to feed due to the release of ozone from the damaged root. But this only occurs on living marigold roots. Once the marigolds have been tilled in, there is no further benefit. Also, the full benefit is only achieved when the whole area is covered with marigolds. In the Dutch test, cover crops of marigolds reduced the numbers of the very common root-lesion nematode (Pratylenchus penetrans) enough in one growing season that other crops susceptible to that pest could be grown for two or three years without suffering. The French Marigold (Tagetes petula) proved to be the most effective, with the variety known as ‘Single Gold’ providing the greatest control (almost 99 percent).


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