Companion Planting Has No Scientific Basis but Planting a Wide Range of Crops Works

Question: The other thing you said was that companion planting has been proven ineffective – by that do you mean that researchers have failed to find that planting certain CROPS in proximity to each other does them ANY good? Or that certain non-crop plants actually do NOT provide benefit to crop plants… Yikes – that’s quite a sacred cow to slaughter (but only a grass-fed happy cow) but you advocate what I will call “diversity planting” – maybe the whole idea of companion planting was just a ruse to get folks to diversify! Anyway – that was a shock to me, so I wanted to know more –

Thanks again so much, and a very productive Spring season to you!

Answer: Tests have been done by the University of California and other entities to see if the idea that one plant, such as basil, for example, can effect the growth or flavor of another plant growing next to it, such as tomatoes, for example and it is impossible to prove any such relationship. Despite this fact and many tests done to try to see if companion planting works, gardeners and even some farmers believe that companion planting works. There are whole books on the subject.

In my opinion, as I stated last night, the reason that many gardeners think companion planting works is that growing many different plants mixed up together has the beneficial effect of cutting down on pests and diseases. This is due to the fact that pests like to concentrate and will come in greater numbers if they find a lot of what they prefer eating concentrated in one place. To some extent the same can be said for plant diseases. They build up in soil unless crops are rotated. Plant chemistry is complicated to the extreme and all the smells and colors of plants when mixed together can muddle up the pests. Some gardeners think caterpillars may be less likely to find basil if it’s alternated with tomatoes than when it’s planted in a solid row. (I tried this once but it didn’t work for me.) Lots of gardeners think it’s good to plant garlic here and there because it smells strong and pests leave it alone. These are not scientific statements and no one could prove them scientifically, but on the other hand it is a true and accepted fact that if one plant is grown in a field exclusively with no other plants mixed in, that plant will be more subject to pests and diseases than plants grown in a mixed environment.

Some interesting experiments, also have been done by the University of California and in England, with strip farming where a cover crop or a hedgerow is grown between areas of an agricultural crops. The practice of strip farming has been shown to help cut down on disease and also on pests because beneficials lived in the cover crop and later when it was dug into the ground as green manure there was a beneficial reduction of plant diseases that can build up in the soil. In the case of hedgerows in England birds and beneficial insects, including bees, live in the hedgerows and help with pollination or with protecting agricultural crops from pest insects when these crops have not been sprayed with pesticides. Crops are not as healthy in places where hedgerows have been destroyed, so there is a strong movement in favor of keeping the ancient hedgerows, not just for beauty’s sake but for wise farming and gardening too.

Last night when I said that companion planting has been proved not to work in scientific experiments done under the auspices of the University of California, I fully expected a storm of controversy. I am glad you spoke up thus giving me a chance to provide more information. Many times unscientific things seem to work and sometimes it turns out to be for the wrong reason. As I stated last night I’ve always planted flowers in and around my vegetables, not only for beauty but to muddle up the pests and this works.