If you read much about organic gardening, you’re likely to hear all kinds of recommendations for companion planting, or which plants should be grown next to other plants. There are many reasons that organic gardeners promote companion planting. For example, vining crops may be promoted as ground cover to suppress weeds around taller crops. Companion planting may also be used as a way to deal with insect pests and diseases. For example, aromatic plants may confuse insects, who are then unable to find their preferred host. Some plants may attract beneficial, predatory insects who feed on pest insects. Companion planting may be used so that pathogens must travel further to find a suitable host, which could slow down the spread of disease.
As I have read several recommendations for companion plants, I have often wondered how much scientific evidence there is for particular plant combinations. The theory behind companion planting seems sound to me, but until particular combinations are tested with proper scientific controls, we don’t really know if those combinations provide a real benefit. Lately, I have been reading the literature to find what evidence I can for (or against) particular companion plant choices.
I’m just going to address a few studies with tomato combinations today. One popular recommendation is that tomatoes do well when intercropped with basil, but are antagonized by being planted with brussels sprouts. But one study (1) compared these combinations with each other, and found that basil as the intercrop was no better than brussels sprouts as the intercrop. When they compared these mixtures to monocultures, they found that the mixture used land more efficiently in a drier, hotter year, but that the monocultures used land more efficiently in the more favorable year. But they do hypothesize that the plants could have been spaced more densely in the mixture, which might have resulted in more efficient land use than the monoculture.
One study (2) compared land use efficiency for tomatoes intercropped with onion, kale, or corn. They found that going by land use efficiency, tomatoes + onions was best, followed by tomatoes + kale, followed by tomatoes + corn. The least efficient land use was the tomato monoculture. The same study was also looking at how these different combinations affect a tomato insect pest called thrips.
They showed that the tomato + corn mixture had the least number of thrips, perhaps because the tall corn impeded the thrips from landing on the tomato plants. There were smaller tomatoes though, because the tomatoes were being shaded out by the corn.
There was another study (3) showing that tomato yield is greater if intercropped with rue, followed by peppermint, followed by basil. However, tomato yield was less if intercropped with fennel than if the tomatoes were monocropped. Perhaps the saying should be, “Tomatoes love rue but hate fennel” instead of “Tomatoes love basil but hate brussels sprouts.”
I also searched a little for the effects of intercropping with cucumber as the primary crop. Intercropping with onions or garlic both resulted in a greater cucumber yield than in cucumber monoculture.
What I gather from these studies is that there are real benefits to using intercropping versus monoculture. But the popularly recommended combinations may not have the evidence to support them and they may not be the best combinations to use. Hopefully more research will be done in these areas. Maybe when I have more garden space to work with, I’ll do some experiments of my own!
1. M.K. Bomford. 2009. Do tomatoes love basil but hate brussels sprouts? Competition and land-use efficiency of popularly recommended and discouraged mixtures in biointensive agriculture systems. Journal of Sustainable Agriculture 33:396-417.
2. R.C. Ramkat et al. 2008. Cropping system influences Tomato spotted wilt virus disease development, thrips population and yield of tomato (Lycopersicon esculentum). Annals of Applied Biology 153:373-380.
3. L.M. Carvalho et al. 2009. Yield of tomato in monocrop and intercropping with aromatics plants. Horticultura Brasileira 27: 458-464.