Aromatic marigolds can serve to repel insects when planted with vegetables in need of protection from bugs and worms that dislike the flowers. Certain kinds of marigolds (Tagetes) discourage microscopically small, root-eating roundworms called nematodes and predatory insects that alight on vegetables. Marigolds also deter insects when used in homemade bug spray. But sometimes the flowers need help warding off their own attackers.
Pretty But Repelling Species
French and African marigolds are often cited as the most powerful Tagetes species for repelling insects. Both contain chemical compounds called thiopenes in their seeds, roots and other plant parts that are offensive to nematodes, aphids, cabbage maggots, white flies and other pests. French marigolds (T. patula) and African marigolds (T. erecta) are also pretty additions to the garden. The flower heads of the French varieties come in single or multiple whorls and in solid or variegated shades of yellow, orange and burgundy. African (sometimes referred to as "American") species are taller than French marigolds and have larger pompom-shaped flowers in solid, sunny colors.
Marigolds can be companion planted amid vegetables to benefit them by discouraging nematodes and predatory insects. One technique involves interplanting rows of vegetables, such as tomatoes, with rows of marigolds. The Arizona Cooperative Extension and Cornell University say that preplanting an area heavily with marigolds is a powerful method for eliminating nematodes. Under the technique, a cover crop of marigolds is planted then most of the seed heads are collected at season's end to avoid an abundance of volunteer plants the following year. The plants are plowed into the soil then vegetables are planted in that location the next season. One suggestion is to split the garden in half the first year, planting marigolds on one side and nematode-resistant vegetables in the other half. Switch sides the following year, planting whatever vegetables are desired in the former marigold area.
Marigold Insecticidal Spray
When added to an insecticidal soap spray, French marigolds discourage insects that feast on vegetable leaves. This includes the huge hornworms that bedevil tomato foliage. The Herb Gardener website offers a recipe combining marigold flowers, leaves and stems with cilantro, distilled water and dish soap. However, unlike pot marigold (Calendula officinalis) — which, similar to Tagetes species, is a member of the aster family — true marigolds aren't edible. They are toxic to humans as well as bugs, so sprays containing their compounds should not be applied to parts of plants that people will eat.
Marigolds and Red Spider Mites
Although marigolds are hardy flowers that are seldom bothered by pests, red spider mites can attack during hot, dry weather. Spider mites are about the size of a pinprick. They make marigold leaves turn a silvery green. The Florida Gardener website says mites are too small to see without a magnifying glass. To identify if mites are present, it suggests holding a piece of white paper under plant leaves and shaking the leaves. Any tiny red dots that move are mites. Mist plants daily in hot, dry weather to avoid the problem. If symptoms are present, hose the plants heavily with water every other day.