What to Plant With Tomatoes (and What the Research Says)
Quick answer
Keep tomatoes 50 to 60 feet from mature black walnut trees (Penn State). Basil alongside may increase total harvest per bed, but in a 3-year West Virginia University study, blind taste tests showed no consistent flavor benefit. Use marigolds only as a dense cover crop at least two months before planting (UF/IFAS).
Updated
What helps tomatoes most is keeping them away from black walnut trees. Basil grown alongside can mean more total harvest per bed, but it did not make tomatoes taste better in blind tests. Marigolds help with root-knot nematodes only when grown as a dense cover crop before the tomatoes, not mixed in.
Tomato companions: the evidence
| Claim | What the research says | Evidence |
|---|---|---|
| Keep 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 |
| Basil raises total yield per bed | Partly. Bomford found land-use “advantages (x̄ = 20%) to planting either tomato or Brussels sprout with basil companions”, mostly at high densities with a lot of basil. But “The highest yields in tomato, basil, and Brussels sprout” came from single-crop plantings. Read it as more total harvest from the bed, not bigger tomato plants (Bomford, West Virginia University dissertation, 2004). | Weakto moderate |
| Marigolds against root-knot nematodes | Only as a cover crop grown first. UF/IFAS: marigold “should be planted at least two months before the desired vegetable crop”, and “interplanting marigold and susceptible crops is very risky and may result in damage to the susceptible crops” (UF/IFAS, Nematode management with marigolds (NG045)). | Moderate(cover-crop method) |
| Sunflowers next to tomatoes | Caution. Penn State lists sunflowers among garden plants that “produce allelopathic compounds”. We found no tomato-specific trial (Penn State Extension, Allelopathy in the Home Garden). | Weak |
| Carrots love tomatoes | No test found. WSU: “There is no scientific basis, however, for any of the several lists that exist describing ‘traditional companion plants’.” (WSU Extension, The Myth of Companion Plantings) | None |
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.
Peppers and potatoes
Peppers and potatoes are in the same plant family as tomatoes, and black walnut is the one well-tested rule for them too: both are on Penn State’s least-tolerant list. We found no qualifying trials for other pepper or potato companions.
Myths
- “Basil makes tomatoes taste better.” Not supported. “Double blind taste tests over three years showed no consistent preference for tomatoes grown with companions over those grown in monoculture” (Bomford, West Virginia University dissertation, 2004).
- “Plant marigolds between tomatoes to stop nematodes.” Interplanting is what UF/IFAS calls “very risky”. The method that works is a dense marigold planting for at least two months before the crop.
See the companion planting hub for other crops, and planting dates by ZIP.
Sources
- Penn State Extension: Allelopathy in the Home Garden
- University of Wisconsin–Madison Extension: UW–Madison Extension, Black Walnut Toxicity
- American Geophysical Union (peer-reviewed journals): Bomford, West Virginia University dissertation, 2004
- UF/IFAS Extension: UF/IFAS, Nematode management with marigolds (NG045)
- 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.