What Companion Planting Actually Does
Companion planting gets tangled up with folklore, but the mechanisms that genuinely work fall into three categories: (1) physical shading or wind protection that benefits neighboring plants, (2) chemical signals or root exudates that affect pest behavior, and (3) nitrogen fixation from legumes that benefits adjacent heavy feeders. The famous Three Sisters (corn, beans, squash) works because of all three simultaneously.
Proven Combinations That Work
Tomatoes + Basil: The research on direct pest repellency is mixed, but basil repels thrips in controlled studies, and the dense aromatic foliage disrupts pest navigation. Practically: they share similar water and sun needs, so growing them together is efficient regardless of any direct companion effect. Brassicas + Nasturtiums: Nasturtiums act as a trap crop for aphids — pests preferentially colonize nasturtiums, clustering them away from cabbage, kale, and broccoli where they'd do more damage. Legumes (beans, peas) + Heavy feeders (corn, squash, tomatoes): Legume root nodules fix atmospheric nitrogen, enriching the soil for adjacent plants. This works — it's the mechanism behind the Three Sisters and behind cover cropping with vetch or clover. Carrots + Onions: Onion scent is thought to deter carrot fly; carrot foliage may confuse onion fly. Evidence is limited but the combination is traditional across cultures, suggesting practical observation.
What Doesn't Actually Work
Many companion planting claims are not supported by evidence. Marigolds repelling nematodes is true only for specific nematode species and requires planting a solid bed of marigolds for a full season — not a border alongside vegetables. Dill attracting beneficial insects is true, but so does any flowering plant. "Negative" companions (like fennel supposedly harming everything) are inconsistently supported.
For growing specific crops well, see our guides on growing garlic and growing microgreens indoors.
The Three Sisters in Practice
Plant corn first in a block (not a row — corn is wind-pollinated and needs block planting). When corn is 6 inches tall, plant beans at the base of each corn stalk. When beans have emerged, plant squash between the corn hills to shade the ground and retain moisture. The spacing: corn plants 12 inches apart in a 4-foot-wide block; beans at the base; squash at a 24-inch radius from each corn plant.
High-Value Companions for Pest Management
The most effective "companion plants" for pest management are flowering plants that attract and support beneficial insects: parasitic wasps, lacewings, hoverflies, and predatory beetles that eat aphids and caterpillars. Best options: dill, fennel, cilantro (allowed to flower), sweet alyssum, phacelia, borage, and yarrow. Plant in 2-foot strips throughout the vegetable garden, not just at the border.
FAQ
Do tomatoes and peppers grow well together?
Yes — they have nearly identical growing requirements (full sun, warm soil, consistent moisture) and neither negatively affects the other. They're good neighbors because of shared needs, not because of any direct companion effect.
What should not be planted near tomatoes?
Fennel (allelopathic — produces chemicals that stunt many plants including tomatoes), brassicas (compete for similar nutrients and attract different pests whose populations can spill over), and corn (shares common pests including corn earworm/tomato fruitworm — the same insect).
Does basil actually repel pests from tomatoes?
Evidence is mixed. Basil does produce aromatic compounds that deter some insects in controlled conditions. Practically, the benefit is modest — good tomato pest management relies on physical exclusion, early detection, and beneficials more than herb planting.
How close do companion plants need to be to work?
For shade/wind effects: direct adjacency (6–12 inches). For pest disruption: within 1–2 feet. For nitrogen fixation: within root range (2–3 feet). For attracting beneficial insects: within 30 feet, since insects forage widely.
What is the best companion plant for squash?
Nasturtiums (trap crop for squash bugs and aphids) and borage (deters hornworms, attracts pollinators). Both are easy to grow from seed directly around squash plants.