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As an obsessive plant collector with little impulse control, I will take any excuse to cram a few more species into my tiny plot. So the concept of companion planting, an old belief that adding certain plants around existing crops can boost pest resistance, yields and even flavour, is one that has always appealed to me. But as a sceptical scientist, I’m curious about how well these claims actually stack up. Let’s dive in.

The tricky thing with examining the science around companion planting is quite how many benefits it is said to have, so I’m tackling only the most common claim here: that tomato yield and flavour is improved by being grown alongside aromatic herbs, like the basil pictured here.

I managed to dig up seven papers that set out to test this hypothesis, all of which had pretty consistent findings. The presence of aromatic herbs like marigolds, for example, seems to notably cut infestations of two of the most dreaded pests to afflict : and .

This isn’t entirely surprising. The ability of marigolds to defend themselves against predation by producing volatile compounds with pest-repellent properties, such as limonene (the molecule that gives lemon zest its scent), has been documented. The fact that trials have shown similar effects from other limonene-producing herbs, like some basil, but not from non-aromatic plants, like mustard, could be seen as further support for this idea.

What is surprising is that this apparent protective effect against pests didn’t always translate into higher yields. While some studies have found a boost in yield of up to 59 per cent when growing tomatoes with herbs, growing them next to peppermint could create a 6.7 per cent drop in yield.

Researchers attributed this outcome to the herbs competing with the tomatoes for nutrients, light and water, which offsets any reduction in pest damage. (Anyone who has ever grown peppermint will be familiar with its megalomaniacal tendency to swamp all before it.)

But what about flavour? Comparatively fewer trials have investigated this aspect, but the results from those that have are pretty disheartening. The three studies I could find reported no appreciable difference in flavour – neither in objective, quantitative like sugar or acidity levels, nor in like .

The bottom line: adding a range of fragrant herbs is indeed likely to reduce your tomatoes’ risk of succumbing to pests. And, as long as you don’t pick super vigorous ones like mint, they may also lift yields. While it doesn’t seem to result in any real flavour boost in the tomatoes themselves, you’ll also have loads of herbs to serve with them. What these companion plants may not do in your plot, they will certainly do on your plate.

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