Companion Planting—An Overview

What It Is and What We Know of Why It Works

Companion planting is yet another powerful tool in the garden. This powerful secret to full green-thumbed gardening is based on the simple observation that certain plants show improved health, growth, and production when planted next to one another while others show marked impairment by such proximity. It enlists plants’ natural affinities, smells, flowers, and biochemical wizardry to promote plants’ health, repel pests, attract beneficial insects, and thwart disease. Plants that have beneficial effects on each other make obvious companions in the garden, purposely planted near each other to take advantage of their mutual benefit, as do plants that benefit only one side of the pairing. Plants that have negative effects on each other, in contrast, are clearly better kept apart as antagonists or adversaries for each other’s growth, health, or production.

Why It Works

Some plants grow well or poorly next to each other for reasons we know, others for reasons we can guess, and still others do better or worse for reasons we just don’t know yet but from which we can still benefit. 

The Reasons We Know

The reasons we know or for which we have good guesses are many: root compatibility, foliage compatibility, nutrient compatibility, and pest deterrence.

Root Compatibility

First, most companion plants do well together, in part, because their roots occupy somewhat different levels in the soil. As such, they don’t compete enough with each other underground to offset the other benefits for each other above ground (e.g. shade for the soil or pest deterrence).

Foliage Compatibility

Second, while their roots surely would still need to be mostly compatible, sometimes the clearest benefit is because of the compatibility of above-ground growth. Just like with roots, for most companions, each plant occupies a different area above the ground, so as not to compete for sunlight, while still helping each other out in some way—perhaps by providing support or pest deterrence or by casting shade on leaves or soil. Sometimes this is possible only because one grows quickly, helping quickly provide a “living mulch” and being harvested before competing.

Nutirient Compatibility

Third, most companions work well together because they need different amounts of nutrients that tend to be in short supply in the soil. Plus, some produce nutrients that their companions need and can’t produce themsleves.

Pest Deterrence

Fourth, some companions work well together mostly because one repels the pests of another, or they both repel the pests of each other. Finally, some companions work well together because one adds something to the soil that the other needs. 

Examples

Corn, beans, and squash, for example, work well together if planted at the correct times and with the correct spacing. Corn, sprouting first and gaining height quickly, provides support for the beans once they later turn and reach for the heights. The beans fix nitrogen in the soil for the corn and squash, furthering their growth. The squash, with its big leaves, shades the soil, reducing moisture lost to evaporation for the all three. Plus, all occupy somewhat different areas above and below ground so don’t compete with each other too much. 

Radishes, as another example, repel two of the pests of squash (striped and spotted cucumber beetles) and do little to affect the growth of the squash if planted on the downward sloping parts of the squash hills, even if left there all season as a deterrent. By the time the radishes are sending stalks skyward to flower and go to seed, the squash leaves are so large they scarcely notice.

Radishes can also be an early-season helper among carrots, as can kohlrabi between broccoli, each providing shade for the soil for the tender young plants (and probably some pest deterrence) and being harvested before competing with the main crops. 

The Reasons We Don’t Yet Understand

For many companions, however, we don’t know for sure why they work together or don’t know enough to explain all of it.

In the past, people have turned to all sorts of possible explanations to explain why certain pairings work—including how their emulsified parts crystallize together.

Still, many are a mystery.

Is it something exuded by the roots of one or both that benefit or cause a detriment to the other or both? Is it pheromones that clash or harmonize, possibly even creating a symbiosis? Is it macro- or micro-nutrients used in the soil that, when used, create the ideal conditions for the companion? Is there something more about the roots, their growth, and placement we haven’t researched yet? Is it a subtle influence on soil microbes, an enhancement or suppression of one or another? Is it any combination of the above or others we haven’t thought of asking?

We’re not sure. But that doesn’t mean we don’t know that they do work. The truth is, for some, we just don’t know, and that’s okay. We can still pair them. Whether we know why or not, we see certain plants do markedly better or worse next to one another and can put this knowledge to work in our gardens.

Observing and Responding Is What Humans Do…and Have Always Done

We are wonderfully adapted to learning from experience and using it. We can observe that something works for hundred of years (say the foods of a culture benefiting one’s health) and decide to continue doing it based on those continuing observations. We can mimic and adapt, observe, and react. We’re great at these! Knowing all the root causes?—not so much. It doesn’t mean we’ll never find out. We can still question, and question our assumptions, and experiment and make changes.

All of that is awesome, and tremendous fun, but we don’t need to wait to know every minute root cause before we simply pair two plants that grow well together. We don’t need to know why it works to know that it works. It works. So you can use it to your great advantage!

Some easy companion groupings of common garden vegetables will help you get started. You can also use my complete chart of companions and adversaries to substitute plants in the groupings or to make your own groupings from scratch. Happy Companion Planting!