Companion plants are those which are beneficial to the ones around them by providing protection from insect pests. Companion plants are chosen purposefully to either attract beneficial insects or to repel harmful pests or, in some cases, to do both. Beneficial insects, such as ladybugs, hoverflies, predatory wasps, and dragonflies are Mother Nature's version of pest control. When you provide appropriate hosts for these allies, they will call your garden home and help to control the harmful bugs.
The gardener's rule of thumb for these aromatic flowers is "plant them everywhere". French marigolds (Tagetes patula) are one of the most valued of companion plants. These plants have a particularly strong scent that repels whiteflies and harmful beetles. Their roots emit a compound that repels nematodes in the soil. The stronger the scent the better. Do not plant French marigolds near bean plants.
Intersperse basil freely in vegetable and flower gardens to repel flies, aphids, and mosquitoes. A companion planting stalwart, basil is a particularly good companion for tomatoes and peppers as it repels hornworms. Basil also acts as a fungicide and growth stimulator for many plants. Butterflies are attracted to their small blooms, so plan to let some of the plants produce flowers for the bees to pollinate. Do not plant near sage.
This fragrant herb attracts beneficial hoverflies, honeybees, and predatory wasps. Swallowtail butterflies rely on dill as a host plant for their pupae, so do plant extras for these valuable insects. Dill is a particularly effective companion plant for lettuce and cabbage. Do not plant near tomatoes as it attracts the tomato hornworm.
Plant petunias near tomatoes, beans, and asparagus for an attractive way to repel tomato worms, Mexican bean beetles, and aphids. Plant freely in your garden among flowers and vegetables as a deterrent for many garden pests in general. Petunia leaves can be steeped as a tea and used as a strong, all-purpose bug spray.
Plant nasturtiums as a companion for melons, cucumbers, beans, tomatoes, broccoli, cabbage, and roses. Nasturtiums both attract beneficial predatory insects and act as a trap crop for harmful aphids, cabbage worms, whiteflies, and cucumber beetles.
This common annual is useful as a trap crop; geraniums attract Japanese beetles and leafhoppers that damage roses, tomatoes, and peppers.
Asters and Zinnias
The bright-colored varieties of these summer annuals attract hummingbirds, which devour whiteflies. The pastel varieties act as a trap plant for Japanese beetles. All asters and zinnias attract bees and other insect pollinators as well as numerous beneficial insects. Plant freely throughout your garden.