Farmers and gardeners of long gone past were aware of the importance of keeping some form of order in their farming or gardening to keep the plants healthy, repel insects and pests with no other tool but the plants and how they are grouped together. Plants that are not friendly together are planted separately.

There were no supportive scientific data to back up their findings. They conducted their experiments, with their lands as the laboratory using the trial and error method year after year. This method of gardening helps to keep this earth healthy, minimize if not eliminate the use of chemicals for growth or pest and disease control. It keeps the soil balanced.

Plants can be good companions to one another. They provide pest and insect control to their neighbors. It can be the scent, hormones and oils coming from their roots, flowers or leaves that help to discourage disease and harmful pests without losing their beneficial assets.

Taller sun-loving plants provide shade for the shorter plants that prefer the shade.

Some plants provide extra nutrients, such as deep- rooted plants that bring out sulfur, potassium and calcium to the surface, for the benefit of the shallow-rooted ones to share.

Some plants act as fungicide, insect and pest repellants. Some plants accumulate sulfur well and this gives an odor that many pests avoid.

Flowers and leaves with strong scent are known to repel flying insects.

Secretions from the roots inhibit weeds and kill parasitic worms and nematodes.

Some gardeners provide a trap plant to absorb the disease or attract harmful pests without causing harm to itself

These plants can be used as borders, ground covers,

backdrops or interplant to keep the plants happy and healthy and in good company.

Some good neighbors and partners

Marigolds top the list. The strong scent from the flowers and foliage repel pests and the roots inhibit nematodes. Plant with vegetables and other flowering plants.

CAUTION: Marigolds are not friendly with herbaceous plants. The root secretions can inhibit the growth of the herbs. If you must plant marigolds with herbs, dont plant them too close, keep them around the edges.

Foxglove ( Digitalis) have beautiful flowers and it is known to stimulate the growth of plants near it. It makes the neigboring plants disease resistant. It improves the storage qualities of fruits and vegetables and root vegetables. probably due to gaseous secretions and minute hormones.

Insect repellants : Many flowers used as border plants repel flying insects, such as: coreopsis, coriander, cosmos, geranium, marigolds, chrysanthemum, marjoram, oregano

Most vegetables are friendly to one another.

Peas and beans make good companions for other plants because the roots fix the nitrogen supply for the other plants,

Onions and garlic and other plants from the alium family are beneficial to plants around them. These plants are known as good fungicides and insecticides. They accumulate sulfur very efficiently and the odor they emit repels many pest and other pesky four-legged critters. Plant with cabbage, tomatoes, peas, corn. Good for roses too.

Unfriendly neighbors: Avoid planting these together for they are unfriendly and definitely not good companions:

rue and basil
runner beans and potatoes
beets and beans
beans and onions or garlic
strawberries and cabbage
dill and carrots
cucumber and potato, no strong herbs
potato no cucumber, pumpkin, raspberry, squash, sunflower, tomato

Fennel prefers to be by itself.

This is just a short list.

Herbs are good companions to most plants. The strong scent repels most pests. Some herbs are also known to bring out potassium, sulfur and calcium to the soil surface for the plants around it to benefit from. Chamomile is especially favored for these qualities.

This can be a topic by itself for herb gardeners.

Reference: Garden Folklore that Works by Charlie Ryrie