Roses and chives: Gardeners have been planting garlic with roses for eons, because garlic is said to repel rose pests. Garlic chives probably are just as repellant, and their small purple or white flowers in late spring and looks great with rose flowers and foliage.
Tomatoes and cabbage: Tomatoes are repellent to diamond back moth larvae, which are caterpillars that chew large holes in cabbage leaves.
Cucumbers and nasturtiums: The nasturtium’s vining stems make them a great companion, rambling among the cucumbers and squash, suggests Sally Jean Cunningham, master gardener and author of Great Garden Companions. Nasturtiums "are reputed to repel cucumber beetles, but I depend on them more as habitat for predatory insects," such as spiders and ground beetles.
Cabbage and dill: "Dill is a great companion for cabbage family plants, such as broccoli and brussels sprouts," Cunningham says. "The cabbages support the floppy dill," while the dill attracts the tiny beneficial wasps that control imported cabbageworms and other cabbage pests.
Corn and beans: The beans attract beneficial insects that prey on corn pests such as leafhoppers, fall armyworms and leaf beetles. And bean vinesclimb up the corn stalks.
Lettuce and tall flowers: Nicotiana (flowering tobacco) and cleome (spider flower) give lettuce the light shade it grows best in.
Radishes and spinach: Radishes attract leafminers away from the spinach. The damage the leafminers do to radish leaves doesn't prevent the radishes from growing nicely underground.
Potatoes and sweet alyssum: The sweet alyssum has tiny flowers that attract delicate beneficial insects, such as predatory wasps. Plant sweet alyssum alongside bushy crops like potatoes, or let it spread to form a living ground cover under arching plants like broccoli. Bonus: The alyssum's sweet fragrance will scent your garden all summer.
Cauliflower and dwarf zinnias: The nectar from the dwarf zinnias lures ladybugs and other predators that help protect cauliflower.
Collards and catnip: Studies have found that planting catnip alongside collards reduces flea-beetle damage on the collards.
Strawberries and love-in-a-mist: Tall, blue-flowered love-in-a-mist (Nigella damascena) looks wonderful planted in the center of a wide row of strawberries.