Gardeners have planted companion flowers with vegetables to deter harmful insects for ages, and with good reason. Many flowers will kill or deter vegetable-feeding insects that may otherwise ravage your vegetable garden and ruin your crop. Some companion flowers have an added bonus of actually attracting beneficial bugs like bees and parasitic insects that will help to pollinate your plants and also feed on the harmful garden bugs.
Marigolds are the most popular companion flower for vegetable gardens, deterring many different kinds of harmful insects. They also kill damaging species of nematodes in the soil. Be sure to plant a fragrant marigold variety for the flowers to be effective. Beware that marigolds will attract slugs and spider mites, and you shouldn't plant French marigolds (Tagetes patula) or Mexican marigolds (T. minuta) beside beans or cabbage.
Plant nasturtiums around your vegetable garden to deter aphids, squash bugs, striped pumpkin beetles, whiteflies, cucumber beetles and other insects. Nasturtiums both trap harmful insects and attract beneficial predatory insects. These flowers are especially helpful in protecting insect infestations of radishes, cabbage family vegetables, cucurbits or squash family vegetables, tomatoes and fruit trees.
There's a reason why you see sunflowers planted beside corn, and it's not because they're tall and pretty. Sunflowers kill aphids while attracting hummingbirds, which feed on whiteflies.
Rich in trace minerals, borages boost the soil nutrients and help to build up nearby plants' resistance to pests and diseases. Borages are especially helpful in deterring cabbage worms and tomato hornworms. Plant borage flowers beside strawberries, tomatoes and squash.
Chrysanthemums, particularly the C. coccineum or "painted daisy" species, kill harmful root nematodes in the soil. Mums with white flowers deter Japanese beetles and extracts from Chrysanthemums are often used in making organic pesticides. Costmary, which is part of the mum family, repels moths.
Dahlias and Four O'Clocks
Dahlias planted beside vegetables help to rid the soil of harmful nematodes. Four o'clocks attract Japanese beetles to feed on the foliage, which poisons and kills the insects. The biggest drawback to planting four O'clocks is that they're also poisonous to humans and animals.
Geraniums planted around corn, tomatoes, cabbage and peppers will deter Japanese beetles and cabbage worms, as well as keep beet leafhoppers off the vegetables. Geraniums are also effective insect repellents when planted around roses and grapes.
Lavender doesn't just smell lovely, but it also repels moths, whiteflies and fleas while attracting beneficial nectar-feeding and pollinating insects. Lavender also repels codling moths when planted beside fruit trees.
Like marigolds, petunias repel a wide variety of harmful garden pests, including aphids, tomato hornworms, Mexican bean beetles, leafhoppers and asparagus beetles. You can also steep the petunia's leaves to make a tea and use it as an insect spray for your vegetable plants.