Companion planting is not an exact science, but certain combinations of plants grown together seem to be an effective way of enhancing food security by reducing the chances that crops will be destroyed by diseases or pests. The smell of certain pungent plants can repel pests. Other plants attract birds and insects that eat garden pests. One plant will feed the soil with the nutrients another needs, creating balance. A few specific examples follow.
The Three Sisters, a name given by Native American tribes to the combination of beans, corn, and squash, grow well together. The corn provides structure for the beans to climb, and the creeping squash vines (carefully guided by the gardener so as not to strangle the beans before they can climb) keep pests out and help the roots stay moist.
Beans are nitrogen-fixers, meaning that they are a great fertilizer for the soil. Some plants need lots of nitrogen in the soil, such as carrots and eggplant; others, not so much – for instance tomatoes. Too much nitrogen will burn them, so planting them with beans is not recommended.
Tomatoes do, however, benefit from French marigolds. Whiteflies do not like the smell of French marigolds and will avoid them, so inter-planting them with your tomatoes is an effective way to protect against whitefly destruction of your tomato crop. It also looks nice.
Cabbage and lettuce are particularly susceptible to slugs, which are repelled by the strong oils present in mint, therefore inter-planting mint with cabbage and lettuce should limit slug damage. Likewise, strongly-scented rosemary repels those pretty little white butterflies that lay their eggs in cabbage leaves…those eggs becoming caterpillars that are capable of destroying an entire crop in record time.
Avoid planting members of the same plant family together, as they tend to attract the same pests. If you do intend to plant members of the same family in your garden, make sure they are a good distance apart, preferably at opposite ends of the bed or in an entirely different section of your garden. This will require some creativity with your 3-5 year crop rotation plan.
Any good nursery should be able to offer advice on companion planting. There are long, detailed lists of companion (and repellent) plants available on the internet and in numerous books on organic gardening…and recommendations differ from gardener to gardener. For a more scientific approach to companion planting (as opposed to anecdotal), click on the link to the book advertised to the left.