There are many herbs that are thought to be companion plants. Hundreds of examples of plant companions are recorded in garden lore.
In planting a vegetable garden, you should use plants that are mutually compatible. Vegetables are divided into four groups - heavy feeders, light feeders, soil-conserving and soil-improving crops. You should plant heavy feeding vegetables in newly fertilized soil. Some examples of heavy feeding vegetables are cabbage, cauliflower, all leaf vegetables such as lettuce, spinach and celery. Also included are leeks, cucumbers, squash, sweet corn and tomatoes.
Most herbs are light feeders that like compost. Root crops like carrots, beets, radishes, turnips and rutabagas are light feeders.
The companion plants chosen must be adaptable to the same soil conditions as those preferred by the crop plants.
French marigolds (Tagetes patula) planted thickly in a vegetable garden help repel nematodes. Keep in mind unscented marigolds will not protect your plants. The cabbageworm is controlled by planting mint nearby. Keep mint in a pot because it does tend to take over. Rosemary, sage, peppermint, thyme and catnip are other helpful plants to control cabbageworms and aphids. Sweet basil (Ocimum basilicum) tends to repel aphids, mosquitoes and mites. It acts as a natural fungicide and slows the growth of milkweed bugs. Planting basil among tomato plants controls tomato hornworms. Thyme planted with cabbage controls flea beetles, cabbage maggots, white cabbage butterflies and imported cabbageworms. Catnip planted with eggplant tends to deter flea beetles. Planting onions in rows with carrots controls rust flies and some nematodes. Growing horseradish with potatoes repels the Colorado potato beetle. Nasturtiums and radishes grown with your cucumbers controls the cucumber beetle. To deter asparagus beetles, plant tomatoes, parsley or basil with your asparagus. Whiteflies and squash bugs are controlled with Nasturtiums; however, the flowers are more often used as a trap crop for aphids. Planting a ring around apple trees limits woolly aphid damage to the trees.
There is some scientific research to support companion planting. A potato plant which has been grafted onto a tobacco plant root becomes resistant to the Colorado potato beetle. Also, asparagus roots left in the soil has led to a decline in the stubby root nematode population. It has been found that Boston fern leaves contain a feeding deterrent effective against the Southern armyworm. Research has found a chemical in tomato plant leaves that is toxic to some weevil species.
Diversity of plants is the easiest and most effective pesticide and fertilizer a garden has.