Companion Planting (CP) can be described as the growing of two or more plant species in close proximity so that some cultural benefit (pest control, higher yield etc) can be derived. Benefits can be mutual but in most instances the improvement is one way only. On the other side of the coin, some plants make bad neighbours; this is known as allelopathy. The concept of CP is not new, being reported in early Greek and Roman writings but with the development of the organic movement it has received more attention over recent years as an "eco-friendly" approach to gardening. Its application is fashionable and we now have seed merchants offering Companion Plants Seed Collections (1).
With such a long history, much of the information handed down and repeated on CP is 'folkloric' in nature and not necessarily supported by rigorous scientific assessment. Articles on the subject therefore often qualify any recommendations by such statements as "it is believed that-"etc. Recent studies in the UK have shown that some remedies work only under certain circumstances. Refs. 2,3 & 4.
There are many ways in which CP may create beneficial plant associations. These include:
Monoculture helps the pests to home-in on their targets. Mixed crops confuse them with a diverse canopy. (but see ref. 3)
Sometimes a neighbouring plant may be selected because it is more attractive to pests. E.g. nasturtiums attract black fly away from other plants such as beans. Seeding Chinese cabbage attracts aphids in preference to adjacent cabbages
Some plants exude chemicals from their roots or aerial parts which suppress or repel pests. Marigolds, for instance, release thiopene from their roots which acts as a nematode repellent and the flower scent also detracts black fly.
CP plants provide a desirable environment for beneficial insects e.g. predators such as ladybirds and hoverflies, and parasites such as flies & wasps. Examples of such plants are poached egg flower (limnanthes douglasii), phacelia, umbelliferous seed heads (carrots, parsley etc) .
Legumes (peas, beans, clover) fix atmospheric nitrogen for their own use and for the benefit of neighbouring plants via a symbiotic relationship with rhizobium bacteria.
Bearing in mind the reservations previously mentioned regarding lack of rigorous scientific proof of many of the associations, some typical examples of quoted combinations are given in the table below for a range of the more popular vegetables.
| CROP | GOOD COMPANIONS | BAD COMPANIONS |
| Asparagus | Basil, Parsley, Tomato | |
| Beans French | Borage, Carrot, Marigold | |
| Beans Runner | Marigold, Nasturtium | |
| Brussels Sprouts | Dill, Hyssup, Potato, Rosemary | Strawberry, Tomato |
| Carrot | Leek, Onion, Sage, Tomato | Dill |
| Corn | Beans, Lupin, Squash | |
| Cucumber | Beans, Lettuce, Radish, Tomato | Sage |
| Leek | Carrot, Celery | Broad Bean |
| Lettuce | Carrot, Radish, Strawberry | |
| Onion | Beetroot, Lettuce, Cabbage, Strawberry | Beans, peas |
| Peas | Carrot, Radish, Corn, Beans | Onion |
| Potato | Beans, Corn, Cabbage, Marigold | Pumpkin, tomato |
| Radish | Pea, Nasturtium, Lettuce | Hyssup |
Much of the impetus for studies on CP derived from work by the Biodynamic Movement in Continental Europe and in North America. Ref. 5 is a very comprehensive summary. Some recent American and Canadian publications on CP, available on the internet, are given in Refs. 6-11. Two very comprehensive UK publications on CP are by Bob Flowerdew (12) and Diana Anthony (13). Finally, Geof Hamilton reports no success with the carrot onion combination (14) and Gardening from Which refers to "organic myths".
Some practical steps to the use of CP in your vegetable plot include avoid monoculture and mix your planting