Brilliant yellow and orange marigolds planted among tomatoes create a fresh, clean appearance that enhances the overall design of your garden. Used historically for both medicinal and ceremonial purposes, marigolds originated in South America where they were bred by the Aztecs to produce large, showy blooms. Introduced to North America in the late 1700s, marigolds soon earned their place as a companion plant for tomatoes.
Although many American gardeners claim that marigolds repel a variety of insects known to be harmful to tomatoes, according to the Alabama Cooperative Extension, marigolds do not repel insects. They do, however, offer protection from nematodes. Nematodes are tiny microscopic round worms in the soil that attack the roots of vegetables, like tomatoes, causing swollen nodes that may lead to total destruction of the plant.
Many gardeners plant marigolds near tomatoes or between individual tomato plants for both ornamental and practical purposes. Marigolds bloom profusely creating a mound of deep green foliage with yellow, white or orange blooms that create a sharp contrast to green and red tomatoes, and release a pungent fragrance thought to deter insects.
Marigolds are available in a variety of sizes from dwarf varieties that reach a size of 6 to 8 inches to massive plants that tower to 4 feet or more. When choosing marigolds for interplanting with tomatoes, smaller varieties are generally desired. Large marigolds make impressive borders or at the back of tomato beds.
Many insects locate specific plants by smell. When flowers and herbs, like marigolds, are interplanted, it may confuse insects or deter them from spreading to the entire crop. Using beds of marigolds as dividers in a tomato bed may provide some protection by confusing insects.
Planting marigolds as a cover crop in an area to be used for tomatoes the following season may be effective in controlling nematodes, because the plant's roots give off chemicals toxic to nematodes in the soil. It is important to note that the release of these compounds occurs during active growth. Tilling the plants into the soil after maturity adds organic matter to the soil and provides nematode suppression in the following year. It is important to note that distributing marigold plant debris in areas, other than where it has grown, is ineffective in suppressing nematodes.