Companion Planting Checker

Companion planting is not garden folklore — most of the mechanisms behind it are well understood. Plants interact through their root systems, their scent profiles, and their physical structure. The three main pathways are nutrient sharing, pest deterrence, and physical complementarity. Legumes (beans, peas, clover) host Rhizobium bacteria in root nodules that convert atmospheric nitrogen into plant-available ammonium — neighbouring plants with high nitrogen demand genuinely benefit from this, though most of the nitrogen becomes available after the legume is cut or dies back and the nodules decompose. Strong-scented herbs like basil, rosemary, and marigolds release volatile compounds that mask the chemical signatures pest insects use to locate their host plants. A carrot fly finds carrots by detecting chlorogenic acid in the air — interplanting with onions floods the airspace with competing sulfur compounds and makes the carrots harder to locate. Physical complementarity is the simplest mechanism: tall crops cast shade that benefits heat-sensitive plants like lettuce and spinach, while low-growing ground covers suppress weeds and retain soil moisture for their taller neighbours.

The Three Sisters: companion planting with centuries of evidence

The most researched companion planting system is the Three Sisters — corn, beans, and squash grown together. This is not a quaint tradition; it is an engineered polyculture developed by Indigenous peoples of the Americas over thousands of years. The corn provides a tall structural pole for the beans to climb. The beans fix atmospheric nitrogen through their root nodules, feeding the nitrogen-hungry corn. The squash spreads large leaves across the soil surface, suppressing weeds, reducing moisture evaporation, and its prickly stems deter raccoons and other ground-level foragers. Modern agronomic studies have confirmed that Three Sisters plots produce more total calories per square metre than any of the three crops grown alone — a genuine yield advantage, not just convenience. In my experience the timing matters: plant corn first and give it a two-week head start before adding beans, or the beans outpace the corn and have nothing to climb.

Beneficial pairings worth trying

Basil + tomatoes: This is probably the most popular companion pairing in home gardens. Research from the UK and Italy suggests basil may reduce whitefly and aphid populations on neighbouring tomatoes — the mechanism appears to be volatile interference rather than direct repellence. The basil's strong terpene emissions (linalool, eugenol) confuse the pest's host-finding behaviour. In my experience the effect is modest but real, and you get pesto ingredients from the same bed regardless.

Carrots + onions: Carrot fly (Psila rosae) locates carrots by scent. Onion fly locates alliums by scent. Interplanting the two means each crop's smell helps mask the other from its specialist pest. This is one of the better-documented companion effects — multiple trials show reduced carrot fly damage in mixed stands compared to monoculture rows, though it does not eliminate the pest entirely. A physical barrier (fine mesh at 60 cm height) is still more reliable if carrot fly pressure is severe.

Beans + sweetcorn: Beyond the Three Sisters context, any legume near a heavy nitrogen feeder makes biological sense. Sweetcorn is one of the hungriest crops in a vegetable garden — it benefits from the nitrogen that bean root nodules release into the surrounding soil as the season progresses. French beans or runner beans climbing the corn stalks also save you the cost and hassle of bamboo canes.

Pairings to avoid

Fennel + most vegetables: Fennel is genuinely allelopathic — it releases compounds from its roots and fallen foliage that inhibit germination and growth in many common vegetables. Tomatoes, beans, brassicas, and carrots all perform poorly near fennel. Grow it in its own isolated spot or in a container. Dill, its close relative, does not have this problem.

Potatoes + tomatoes: Both are Solanaceae and share the same diseases — most critically Phytophthora infestans (late blight). Growing them adjacent increases the chance that blight spores jumping from one crop find a susceptible host right next door. The same logic applies to potatoes near outdoor-grown aubergines or peppers, though blight is less aggressive on those crops. Keep at least a few metres between any Solanaceae family members if you can.

Beans + alliums (onion, garlic, leeks): Alliums release sulfur-containing compounds that appear to inhibit the nitrogen-fixing Rhizobium bacteria in bean root nodules. The effect is not dramatic in a single season, but trials consistently show slightly reduced bean yields when interplanted with onions or garlic compared to beans grown alone or with non-allium neighbours.

How to use this tool

Select the plants you are planning to grow together and the checker will show you which combinations are beneficial, which are neutral, and which you should avoid. Each pairing includes a short note explaining the mechanism behind the recommendation — whether that is pest confusion, nutrient competition, disease crossover, or growth inhibition. Use it when planning your bed layouts to catch problem pairings before you commit seeds to soil, and to spot opportunities for beneficial neighbours you might not have considered.