Print edition: 
Planting certain combinations of plants together is sometimes touted as a natural way to repel insects. For example, worms are supposed to turn up their nose at catnip, so planting catnip alongside cabbage is supposed to protect the cabbage from worms. But studies are showing that it doesn't work, Ward Upham of K-State says.
The University of California looked at planting cabbage with catnip, nasturtium, marigold, summer savory and basil, Upham said. The cabbage-catnip plots had reduced cabbageworm eggs and larvae, but the amount of worm injury was the same, he says. Also, the average weight per cabbage head was reduced, probably because of competition from the catnip for sunlight, water and nutrients. "The other cabbage companion plantings also failed to show positive results," Upham says.
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," Upham says.
A twist on companion planting involves planting a cover crop that is supposed to repel insects in the food crop that will be planted afterward. Upham says that Dutch researchers looked at the effectiveness of more than 800 varieties of marigolds on nematodes.
"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," Upham says. "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)."
