Companion planting means growing plants near each other which interact and do each other good. Marigolds (Tagetes), for example, have been grown for hundreds of years in European vegetable gardens. Their scent seems to repel insects and the root excretions inhibit nematodes. There are many other documented or experimental examples of successful relationships between growing plants, sometimes because one is shallow-rooted and another deep-rooted, or because of insect repelling scents, or exudations from leaves, and roots. This list is suggested by various organic gardeners.
It can be decorative and good sense to grow flowers, herbs and vegetables together, as they grow naturally in the wild. You will have more birds and a variety of insect activity of a useful kind, such as pollinating insects, and perhaps an increase in hover flies and predatory wasps, which are attracted to some seeding vegetables like carrots and parsnips. On the other hand, monoculture is more likely to allow a buildup of pests, and yields are often not as good.
Try planting together:
• Asparagus, tomatoes, parsley, basil, strawberries and peaches.
• Beans, carrots and cauliflower.
• Beetroot, cabbage, dwarf beans and kohlrabi.
• Broccoli, and the rest of the cabbage family, with aromatic plants such as peppermint, chamomile, rosemary, and thyme.
• Cabbage, potatoes, celery, dill, chamomile, lavender, rosemary, beetroot, marigolds and sage.
• Carrots, salsify, rosemary, leeks, onions, lettuce, tomatoes.
• Cauliflowers and celery.
• Cucumbers and sweet corn.
• Grapevine and mulberries.
• Hyssop and grapevines.
• Lettuce, onions, cucumbers, carrots, strawberries and radishes.
• Onions, chives, leeks, garlic, lettuce, beans and carrots.
• Potatoes and marigolds.
• Strawberries, spinach, a few plants of borage, dwarf beans and lettuce.
• Sweet corn, broad beans, peas, potatoes, squash, cucumbers and pumpkins.
• Tomatoes, marigolds, asparagus, parsley, cabbage and all brassicas, carrots, chives, onions and nasturtiums.
• Turnips and peas.
• Wallflowers and apple trees.
There are other plants that do better when separated.
• Beans from onions, garlic, shallots or fennel.
• Cabbage from onions, garlic and strawberries.
• Potatoes from tomatoes and herbs.
• Gladioli from peas, beans and strawberries.
• Potatoes from apples.
• Pumpkins and raspberries from potatoes.
• Tomatoes and strawberries from cauliflower.