When we speak of complementary plants, they can include plants that are used in recipes to complement other ingredients and also plants that complement each other in the garden by repelling insects or creating favorable conditions from which both herbs will benefit.
Basil is an essential ingredient in many Italian dishes, from spaghetti sauce to pesto. It complements the taste of garlic, oregano, marjoram and rosemary, so you can grow it in a section of your garden along with these herbs to make an Italian garden. Basil is a companion plant with tomatoes, peppers and asparagus because it helps to improve their flavor and growth. It is also said to repel certain insects, such as thrips, flies and mosquitoes. As with many other herbs, basil also has its enemies: it is not complementary with sage or rue, so plant basil and these herbs at opposite ends of your garden.
Both German and Roman chamomile are favorites for relaxing tea blends and their pretty little daisy-like flowers serve as hosts to beneficial insects such as non-stinging predatory wasps and hoverflies. Both types of chamomile are complementary with most plants, so you can grow it wherever you want a soft, low-growing plant with small, attractive flowers. Chamomile is said to improve the flavor of cucumbers, cabbages and onions, so don't limit it to your herb garden, but rather plant it near these vegetables.
The young, leafy portion of the coriander plant is often called cilantro, while the seeds, which develop by mid summer, are ground and used as the spice called coriander. Cilantro is a favorite ingredient of Mexican style salsa, guacamole and other South of the Border dishes. It complements other salsa ingredients, such as tomatoes, garlic, onions, hot chile peppers and avocados. As a companion plant, cilantro/coriander makes a good neighbor for anise. If you make tea from this plant's leaves and then spray it on affected plants, it can help to control an invasion of spider mites, aphids and the potato beetle.
Dill pickles contain cucumbers, dill, vinegar, water and salt, so you can grow dill to complement your cucumber plants anywhere in the garden. But keep it away from carrots, tomatoes and caraway because it is not a companion to these plants; dill might attract the tomato horn worm, so it's a good idea to plant it far from your tomato patch. Dill is said to repel spider mites, squash beetles and aphids. It attracts beneficial insects such as parasitic wasps. It's a companion with onions, lettuce, corn and cabbage, in addition to cucumbers.
Sweet marjoram is an Italian herb that complements the flavor of basil, oregano, onions, rosemary and thyme in spaghetti and other pasta sauces. It is also a companion plant to these herbs as well as to the other ingredients used in Italian dishes, such as sweet peppers, tomatoes and eggplant. It is believed that marjoram improves the flavor of these vegetables and other herbs used in recipes that contain it.