Review by Susan Wilhelm, Extension Master Gardener
If you are looking for planting strategies with the potential to improve the productivity of your vegetable garden, or if you have heard about “companion planting” and want to know what science supports, Plant Partners by Jessica Walliser is a good resource.
Walliser defines companion planting as “planting two or more plant species in proximity for some benefit to one or both of them.” In Chapter 1, she explains that vegetable gardens are their own ecosystems where plants interact with one another in a variety of ways and that by understanding these interactions gardeners can build healthier, more productive gardens. She discusses the ways that plants interact, such as improving nutrient availability, chemical messaging, or releasing chemicals that suppress the growth of other plants (allelopathy). Next comes an overview of what the emerging science of companion planting (or the related terms of intercropping and polyculture) suggests are the possible advantages of this practice for improving soil, reducing pests and weeds, and attracting pollinators and beneficial insects.
Subsequent chapters delve into these potential benefits more deeply and identify specific plant combinations which research suggests have a positive impact—for example, planting peas and lettuce together with the peas fixing nitrogen in the soil and shading lettuce as the temperature rises, or planting zucchini next to nasturtiums resulting in significantly fewer squash bugs and less squash bug damage compared to zucchini grown by themselves on bare soil. A bibliography at the end provides citations for the studies underpinning the plant combinations. There is also a glossary of terms.
Detailed photographs accompany each plant combination, and, in the case of plant combinations for reducing pests and diseases, there are photographs of the disease damage or pest. The text and sidebars are full of practical advice. For instance, the author describes techniques to ensure cover crops do not increase certain diseases and indicates how close companion plants need to be and when to use transplants when a plant partner has allelopathic properties that can suppress seed germination.
The only chapter without a research base covers plant combinations where a tall, sturdy plant provides structural support for a vining plant, what Walliser calls a “living trellis.” Walliser includes these plant combinations as a practical technique for taking advantage of vertical space “to grow more edibles in less space” and clearly states she is making no claim of any benefit beyond structure.
Walliser cautions there is still much to know about companion planting, and, while she purposely looked for techniques that were backed by studies conducted in environments similar in size to home gardens, home gardeners may not get the same results. This is because there are so many variables, including weather, topography, and soil conditions, that can affect plant studies conducted outdoors. Therefore, she recommends gardeners trying these combinations evaluate and record their own results to determine what works best in their individual growing conditions. Walliser also mentions a companion planting potential downside and stresses the importance of selecting and managing plant combinations so a companion plant does not overtake the garden or otherwise compete for resources with the plant the gardener is trying to grow.
Plant Partners’ biggest takeaway is diversity—the greater the variety of plant species in your garden, the healthier it is likely to be.