Most companion-planting chart entries are tradition, and few were ever tested. Here's the short list of pairings that hold up, the famous ones that don't, and how to tell which is which.
Photo: Ezhuttukari · CC BY-SA 3.0, via Wikimedia Commons.
What you need
A healthy skepticism toward the neat companion-planting chart
The willingness to ask “by what mechanism?” of any pairing
Companion planting is the most popular garden idea with the least evidence behind it. The familiar charts (carrots love tomatoes, this plant “protects” that one) are largely tradition and hearsay copied from book to blog to book, and most of the pairings have never been tested. Some of it is still real. You have to sort the few real effects from the many folk ones, and this planner is built to do that.
Why don't you have a companion planting chart?
Start with what the charts assume, that a grid of “good neighbors” encodes real relationships:
Companion planting charts encode real relationships.
Most companion-planting advice you'll find online is folklore, and the charts have no scientific basis. A few pairings do have real mechanisms. Here's what we can actually defend, and why.
What is known
Companion-planting charts are not a record of tested relationships. The idea is old. Plants were once matched by the four elements and the signs of the zodiac, and later by pseudoscientific methods such as sensitive crystallization of plant extracts, and there is no scientific basis for any of the lists. A minority of pairings do have real, named mechanisms: mixed plantings confuse pests hunting for their host plant by sight and smell, a squash canopy works as a living mulch, and diverse plantings keep beneficial predators and parasitoids around. Those are recorded here as a good hunch. Most chart entries have nothing behind them.
The planner won’t plan around this claim. What is known instead is below.
What is known: Some plant teams rest on a real way one plant acts on another. A kitchen bundle says only that you'll want these things in the same bowl. Both are good ways to plant. Only the first has plants working on each other.
A real plant team names a mechanism you can state and check. A chart names a pairing and asks you to trust it.
Which companion pairings hold up?
A few things in this territory are supported, and they earn their place by naming a mechanism:
The Three Sisters’ structure is real. Corn is a living trellis, and the squash’s sprawl shades out weeds. Those are physical facts, and the milpa plant team is built on them.
Flowers that pull in beneficial insects help, because more pollinators and predators visit.
Perimeter trap cropping, a sacrificial border crop that draws a pest off the main one, works in specific, tested systems.
The planner carries hedged versions of the companion claims that have some support, each graded for exactly how sure we are:
Insectary plants near fruiting crops increase pollinator and predator visitation.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: The flowers supply nectar and pollen that adult predators and parasitoids feed on, so a patch of them near a fruiting crop keeps those beneficial insects fed and close by to work the crop. The effect is real but not guaranteed. An insectary patch does not always suppress pests.
Evidence: checked against 1 source in the research-based literature.
Insectary plants are those grown to attract, feed, and shelter insect parasites (parasitoids) and predators to enhance biological pest control. Insectary plants provide nectar and pollen, which the adults of many natural enemy species need to consume.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: Unresolved, and we don't pick one. Two explanations have been proposed. The herbs' volatile oils (their scent) may mask the cues pests use to find the host plant, or they may disrupt the pests' landing. The trials that report an effect measured damage, which can't separate fewer eggs laid from fewer larvae surviving or more predators arriving. So no mechanism here is established; there are only candidates.
Evidence: checked against 1 source in the research-based literature.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: The carrot rust fly finds its host by the smell of carrot foliage. Onion and other allium volatiles growing among the carrots mask that scent, so fewer flies locate the row to lay eggs. Because the protection is the smell, it lasts only while the alliums are in the ground. Once they are pulled, the masking goes with them.
Evidence: checked against 1 source in the research-based literature.
This claim only partly holds up. The planner won’t plan around this claim. What is known instead is below.
What is known: Rhizobia in legume root nodules do fix atmospheric N. Most of it is incorporated into the legume's own tissue. Some does move to a neighboring non-legume the same season via direct root contact and rhizodeposition, but the amount is small (a few percent of the neighbor's N uptake) and does not meet a heavy feeder's demand. The bulk of the benefit arrives the following season, after residues decompose.
Evidence: checked against 3 sources in the research-based literature.
Most of the nitrogen converted by the beans will not be available to the corn and pumpkins the first year; the bean roots have to break down to release nitrogen. Corn is a heavy nitrogen [feeder].
Most of a legume’s fixed nitrogen goes into the legume itself and reaches its neighbors only when its residue breaks down, largely the following year. The beans improve the ground, but they aren’t fertilizing this year’s corn.
The marigold’s reputation as a general pest-repellent is mostly folk too. Its one real effect, suppressing certain nematodes, needs it grown as a full-season cover crop and tilled in. Dotting it along a border doesn’t do that.
Are there pairings I should actively avoid?
Some of this does work. Allelopathy, one plant chemically suppressing another, is real in a few well-documented cases, and the strongest is a plant to keep your vegetables away from:
Don't plant anything sensitive to walnut toxin (juglone) within a walnut tree's root zone. Nightshades are sensitive, and so are apple and cabbage.
When a chart claims plant X protects plant Y, ask one question: by what mechanism, and has it been shown? Usually no one knows, and the pairing is tradition. The explainer why there’s no companion-planting chart here makes the fuller case. In short, grow the Three Sisters for its real, structural mutualisms, because those are what hold up.
Further reading
Good places to read more about the technique on this page. The grade on each card comes from that card’s own sources.
