Companion Planting: What Actually Works
Quick answer
Most companion planting charts aren’t backed by research; WSU Extension says they have entertainment, not scientific, value. What is supported: keep tomatoes, peppers and potatoes 50 to 60 feet from black walnut, use a treated Hubbard squash border as a cucumber beetle trap crop, grow marigolds as a cover crop before the vegetables, and plant sweet alyssum with lettuce, which was linked to fewer aphids in California trials.
Updated
Most companion planting charts aren’t based on tests. WSU Extension says traditional charts have “entertainment, not scientific, value”. But some ways of planting crops together do have research behind them: trap crops, flowers that help with pests, keeping sensitive crops away from black walnut, and cover crops. Every claim below carries an evidence label.
Crop guides
- Tomatoes (with peppers and potatoes): black walnut, basil, marigolds.
- Cucumbers, squash and pumpkins: the Hubbard squash trap border and Three Sisters.
- Cabbage family (cabbage, broccoli, Brussels sprouts): collard trap rows and basil, both weak evidence.
What research supports
| Claim | What the research says | Evidence |
|---|---|---|
| Keep tomatoes, peppers and potatoes away from black walnut | Keep them 50 to 60 feet from a mature black walnut. Penn State: “Vegetables in the solanaceous group, including tomatoes, peppers, and potatoes, are some of the least tolerant of juglone.” Plant “past the root zone (50 to 60 feet away for a mature black walnut)” or in lined raised beds with fresh topsoil, rake up leaves and hulls, and don’t mulch with walnut leaves (Penn State Extension, Allelopathy in the Home Garden; UW–Madison Extension, Black Walnut Toxicity). | Strong |
| Hubbard squash border for cucumber beetles | Worked in UMass farm trials when the border was treated with insecticide; an unmanaged border is untested (UMass Extension, Using Perimeter Trap Crops to Manage Striped Cucumber Beetle). Separately, a hand-picked Hubbard border cut squash bugs on nearby pumpkins in one Utah State trial (Utah State University Extension IPM). | Strong(treated border) |
| Sweet alyssum with lettuce | In California lettuce trials, sweet alyssum planted alone with lettuce was linked to fewer aphids; mixing in more flower types didn’t help. “Hoverfly numbers were similar on sticky traps and in the lettuce for all fields” (UC IPM, Insectary plants in lettuce (2019)). | Moderate |
| Marigold cover crop for nematodes | Grow a dense planting at least two months before the crop; interplanting is “very risky” (UF/IFAS, Nematode management with marigolds (NG045)). | Moderate |
| Basil with tomatoes | More total harvest per bed in one study, but no better flavor in blind taste tests (Bomford, West Virginia University dissertation, 2004). | Weakto moderate |
| Aromatic herbs (mint and similar) confuse pests | WSU cites a study where aromatics “including aromatics like Mentha spp., had little or no disruptive effect on insect behavior” (WSU Extension, The Myth of Companion Plantings). | None(not supported) |
Evidence labels: Strong = several field trials or a clear extension recommendation built on trial data. Moderate = one or two trials, or extension guidance with limits. Weak = suggestive only. None = no qualifying test found.
Why plant diverse beds anyway
WSU says documented benefits from plant associations “include physical, chemical, and biological alterations that can improve the establishment and survival of desired plant species”, and diverse plantings “attract and retain beneficial insects” (WSU Extension, The Myth of Companion Plantings).
Not covered yet
We also found no qualifying tests for carrots with onions, nasturtiums as aphid traps or radishes for cucumber beetles. These are gaps, not proof the pairings fail.
Sources
- Penn State Extension: Allelopathy in the Home Garden
- University of Wisconsin–Madison Extension: UW–Madison Extension, Black Walnut Toxicity
- UMass Extension: Using Perimeter Trap Crops to Manage Striped Cucumber Beetle
- University of California Agriculture and Natural Resources (UC ANR): UC IPM, Insectary plants in lettuce (2019)
- UF/IFAS Extension: UF/IFAS, Nematode management with marigolds (NG045)
- American Geophysical Union (peer-reviewed journals): Bomford, West Virginia University dissertation, 2004
- Washington State University Extension: WSU Extension, The Myth of Companion Plantings
Organic options Growing without synthetic sprays? See our organic gardening guides: pest control and fertilizer.