If your garden is rife with aphids, a smart way to protect your plants is by attracting their predators such as ladybugs, lacewings, or one of the best, the hover or syrphid fly. Adult hover flies resemble wasps and yellow jackets, but there's no need for you to be alarmed; they can't sting and are perfectly harmless. The wasplike adults feed primarily on flower nectar, but during their week-long stint as crawling larvae, each juvenile can consume hundreds of aphids.
Researchers at Oregon State University tested many plants to determine which attract hover flies. The winner was sweet alyssum (Lobularia maritima), which is easy to grow from seed or nursery plants, and prefers the cool temperatures of spring and fall. Runners-up include cilantro (Coriandrum sativum), buckwheat (Polygonum fagopyrum), common fennel (Foeniculum vulgare), and yarrow (Achillea).
For best results, keep the plants in flower with successive plantings and regular deadheading.
Currently garden channel editor at marthastewart.com, Shila Patel is a former managing editor at the National Gardening Association.