Basil: Plant with tomatoes to improve growth and flavor. Basil also does well with peppers, oregano, asparagus and petunias. Basil can be helpful in repelling thrips. It is said to repel flies and mosquitoes. Do not plant near rue or sage.
Opal Basil: An annual herb that is pretty, tasty and said to repel hornworms! Like the other basils it also does well with peppers, oregano, asparagus and petunias. Keep away from rue and sage.
Borage: Companion plant for tomatoes, squash, strawberries and most plants. Deters tomato hornworms and cabbage worms. One of the best bee and wasp attracting plants. Adds trace minerals to the soil and a good addition the compost pile. The leaves contain vitamin C and are rich in calcium, potassium and mineral salts. Borage may benefit any plant it is growing next to via increasing resistance to pests and disease. Borage and strawberries help each other and strawberry farmers always set a few plants in their beds to enhance the fruits flavor and yield. Plant near tomatoes to improve growth and disease resistance. After you have planned this annual once it will self seed. Borage flowers are edible.
Chamomile, German: Improves flavor of cabbages, cucumbers and onions. Host to hoverflies and wasps. Accumulates calcium, potassium and sulfur, later returning them to the soil. Increases oil production from herbs. Leave some flowers unpicked and German chamomile will reseed itself. Growing chamomile of any type is considered a tonic for anything you grow in the garden.
Coriander (Cilantro, Chinese Parsley etc.): The leaves of this plant are Cilantro. When left to flower and go to seed the dried tan seeds are Coriander, a familiar spice. It is a member of the carrot family. Repels harmful insects such as aphids, spider mites and potato beetle. A tea from this can be used as a spray for spider mites. Partners coriander are for anise, caraway, potatoes and dill.
Dill: Improves growth and health of cabbage. Do not plant near carrots, caraway, lavender or tomatoes. Best friend for lettuce. The flower heads of dill are one of the best nectar sources for beneficial insects in the garden attracting hoverflies, predatory wasps and many more. Repels aphids and spider mites to some degree. Also may repel the dreaded squash bug! (scatter some good size dill leaves on plants that are subject to squash bugs, like squash plants.) Dill goes well with lettuce, onions, cabbage, sweet corn and cucumbers. Dill does attract the tomato horn worm so it would be wise to plant it somewhere away from your tomato plants. Do plant dill in an appropriate spot for the swallowtail butterfly caterpillars to feed on.
Lavender: Repels fleas and moths. Prolific flowering lavender nourishes many nectar feeding and beneficial insects. Lavenders can protect nearby plants from insects such as whitefly, and lavender planted under and near fruit trees can deter codling moth. Use dried sprigs of lavender to repel moths. Start plants in winter from cuttings, setting out in spring.
Marigold: (Calendula): Given a lot of credit as a pest deterrent. Keeps soil free of bad nematodes; supposed to discourage many insects. Plant freely throughout the garden. The marigolds you choose must be a scented variety for them to work. One down side is that marigolds do attract spider mites and slugs.
-French Marigold (T. patula) has roots that exude a substance which spreads in their immediate vicinity killing nematodes. For nematode control you want to plant dense areas of them. There have been some studies done that proved this nematode killing effect lasted for several years after the plants died back. These marigolds also help to deter whiteflies when planted around tomatoes and can be used in greenhouses for the same purpose. Whiteflies hate the smell of marigolds. Do not plant French marigolds next to bean plants.
-Mexican marigold (T. minuta) is the most powerful of the insect repelling marigolds and may also overwhelm weed roots such as bind weed! It is said to repel the Mexican bean beetle and wild bunnies! Be careful it can have an herbicidal effect on some plants like beans and cabbage.
Oregano: Can be used with most crops but especially good for cabbage. Plant near broccoli, cabbage and cauliflower to repel cabbage butterfly and near cucumbers to repel cucumber beetle. Also benefits grapes.
Sage: Use as a companion plant with broccoli, cauliflower, rosemary, cabbage, and carrots to deter cabbage moths, beetles, black flea beetles and carrot flies. Do not plant near cucumbers, onions or rue. Sage repels cabbage moths and black flea beetles. Allowing sage to flower will also attract many beneficial insects and the flowers are pretty. There are some very striking varieties of sage with variegated foliage that can be used for their ornamental as well as practical qualities.
Tarragon: Plant throughout the garden, not many pests like this one. Recommended to enhance growth and flavor of vegetables.
Thyme: Deters cabbage worms. Wooly thyme makes a wonderful groundcover. You may want to use the upright form of thyme in the garden rather than the groundcover types. Thyme is easy to grow from seeds or cuttings. Older woody plants should be divided in spring.
Yarrow: Yarrow has insect repelling qualities and is an excellent natural fertilizer. A handful of yarrow leaves added to the compost pile really speeds things up. It also attracts predatory wasps and ladybugs to name just two. It may increase the essential oil content of herbs when planted among them.