Photograph (1) © Copyright Jonathan Billinger and licensed for reuse under this Creative Commons Licence.

Dear Readers, I have always loved marigolds, with their cheerful faces,  tolerance of nearly every kind of soil and weather condition and habit of popping up unexpectedly all over the garden. But I have always wondered if the story about companion planting was true. This week in New Scientist, botanist James Wong took a look at the research, and very interesting it was too.

Planting marigolds next to tomatoes definitely did have an impact on nematodes and whitefly: Wong points out that this isn’t surprising, as the volatile chemicals in marigolds (notably one called limonine, which gives lemon balm and citrus fruits their scent) help to protect the plants themselves from pests. Most of the experiments used French marigolds,

Further experiments showed that plants which contained these volatile chemicals, such as basil, reduced whitefly infestation, while other plants, such as mustard, did not. In one whitefly experiment, whitefly infestation was reduced by almost 69%, so well worth trying, I’d have thought.

However, companion planting did not necessarily increase yields – it very much dependent on which ‘companions’ were chosen. When mint was used, tomato cropping actually dropped – this is probably due to mint’s vigorous nature, meaning that it competed with the tomatoes for nutrition, water and even light. I’m very fond of mint, but it can be a little devil, so choose your ‘friends’ carefully!

And finally, Wong found that the scientific studies showed no improvement in the taste of tomatoes at all – not in taste-tests, and not in objective criteria like sweetness or acidity either. So the main reason for companion planting is its demonstrable ability to reduce infestation by pests like whitefly. And, if I was growing tomatoes, that would be good enough for me!

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