The idea that some plants exert a beneficial effect on others growing nearby and other plants exert a malign influence is an ancient one. Many plants do defend themselves against insects by being poisonous to them or developing a strong scent that frightens them away, and it is possible that a plant growing close by might benefit from being in this bug-free zone.

For example, French marigolds (Tagetes patula) secrete an enzyme or a hormone into the soil that deters nematodes from infesting their roots, and it does seem that tomatoes or other nematode susceptible plants growing next door will be protected also. It may be significant that most of these beneficent plants are strongly aromatic.

On the other hand very few plants will grow under a walnut tree, and it's not just that the walnut casts deep shade: its fallen leaves decay to make substances that kill off the walnut's competitors. And it is very difficult to grow grass under most eucalypts. Here the culprit is the lerp insect, which feeds on the gum leaves. Its droppings fall to the ground and poison the grass. Kill off the lerp, and the grass grows green again.

However, in many cases there appears to be no firm evidence for the claims and when you trace them back you find they originate to medieval books where botany and astrology are marvellously entangled. Every plant was then believed to be under the governing influence of one of the planets, and it often turns out that the supposed sympathy or antipathy is based on nothing more than the compatibility of horoscopes.

The same celestial influences were also believed to indicate the medicinal 'virtues' of every plant, and although some medieval herbs are still used in medicine, many more have been shown to have no curative powers.

Roses, for instance, were held to cure almost anything you could think of but modern medicine denies them any curative powers at all. Bay trees wereparticular favourite of Apollo, and it was believed by the Greeks and Romans and for long after-that if you had one no harm could come to your house. Presumably it protected the garden too, but today its garden-protecting powers seem to have declined.

However, it can be great fun to try out the old ideas and, if nothing else, it can give you a happy feeling of kinship with the many generations of gardeners before you.

GOOD companions - Try them and see what happens.

• Basil with tomatoes, asparagus, beans, grapes, apricots and fuchsias
  Beans with potatoes and sweet corn
  Borage with strawberries
  Chives with carrots, cucumbers and tomatoes
  Citrus with guavas
  Cucumbers with potatoes
  Garlic with roses, apples, apricots and peaches
  Geraniums with grapes
  Grapes with mulberries
  Horseradish with almost any fruit tree
  Hyssop with cabbages and grapes
  Irises with roses
  Leeks with celery
  Lettuce with carrots, onions and strawberries
  Marigolds (French) with tomatoes, roses, potatoes, daffodils and beans
  Melons with sweet corn Mignonette with roses
  Mint with cabbages and other brassicas, and peas
  Nasturtiums with cucumbers, zucchini, squash and apple trees
  Onions with carrots, kohlrabi and turnips
  Parsley with roses, asparagus and tomatoes
  Peas with carrots
  Sage with cabbages
  Sunflowers with squash and sweet corn
  Thyme with any Brassica
  Wallflowers with apples

BAD companions - Some of the unhelpful combinations:

• Apples with potatoes
  Beans with garlic
  Cabbages with strawberries
  Gladioli with strawberries, beans and peas
  Hyacinths with carnations
  Mint with parsley
  Sunflowers with any vegetable but squash
  Wormwood with just about everything