Most companion planting charts pinned up in sheds are copies of copies. Somebody scanned a page from a 1970s paperback, somebody else redrew it, and forty years later the same unverified pairings are still circulating as fact.
A chart on your shed wall is genuinely useful, but only if what is printed on it survives contact with evidence. Here is what belongs on one, what should be cut, and how to build a version you can trust while standing in the garden with muddy hands.
Four things, in descending order of usefulness: plant spacing, crop family groupings for rotation, the handful of pairings with real evidence, and the antagonisms that have a documented mechanism. Notice that the pairwise "friends and enemies" grid, which is what most charts consist of almost entirely, comes third.
What you'll get out of this guide:
Key Takeaway
Print spacing and crop families first. Those two are unambiguous, hold up everywhere, and are the details you actually forget while planting. Companion pairings are the decorative part of the chart, not the load-bearing part.
The ones with a stated mechanism. University of Maryland Extension frames companion planting as intentional interplanting for a specific purpose, and that framing is the filter: if nobody can say how a pairing works, it does not earn wall space.
Basil and tomatoes is the interesting case. It is on every chart as pest control, and the hornworm claim does not hold up. But basil and tomatoes want identical conditions, occupy different heights, and basil covers soil the tomato does not use. That is a legitimate space-efficiency pairing, just not the one the chart claims.
| Pairing | Mechanism | Confidence |
| Carrots + onions | Onion volatiles disrupt carrot rust fly host-finding | Measured |
| Corn + beans + squash | Trellis, nitrogen, living mulch | Historical and documented |
| Broccoli + lettuce or spinach | Brassica canopy shades cool-season greens | Extension endorsed |
| Marigolds + root crops | Root exudates suppress soil nematodes | Measured, root zone only |
| Flowering herbs + anything | Beneficial insect and pollinator habitat | Extension endorsed |
| Tomatoes + basil | Space efficiency, shared conditions | Practical, not pest control |
Sources: Uvah & Coaker (1984), Penn State Extension, UMN Extension, USDA National Agricultural Library.
University of Florida IFAS makes the point that extension research across the country has now identified strategies that are demonstrably effective, which is a real shift from the era when the whole subject was hearsay. Print those. Leave the rest.
Antagonisms are actually the more reliable half of the chart, because the mechanisms are concrete: shared diseases, allelopathy, and competition. These are worth more shed-wall space than most charts give them.
| Avoid | Why |
| Tomatoes near potatoes | Both nightshades, share late blight |
| Anything under black walnut | Juglone is allelopathic to nightshades |
| Fennel near most vegetables | Documented allelopathic effects |
| Brassicas following brassicas | Shared pest and disease complex builds |
| Corn shading short crops | Straightforward light competition |
| Two squash varieties for seed | Cross-pollination ruins saved seed |
Sources: UC Master Gardeners of San Diego County, University of Wisconsin Extension.
The Onion and Bean Rule Is Weak
Nearly every chart says alliums stunt legumes. The evidence for this in a home garden bed is thin, and gardeners grow them adjacent successfully all the time. Treat it as unproven rather than a rule. This matters because a chart full of unverified prohibitions makes you waste bed space avoiding problems that were never demonstrated.
Here is the single most useful thing you can print, and almost no companion chart includes it. Group your vegetables by botanical family and rotate those groups.
University of Wisconsin Extension notes rotation reduces insect damage, limits disease development and helps manage soil fertility. Families share pests and diseases, so moving a whole family is what actually breaks those cycles. Five groups cover most home gardens: nightshades, brassicas, legumes, alliums and cucurbits. Rotation also spreads the fertility demand around, which is why it pairs naturally with building living soil rather than feeding individual plants.
Why This Works: Disrupting the Pattern
A pest that overwinters in your soil emerges expecting to find the same host it fed on last year. Rotating by family means it emerges into a crop it cannot eat. This is the permaculture principle of designing for the whole system rather than treating symptoms, and it is why a rotation plan outperforms any amount of pairing optimisation within a single season.
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Send Me the ChartThere is no research on this, so treat what follows as practical advice rather than evidence. It is, however, advice earned from several charts that dissolved.
Print landscape, one sheet
If it runs to two pages you will only ever read page one. Force it onto a single sheet at 11 by 17 inches (28 by 43 cm) if your printer allows, or letter size if not.
Put spacing at the top
It is the thing you check most and the thing you forget most. Pairings go at the bottom where you will glance at them once a season.
Sheet protector, not lamination
Around $2 and it slides in and out, so you can update the chart as you learn what works in your own beds. Laminating freezes a document you should be editing.
Write on it in pencil
Note what actually happened in your garden this season. After three years your annotations will be worth more than the printed content.
That last step is the real point. A generic chart is a starting position. Your own records of what performed in your soil, in your climate, beat any published pairing table. For the full reference behind the printed version, our complete companion planting chart covers every common vegetable, and what not to plant together goes deeper on the antagonisms.
Several university extension services publish free charts, and those are the ones to start from because they carry institutional review. UC Master Gardeners of San Diego County publish a comprehensive pairing chart including explicit incompatibilities. Compare any chart you find against a second source before printing it, because the commercial charts that circulate on social media are usually redrawn from the same unattributed 1970s original and repeat its errors faithfully.
Mixed, and the accuracy is not evenly distributed. Spacing data and crop family groupings are reliable, because they come from published horticultural reference sources. Pest-repellent pairing claims are much weaker and are where most errors live. A useful test: if the chart states a mechanism, it is probably drawing on real sources. If it just lists friends and enemies with no explanation, it is a copy of a copy.
No, and this is the most freeing thing to understand. Use it for spacing and rotation, which are near-universal, and treat pairing suggestions as low-cost experiments. Interplanting basil with tomatoes costs nothing and uses space you were not using anyway. Rearranging an entire bed to avoid an unproven antagonism costs you a season of production. Weight your effort accordingly.
Whatever your regional extension service publishes for your area, because spacing recommendations vary with climate and season length. Include both in-row and between-row figures, since companion planting only works when the spacing supports it. Crowding two crops into the space meant for one produces competition, not cooperation, which is the most common way a well-intentioned interplanting scheme fails.
Sometimes, and usually through space efficiency rather than magic. Filling the ground between slow-growing crops with fast ones raises total production per square foot because you are harvesting light that would otherwise hit bare soil. Pest-reduction benefits are real but narrower than charts suggest, and mostly come from diversity and beneficial insect habitat rather than any specific pairing. Our guide to three sisters planting shows what a genuinely productive polyculture looks like.
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