Companion planting is based on the idea that certain combinations of plants affect each other in terms of growth or infestation by pests. All plants leak substances into the soil from their roots, and some release substances into the air. These chemicals may harm nearby plants, a form of competition called “allelopathy”. Black walnuts are notorious for soil contamination, although not all plants are adversely affected, and there is also evidence from studies on sagebrush that airborne chemicals can stunt the growth of some nearby species. Clover and many other legumes have root bacteria that fix nitrogen and improve the soil if tilled in — a natural way to fertilize. But will growing marigolds next to tomatoes reduce tomato worms? Will tansy repel Cabbage Whites? Does parsley make your asparagus grow better, and do beets grow badly next to pole beans? There is very little scientific evidence about this subject but some recent work is quite interesting.
Stan Finch and Rosemary Collier in the UK grew cabbages in bare soil, in soil covered in clover and in soil covered in model plants made out of green paper. They found that there was significantly less damage due to 8 different pest species in the plots covered with clover or plant models. Tests using 24 other plant species, including aromatic ones, as ground cover showed no difference from using clover or the models. It looks like the simple fact of camouflaging the cabbage plants with other green stuff is enough to deter pests. Finch and Collier suggest that the pests detect their target plant odor, fly there and then land on something green. The insects test several leaves, and if not enough of them are cabbage, they fly to another green area before laying eggs. They also seem to spend more time on the non-host plant leaves, which means less time is available to infest the host plants. So it looks like anything else green will help deter pests (a later study suggested gray-leaved plants or ones with lots of flowers that hide the leaves are less effective).
Another group of experimenters tested the effect of companion plants or bags of aromatic substances on Japanese beetle attack on roses. None of the treated bushes showed less damage than the control plants and geraniums actually increased beetle numbers, as did sachets of a number of aromatics such as fennel seeds, cedar and red pepper. Similar results occurred in a study evaluating companion planting and plant-based sprays on infestation of potatoes by Colorado potato beetles. Of the plant-based sprays, only neem spray was effective.
So, the take-home lesson: although gardening lore includes the idea that certain combinations of plants improve growth, health, and/or yield of nearby plants there isn’t a lot of scientific evidence for this. However, it’s also true that there is no reason not to try different groupings out anyway and see what happens — at the very least it can make the vegetable garden prettier! Although a ground cover plant may use additional water, it may also provide a living mulch and reduce both water loss due to evaporation from bare soil and the number of weeds so the effects may balance out somewhat. A ground cover might also be chosen to produce another crop. If you decide to experiment with some companion plantings or ground covers, consider including a control group along with the test group. Plant two sets of your test species as far apart as you can and then add the companion plants only near one set. Not a truly scientific experimental design, but it should give some information. Record your observations about growth and visits by pests and pollinators, take photos, and let us know (firstname.lastname@example.org) if something seems to work. We’ll get the information out. Some possible combinations that might affect growth or deter pests can be found at this site hosted by Cornell University: http://counties.cce.cornell.edu/chemung/agriculture/publications/companion-planting.pdf.
Demystifying Sustainability is an initiative of Sustainable Claremont (sustainableclaremont.org).