Why this planner will never give you a companion-planting chart
The grid of good and bad neighbors is the most-copied thing in garden writing, and one of the least examined. Here's what happens when you check it.
Photo: George Chernilevsky · Public domain, via Wikimedia Commons.
The chart is the product everywhere else
Open almost any garden planner and you’ll find a companion-planting chart: crops down one side, crops across the top, and every cell marked “plant together” or “keep apart.” Basil with tomato. Carrots love tomatoes. Never plant beans near onions.
It’s the easiest feature in the category to build. The data is a list of pairs, nobody asks where the pairs came from, and anyone who follows it and has a good season gives the chart the credit.
This planner doesn’t have one, and it never will. We left it out on purpose.
Where the pairs actually come from
Most of the modern companion-planting canon traces back to early-twentieth-century biodynamic and anthroposophical writing, spread later through mid-century gardening books. It wasn’t put together from trials. The individual pairings are rarely tested, and when someone does test them, most of them don’t hold up.
The corpus records that finding as a claim in its own right, with the same grade and the same sources as anything else it says:
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.
There is no scientific basis, however, for any of the several lists that exist describing
A grid can’t tell you why, and that’s the whole problem with it. You can’t check “plant together”, because it never says what is supposed to happen, to which plant, by what route, or how much. The cell can’t be proven wrong, which is exactly how it has survived a century without anyone really examining it.
What replaced it here
Every rule in this planner has to name the reason it works in one sentence, without leaning on the word “helps”. If it can’t, it gets downgraded or dropped. That one rule is what makes the chart impossible: almost no cell in a companion-planting grid can pass it.
What does pass is a much shorter list. Corn shades and supports. Squash leaves catch the light at soil level and slow evaporation. A legume pulls nitrogen out of the air and leaves it in the soil, through a partnership with bacteria on a timescale you can name. Those are real reasons. They’re modeled here, and each one carries its evidence.
Some of these reasons are weak, and we show them as weak. We don’t drop them. The one about aromatic herbs among brassicas is the edge case: something gets reported, nobody has pinned down why, and the trials disagree with each other.
Aromatic species interplanted with brassicas are reported to reduce cabbage-worm damage. The mechanism is unresolved and trial results are mixed.
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.
That’s what a real companion claim looks like when you write it down plainly. It’s a lot less satisfying than a green cell in a grid, and it’s the truth.
The specific beliefs, answered
A chart is also where a lot of individual folklore hides. The corpus keeps 35 of these as records in their own right, each with our answer and, where one exists, a source that flatly refutes it. Two of the most commonly charted:
Marigolds repel most garden pests and protect the whole garden.
Marigolds don't protect the whole garden. Their best-known trick is underground. A dense planting of the right kind, grown at least two months before your crop, suppresses root-knot nematodes. Dotted among the vegetables, it doesn't work. Against insects the results are mixed. Some pests, like thrips and whiteflies, drop in trials; others, like Colorado potato beetles, don't budge. Grow them for color and treat any pest help as a bonus.
What is known
Marigolds are not a garden-wide shield. Their best-established effect is on root-knot and some other nematodes in the soil. A dense stand of the right Tagetes, planted at least two months before the crop, suppresses them. Whether that is through the root compound alpha-terthienyl or because marigold is not a host is not settled. Dotting marigolds among the vegetables does not appear to work, and a susceptible variety can make nematodes worse. Against insects the evidence is mixed. Studies have found fewer thrips, cabbage worms (in mixed plantings) and squash bugs, and a glasshouse study found marigold scent slowed whiteflies, but marigolds do not deter Colorado potato beetles and there is little research behind the flea-beetle claim.
The one trial we know of found no difference in flavor, and nobody has proposed how it would work. Plant basil next to tomatoes anyway. You'll want them at the same time in the kitchen.
What is known
No proposed mechanism. One garden trial is known: a 2004 West Virginia University study whose blind taste tests over three years found no consistent preference for tomatoes grown with companions. Extremely widely believed.
Double blind taste tests over three years showed no consistent preference for tomatoes grown with companions over those grown in monoculture.
Neither one is a scandal. Marigolds do something real and narrow. Basil is lovely next to a tomato, but the claim that it improves the flavor isn’t supported. Here you can see which is which. A cell that just says “companion” leaves you to fill in a meaning nobody ever claimed.
What you get instead
When this planner tells you to do something, it points to the rule behind it, and that rule names the reason it works, how strong the evidence is, and where it came from. When the evidence is thin, the page says thin. When nothing is known, it says that too, and treats the silence as the finding.
That’s slower to build and harder to sell than a grid. It’s also the only version of this worth trusting.
