Have you ever wondered if companion planting really works? In case you are unfamiliar with the concept, companion planting is a procedure that consists of planting two types of plants close to one another with the belief that it can naturally reduce pest problems. One example is planting catnip with cabbage to reduce worm damage on the cabbage. University professors wondered also, and there are now controlled studies that have been conducted that provide “scientific” answers to this question.
The University of California looked at the effect of combining cabbage with several supposedly pest-reducing plantings that included catnip, nasturtium, marigold, summer savory and basil. The results were that the cabbage-catnip plots did have reduced cabbageworm eggs and larvae, but the amount of worm injury was the same. None of the other cabbage/companion plantings showed positive results either.
The University of Georgia has also conducted similar studies. They grew companion plantings of beans-marigolds, cucumber-nasturtium, cabbage-thyme, eggplant- catnip, tomato-marigold, and tomato-basil. In these tests also, none of these combinations prevented insect damage from the major garden insect pests.
Despite these rather disappointing results from an “Old Farmers Almanac” standpoint, there has been some research that has shown that marigolds can be effective in controlling nematode populations.
Dutch researchers did cover crop studies looking at the effectiveness of over 800 varieties of marigolds on nematode populations. The scientists found that apparently nematodes are attracted to marigold roots but are killed when they try to feed due to the release of ozone from the damaged root. There are two caveats. One is that the effectiveness of killing nematodes is only with living marigold roots, once the marigolds have been tilled in, there is no further benefit. The second is that these were not companion plantings because two crops were not interplanted.
The conclusion of these Dutch studies is that when an entire area has been covered with marigolds, cover crops 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 from nematode damage. The French Marigold (Tagetes patula) was the most effective, with the variety ‘Single Gold’ providing the greatest benefit with almost 99 percent control.