The Plant Lady: What’s the reality with companion planting? It’s not rooted in science

If any aspect of gardening can be controversial, companion planting is at the top of the list. The general idea of companion planting is growing certain plants together to aid one or both — in a sense, a potential symbiotic relationship. Companion planting is a broad topic and can cover categories such as pest control, plant health and fruit quality. As a horticulturalist, I look to see what the science and data is behind the specifics of companion planting. This is a big topic, so I’m going to just touch upon some of the more common ideas.

First, let’s get the obvious question out of the way: Is companion planting a myth? Technically no. When you plant flowers to attract pollinators to vegetables, fruit trees and edibles, this is a form of companion planting. Similarly, planting taller plants near smaller ones to provide shade is also companion planting. The issue is an abundance of false information about growing certain plants together to make them sweeter, grow better and ward of pests.

Pest control ~ marigolds won’t help

I see many people planting marigolds simply because it has been ingrained in them to do so. Why? Having heard from many gardeners, they know it has something to do with pest control. The idea behind planting marigolds is to ward off nematodes, which are microscopic worms that attack tomatoes and other vegetables. If you do have nematodes, planting a few marigolds here and there will not help. Studies have shown that French marigolds (Tagetes patula) have properties that can suppress nematodes — however your entire yard would have to be planted with them to be effective.

Garlic is another plant that has some scientific foundation behind its status as a beneficial companion plant, but the data is inconsistent. It has been shown to slightly increase yields in some crops (note that in strawberries the yields are actually reduced). It contains sulfur, a fungus-fighting compound, but there is sparse data as to whether this results in acting as a fungicide. In a few studies, the volatile oils in garlic have shown to ward off pests when numbers were high — but a big question not discussed is if it also wards off beneficial insects.

Trapping plants are those that are said to attract negative insects away from your desired ones. Nasturtiums are a common one. Are nasturtiums diverting pests away from your other plants or are they attracting pests into your garden? Last year I sacrificed a crop of spinach to aphids. The kale in the bed next to it was aphid-free, so I did not bother to try to control the aphids, with the worry they would migrate to my kale. Would this happen again if both were planted next to each other? Or did it just happen to be this species of aphids, that cultivar of spinach or kale they liked/didn’t like? In this example, there are so many uncontrolled variables that it has to be rolled up as a coincidence more than a scientific fact.

Plant growth ~ Nothing is proven

The Three Sisters Garden is the most well-known combination for plant health and production, having its roots (sorry for the pun) in Native American cultures. This method consists of corn, squash and beans planted in close proximity to each other. The idea is the corn will shade the smaller plants and allow the beans to climb. Nothing wrong with that. However, we get into a bit of trouble when the beans are said to provide nitrogen to the soil. Yes, legumes are nitrogen fixers, but to get this nitrogen back into the soil the plants need to be tilled in at flowering. There goes your bean harvest.

There are some obvious don’ts as far as planting. Black walnuts produce a chemical(s) that will stunt or prevent plants from growing in close proximity. In fact, many plants contain allelopathic chemicals which prevent others from growing well nearby.

For the most part, planting certain vegetables close to each other for health or sweetness is not well-proven. I am a bit horrified to see Cornell University has published a site of plants that “go together” and “do not get along”. Their source is the book Carrots Love Tomatoes by Louise Riotte. This book is said to have started the companion planting trend. But the science is not there – more often than not it is coincidence.

One big issue with the companion planting list is seasonal compatibility. Many of the plants that are said to benefit each other are not planted at the same time in Zone 9.

The takeaway is yes, we have known certain plants can benefit surrounding plants, just as we know certain others can have a negative effect. The key is researching the science behind it before wholeheartedly buying into one of these statements. Scientists do not know the full extent of plant interactions, so in all likelihood, yes, some of these are true. However, for the most part they are likely coincidental. For more specifics on studies and a great narrative on companion planting, Robert Pavlis of gardenmyths.com does a fundamental breakdown.