Book Review – A Scientific Approach to Companion Planting

Plant Partners: Science-Based Companion Planting Strategies for the Vegetable Garden , Jessica Walliser: (Storey Publishing, 2020). ISBN 978-1635861334

Book Review by Linda Dietrick

Companion planting was popularized in the 1970s by two books: Carrots Love Tomatoes and Roses Love Garlic by Louise Riotte. I once owned the first book; it was essentially a well-meaning compilation of folk wisdom, with many contradictions and no references to any science that could substantiate and explain the recommendations. In a 2015 article, Linda Chalker-Scott, one of The Garden ProfessorsTM, criticized the anthropomorphism of seeing “love” or “companionship” between vegetables, an attitude that often leads to mystical or at least unsubstantiated claims. Chalker-Scott prefers the more neutral term “plant associations” for ecological relationships that scientific studies have shown to be beneficial. Nevertheless, among the gardeners I know, “companion planting” is still the accepted term for any practice that puts two plants together in the belief that it will be good for at least one of them.

Respected garden writer Larry Hodgson concedes that Riotte’s books do contain some useful ideas, but they are mostly unrelated to her complicated do’s and don’ts. His own experience has shown that carrots grown next to tomatoes end up stunted, and that nasturtiums attract aphids instead of repelling them, as Riotte claims. He recommends using your common sense and powers of observation, which will tell you that a diverse mix of plants is better at attracting pollinators while minimizing pests and diseases, which are more likely to attack monocultures.

Jessica Walliser thoroughly agrees with this, but as she shows in her new book, we can now go beyond common sense and good garden fundamentals. In recent years, there have been many new scientific studies on plant partnering. They have mainly been in the service of agriculture, where techniques that increase yields and reduce or eliminate pesticide and fertilizer inputs offer major environmental and economic advantages. In agriculture, “companion planting” is more commonly called “polyculture” or “intercropping,” but the same principles and plant associations can be put to work in home gardens. Walliser has done the research and translated it all into ordinary language with specific recommendations that small-scale gardeners can use.

Based in Pittsburgh, Pennsylvania, Jessica Walliser is a writer and broadcaster who, along with Canadians Tara Nolan and Niki Jabbour, authors the popular blog Savvy Gardening. She holds a degree in horticulture from Penn State and is the former owner of a 25-acre organic market farm. This is her fifth book, and it belongs in every vegetable gardener’s library.

While it offers a large amount of information that will be new to many gardeners, it does so in a well-organized, easy-to-understand manner. After an opening chapter describing in general, the ways that plant partnerships work, the remaining chapters each treat a different “service” that modern companion planting can provide. Chapter 2 covers soil preparation and conditioning, showing how you can improve soil fertility by using cover crops and break up compacted soil by using “rototiller” crops like forage radish. Non-harvested cover crops like oats, buckwheat, winter rye, crimson clover, and cow peas improve soil before or after a harvested crop. The best one for beginners seems to be oats, which dies over winter and allows for vegetable starts to be planted right into the residue in the spring. In general, Walliser favours a no-till approach to preserve beneficial soil organisms, but concedes that some cover crops like winter rye may need to be turned under to stop their growth. Soil can also be fed by nitrogen-fixing companion plants, for example by fava beans with interplanted rows of sweet corn.

Chapter 3 goes into weed management: using living mulches to crowd out weeds and allelopathic plants to inhibit weed growth. For example, smaller white clovers and oats have been shown to keep down weeds around taller plants like tomatoes and peppers. Again, the oats will eventually die, while perennial clover will need to be mowed to prevent it from setting seed. A companion plant that has actual weed suppressing chemicals is cucumber, which also shades out weeds with its dense foliage. Chapter 4 talks about plants as support structures, for example Indigenous people’s traditional use of corn as a “trellis” for beans, which in turn supply nitrogen to the corn.

Chapter 5, on pest management, explains five companion strategies for pesticide-free vegetable culture: luring insects away by trap cropping, masking or hiding host plants, interfering with egg laying behaviour, impeding pest movement, and general polyculture. Here are just a few examples. If you don’t want to use row covers to protect your Brassica crops from flea beetles, Chinese mustard greens can lure them away; then you can just vacuum them off the lure plants. Potatoes have been found to be less plagued by Colorado potato beetles when closely surrounded by tansy or catmint because these seem to mask the visual and/or chemical signals the beetles follow for food. (These perennials have to be removed temporarily for potato harvesting.) Interplanting cole crops with various herbs, or tomatoes with basil, interferes with signals to cabbage butterflies or hornworm moths in search of hosts for their eggs. Marigolds provide similar signal-scrambling with respect to root maggot flies. More generally, you can inhibit the circulation of insect pests by blocking them with permanent hedgerows and by planting diversity instead of large swaths of single species.

Chapter 6 treats disease management strategies. For example, underplanting potatoes with oats was found to inhibit verticillium wilt, while mustard greens inhibited potato scab. Chapter 7 shows how you can use companion plants to support the biological control of pests, i.e. attracting beneficial insects to prey on the bad bugs. It turns out the traditional advice to fill your garden with lots of flowering plants and herbs was right. That’s because predator species like parasitic wasps, which feed on aphids and parasitize pest caterpillars, rely on flower nectar for their energy needs. Low-growing plants around your veggies can provide cover for other beneficials like ground beetles.

The final chapter is about pollination, an important service that we need our native bees to perform. Walliser stresses the importance of nectar, pollen, and habitat to these creatures. Flowers in the aster or daisy family are especially attractive to them. Many bees require hollow stems, other plant debris, or a bit of bare ground for winter nesting, and so garden cleanup should wait until after they are out and about in the spring. And of course Walliser reminds readers not to use pesticides, which kill everything, including the good bugs.

Even if you (like me) are not a dedicated vegetable gardener, you will learn lots of new things from this book both for yourself and for your work assisting other gardeners. Because it is science-based, you can use its recommendations with confidence.

References
Chalker-Scott, Linda. “The Myth of Companion Plantings.” Online at s3.wp.wsu.edu/uploads/sites/403/2015/03/companion-plantings.pdf.
Hodgson, Larry. “Companion Planting: Myth or Reality? The Laid-Back Gardener. Blog. laidbackgardener.blog/2016/06/02/companion-planting-myth-or-reality.
Riotte, Louise. Carrots Love Tomatoes (1st ed. 1975; 2nd ed. Storey Pub. 1998) and Roses Love Garlic (2nd ed. Storey Pub. 1998).