When pests begin to eat your garden plants, it's tempting to apply an all-purpose spray to eradicate them. Unfortunately, you will also kill insects that are beneficial to your crops. Instead, use plants that are known to either repel garden pests or poison them when they eat the plants. These are known as “beneficial plants.” Some of these beneficial plants are flowers that make attractive additions to your garden.
A spiky blue annual flower, larkspur (Consolida ambigua) attracts Japanese beetles away from your desired garden plants. The foliage of the plant is poisonous to the beetles. Larkspur is also poisonous to humans and domestic pets. Larkspurs grow in full sun, and they are an excellent addition to a perennial border, or when planted at the edges of vegetable gardens.
This flower is sometimes called “marvel of Peru” (Mirabilis jalapa). It attracts Japanese beetles, who love to eat the poisonous foliage. Four O'Clocks should be used for population control. They are so-named because their flowers open at about 4 p.m. every day and remain open all night, closing shortly after dawn. A tender, bushy perennial, four o'clocks are grown as annuals in the North. The plants are prolific reseeders, and they can become invasive in warm climates.
Geraniums (Pelargonium spp.) are repulsive to Japanese beetles. They will deter insects from coming into the general area where they are growing. Geraniums are heat-loving flowers native to South Africa, where they grow as perennials. They are widely grown as annuals in temperate zones, and they flower continuously from late spring until killed by frost.
A perennial herb, tansy (Tanacetum vulgare) repels a variety of insects. Plant the herb in or near your garden, and Japanese beetles will avoid it. Bunches of the herb hanging to dry have also been shown to repel a variety of insects, and they may work on Japanese beetles as well. Tansy grows about 3 feet high and produces bunches of flat, yellow flower clusters in late summer. The flowers are suitable for drying.