Does Companion Planting Work?

Companion planting is a method of preventing pest damage to vulnerable crops by growing plants nearby which block, distract or repel those pests. The theory is that pests find their host plants through taste and smell. Mixing different crops together, or combining them with ornamentals, makes it harder for pests to find their host plants and travel through the crop. But does it work in reality?

Gardening Which? tested the concept in 2004.and found that it does actually work in practice. The planting trials looked at three crops - cabbages, carrots and leeks. They used an organic plot and, therefore, used organic seed:

• Carrot: Starca
• Leek: Rami
• Cabbage: Advantage

The yield and levels of pests and diseases were recorded for each of the crops planted on their own and with two different companion plants.

Growing onions with carrots provided some of the best results - carrot fly damage was cut by 20% with no effect on the yield, Growing carrots with leeks led to a significant reduction in the levels of thrips and rust on the leeks. And, growing nasturtiums with cabbages produced a large increase in the numbers of beneficial insects.

But the Gardening Which? trial results did come with a word of caution - growing plants too closely together can really hamper their performance.

Some 200 Gardening Which? members also took part in a GroundBreakers trial to investigate the benefits of companion planting. In this trial:

• Tomatoes, susceptible to white fly, were grown with French marigolds.
• Roses, prone to rose aphid and blackspot, were underplanted with chives.

Results:

• Nine out of ten trialists who grew the tomato/French marigold combination, felt that they had fewer problems with whitefly compared to previous years, and 86% said the spider mite populations were significantly reduced by the companionplanting.
• Half of the 98% who said that their roses suffered from blackspot in the past reported that their roses fared better when planted with chives. 60% said chives were a good companion and that they would definitely grow them with their roses again.