Companion Planting Chart

Quick answer: a handful of pairings have been tested and held up — a trap crop of arugula or mustard for flea beetles on the cabbage family, Blue Hubbard squash to pull squash bugs and cucumber beetles off the rest of the squash, nasturtiums and marigolds against squash bugs and cabbage worms, basil and marigolds against thrips on tomatoes, and flowering buckwheat or cowpeas beside sweet corn. Several favourites have been tested and did not hold up, like marigolds against Colorado potato beetles. Most of the rest of any companion chart, including the “keep these apart” column, is tradition that nobody has trialled. The tool shows which is which for the plants in your bed, and who says so.

How to use it

Tap every plant that is in the bed, or that you are thinking of putting there. Each pair a source names appears with its claim, a badge for what stands behind it, and the source’s own words one line down, with a link. Under that, “worth adding” lists the companions a study backs for what you already grow — tap one and it goes in the bed. The link at the bottom keeps your bed, so you can send it to whoever you garden with or open it again in spring. Below the tool is the whole chart: every pair the six sources name, all at once.

What the badges mean

• Research: it helped — a study tested this pair against a pest and it made a difference. The source says which pest, and whether it was in a field or only in a greenhouse.
• Research disagrees — one source reports a study that worked, another says the research does not bear it out. Both are shown. Basil with tomatoes is the famous one.
• Research: mixed — there are results, and they point both ways. The Three Sisters sits here.
• Tested: no clear benefit — it has been tried and did not do what the charts say.
• A spacing trick — not a pest claim at all: an early crop before a late one, or a deep root next to a shallow one, so two crops share a bed without fighting over it.
• Tradition — no trial cited and Tradition says keep apart — the pair is on a published chart, with no study behind it that the source names. That does not make it wrong. It means nobody has shown it is right.

Where the claims come from

Six sources, all public universities, each quoted word for word: University of Minnesota Extension (a 2026 review of the pairings with research behind them), Washington State University Extension (Linda Chalker-Scott’s 2023 publication on the science and the myths), University of Illinois Extension, West Virginia University Extension (its “Plant Friend or Foe?” chart, the traditional kind), University of Tennessee Extension (trap crops and intercropping) and University of Florida IFAS (marigolds and nematodes). We did not add a single pair of our own, and a pair none of them names is not in the tool.

Washington State’s verdict on the popular charts is blunt: “most lack any credible, supporting science”. Minnesota puts it more gently — the charts online “are not always accurate or backed by research” — and then lists the pairings that are. This tool is those two points of view set side by side, pair by pair.

The Three Sisters

Corn, beans and squash, grown together for hundreds of years by Indigenous farmers from Mesoamerica to the Iroquois. Minnesota describes how they are meant to help each other: the corn is a pole for the beans, the beans fix nitrogen, and the squash puts raccoons off the corn; Washington State adds that the squash vines carpet the soil and hold down the weeds. Washington State went looking for the field trials and found few. Those it found report no difference in the soil or in the plants’ nutrients, lower yields of beans and squash in the mix, and more total food from the same ground when it is measured as a land equivalent ratio. Its conclusion: there may be historic and cultural reasons to plant them, but “no compelling, published evidence” that it improves yields or the soil. Plant them for the history, the look or the space, not because a chart promises a bigger harvest.

Marigolds, the most over-promised flower in the garden

Marigolds are on every chart, and the research on them goes both ways. Minnesota’s sources found marigolds cut thrips on tomatoes, squash bugs and cucumber beetles on squash, and cabbage worms on cabbage. But multiple studies found they do nothing for Colorado potato beetles, and Minnesota says there is little research behind marigolds against flea beetles. Washington State adds a catch: French marigolds put off beneficial insects as well as pests. And for root-knot nematodes, Florida IFAS says marigolds planted among the vegetables probably do nothing. Grown on the spot as a cover crop for at least two months before the vegetables go in, they can help.

Trap crops: plants grown to be eaten

A trap crop works the opposite way to what most people expect from a companion. It does not repel anything; it is more attractive than your crop, so the pests go there first. Arugula and mustard for flea beetles, Blue Hubbard squash for squash bugs, vine borers and cucumber beetles. Illinois Extension says to plant it before the crop it protects, and then deal with the pests that collect on it. Otherwise it becomes a nursery for them. Tennessee puts a trap crop at typically 10 to 20 percent of the main crop, and Minnesota found that three or more trap species together work better than one.

What the tool leaves out, and why

West Virginia’s chart also says beans go with “most vegetables and herbs”, and that cucumbers should be kept away from “aromatic herbs”. Neither names a plant, so neither is turned into a pair here. A few plants on that chart are not in the tool at all (gladiolus, chard, horseradish, raspberries, borage, summer savory). And the chart contradicts itself once. It lists dill as a companion of the cabbage family and, in the next column, as a plant to keep away from it. The tool shows both, because that is what the chart says.

Spacing is not here either: how far apart and how many per square foot is a different question from which plants belong together. For the soil the bed needs, the raised bed soil calculator gives the cubic feet and each part of the mix. When something in the bed is already chewed, spotted or wilting, the plant doctor lists the causes that fit, water and weather first. And before you reach for a spray, we compared the neem oils that are sold for vegetable gardens.