Tomato Companion Plants: What Has Evidence, What Is Folklore, and What to Skip
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Most tomato companion lists promise “repels pests” for every herb, but only a few pairings have been tested on tomatoes, and those tests are small. French marigolds grown with tomatoes from the start slowed whitefly build-up in a UK glasshouse trial, basil appears in a few studies on thrips and growth, and flowers that feed predators work as a general principle. The best-supported rules are about the site, not the neighbor: keep tomatoes away from black walnut, and do not repeat the tomato, potato, pepper and eggplant family on the same ground for three to four years. Below, each common companion is graded by what extension and research sources say, followed by what to skip and how to space the bed.
| Companion | Claimed job | Evidence level | What the sources say, and the catch |
|---|---|---|---|
| French marigolds | Repel whiteflies | Tested on tomatoes (glasshouse) | In UK glasshouse trials (Conboy et al., PLOS ONE 2019), short-vine tomatoes grown with French marigolds from the start had slower whitefly population growth; adding marigolds after whiteflies had built up gave only minimal reduction. A closed glasshouse is not an outdoor bed. |
| Marigolds | Stop root-knot nematodes | Supported only as a cover crop | Works as a dense stand grown for months and dug in; interplanting between crops does not (Penn State, UF/IFAS). Details in the nematode section below. |
| Basil | Repels thrips, helps growth, “improves flavor” | Tested on tomatoes (a few studies) | UMN cites “a few studies” in which basil and marigolds reduced thrips on tomatoes in field and greenhouse. In a Japanese pot study (Ahmad et al. 2020), one basil per tomato raised shoot dry weight after four weeks, while higher herb density suppressed growth. I found no trial on fruit flavor. See the basil companion guide. |
| Nasturtium | Trap crop for aphids | Method documented, no tomato trial found | WSU Master Gardeners describe nasturtium as an aphid trap crop: plant it before the main crop flowers, restock every 2 to 3 weeks, and remove infested parts, or it becomes a pest nursery. It sprawls, so keep it at a distance. More in the nasturtium guide. |
| Sweet alyssum, cilantro, dill (allowed to flower) | Feed predators of aphids | General principle, not a tomato trial | UMN says syrphid fly larvae eat large numbers of aphids and the adults visit a wide variety of flowers. UC Davis found sweet alyssum the most effective of 22 plants in coastal California lettuce fields, with aphids suppressed up to 50 feet away (1992). UMD lists cilantro, basil and dill as herbs to let flower. It only works once they bloom, so a harvested-to-the-ground herb does nothing here. |
| Borage | Deters hornworms, helps aphid wasps | No tomato test found | Popular on competitor lists; I found no extension or journal trial on tomatoes. The flower-for-predators effect above would apply to any blooming plant. UConn’s hornworm advice is hand picking. |
| Onions, chives, garlic | Repel aphids and soil pests | Listed as research, not tested on tomatoes | UMD lists volatile compounds from alliums and other herbs as making it harder for pests to find hosts, without a tomato trial; UMN says repellent claims such as green onions against flea beetles have little research. One competitor says onions inhibit Verticillium wilt; I could not confirm it. |
| Peas, beans | “Free nitrogen” | Weak for tomatoes; useful for wind | UMN warns that too much nitrogen gives bushy, leafy plants slow to bear fruit, so tomatoes do not need a nitrogen helper. UMD gives a different use: sow peas on the north side of tomato cages in early spring, and the vines act as a wind shield after tomatoes go in. Check the plant food vs fertilizer guide before adding more. |
| Lettuce, spinach, salad greens | Living mulch, shade | Extension practice, not pest control | UMD suggests low-growing, shade-tolerant salad greens on the north side of tomatoes. UMN suggests growing lettuce, spinach or basil early in the bed, then transplanting tomatoes into it as the early crop matures. |
| Carrots, celery, asparagus | Loosen soil, feed earthworms, deter beetles | No source found | Repeated on competitor lists (Kellogg Garden, Southern Living). I found no extension test on tomatoes for any of these claims. |
What not to plant with tomatoes
| Skip or limit | The usual reason | What the sources say |
|---|---|---|
| Black walnut within its root zone | Juglone poisons tomatoes | Best-supported. UW-Madison Horticulture lists tomato among the juglone-sensitive vegetables. Toxic effects can reach 50 to 80 feet from the trunk, strongest within the dripline, and appear within days of transplanting or years later as the tree grows. Options: plant elsewhere, or use a raised bed with a barrier that limits roots, and keep walnut leaves, hulls and wood out of the mulch. |
| Potatoes, eggplant, peppers on the same ground, year after year | Shared diseases | Best-supported rule overall. UConn: do not grow tomatoes and their close relatives in a site more often than once in three years. UMN: three or four years. UNL: minimum three, ideally four to six, because Fusarium and Verticillium spores survive for years. NC State: Verticillium can survive in soil without a host for up to 10 years. Peppers get their own page: pepper companion plants; the same rule applies to eggplant. |
| Strawberries, cucumbers, pumpkins | “Share fungal disease” | Partly real. NC State lists the cabbage, cucumber/pumpkin and strawberry families among Verticillium hosts, so they are not a clean break in a bed with a wilt history. That is a rotation rule, not proof that neighbors harm each other. |
| Brassicas (cabbage, broccoli) | “Stunt” tomatoes or compete for nutrients | No source found for stunting. The UConn and UMN tomato guides do not list brassicas as a problem; the claim appears on Southern Living and Epic Gardening lists without a citation. The disease-host overlap above is the documented concern. |
| Fennel | “Allelopathic”, stunts neighbors | Sources disagree. An Ask Extension (Michigan) expert says there is not enough research to call fennel truly allelopathic, only some evidence that it can inhibit seed germination. Southern Living quotes Colorado State Extension saying fennel inhibits tomatoes (I did not open the original). Keep it in a pot or at a distance. |
| Corn | Shared pest, shade | Half confirmed. UNH: the corn earworm is a pest of tomato too, where it is called the tomato fruitworm. I found no test showing that neighboring corn raises damage, but corn is tall and tomatoes need sun most of the day (UConn). |
Marigolds and nematodes: the version that works
Root-knot nematodes cause galls on roots, and affected plants are stunted, yellow or wilt (UF/IFAS, MSU). Marigolds help only in one form: a dense cover crop grown at least two months before the vegetable, on the exact site where it will go (UF/IFAS), for several months and then dug in (Penn State). MSU suggests French marigolds such as Tangerine, Petite Harmony or Petite Gold, 7 inches apart in 7-inch rows. Interplanting does not do the job: Penn State says it does not affect root-knot nematode populations, UF/IFAS calls it “probably not” effective, and Texas A&M warns that interplanted marigolds shade tomatoes or lose out themselves. Marigolds also suppress some nematodes and increase others, so UF/IFAS suggests a nematode test first.
The simpler tool is the plant itself. MSU’s table of varieties that root-knot nematodes cannot feed on includes the tomatoes Better Boy, Big Beef and Celebrity. My reading: if you find galls, a dug-in marigold season or a resistant variety beats a few marigolds between plants. For borders, see the pest-fighting marigolds guide.
Can you plant tomatoes and peppers together?
Yes, with a catch that is about next year. Neither the UConn nor the UMN tomato guide says “never.” They say not to repeat tomatoes, potatoes, peppers and eggplant on the same ground within three to four years, which, in my reading, makes a mixed tomato and pepper bed one rotation group. Move the whole group together next season and clear plant residue at the end of the year. If the leaves already look wrong, the yellow leaves on tomato plants guide helps sort wilt from other causes (UW-Madison notes walnut toxicity can also be confused with wilts).
Layout: how to avoid overdoing it
- Start with the tomato’s spacing. UMN says two to three feet in all directions for vining plants; UConn says five to six square feet per plant, for example two feet apart in rows three feet wide with cages, and closer for staked and pruned plants. Companions go in aisles, on the north side or along an edge, not between plants. Put supports in at planting (UMN), such as tomato cages.
- Draw the rotation map first. Mark where tomatoes, potatoes, peppers and eggplant grew in the past three to four years. UNL adds that moving to another corner of a tilled garden does not help; use separated beds or containers.
- Keep sun for the tomato. UConn wants direct sun most of the day, so keep tall companions off the south and west sides (my reading); UMD puts low greens on the north side. UConn also lists air temperatures above 85 °F among the causes of blossom drop, which is worth knowing in a hot climate.
- Use timing instead of crowding. Grow lettuce, spinach or basil early and transplant tomatoes in as they finish (UMN); sow peas on the north side of cages for wind protection (UMD). UConn says to transplant when soil reaches 65 °F, which a soil thermometer confirms.
- Plant the trap crop first. WSU says to plant nasturtium before the tomatoes flower, restock every 2 to 3 weeks and remove infested parts.
- Watch water and density. UMN says to soak the soil thoroughly, keep moisture steady to avoid blossom-end rot, and water at the base. In the pot study, higher herb density suppressed tomato growth; that is one study, so treat one basil per tomato as a ceiling, not a rule. Basil’s schedule differs: how often to water basil.
- Pick one or two companions, not eight. A marigold border from French marigold seeds started early, plus basil, covers the pairings with the most evidence.
Sources: UConn Home & Garden Education Center, Tomatoes; University of Minnesota Extension, Growing tomatoes; University of Minnesota Extension, Companion planting in home gardens; University of Maryland Extension, Vegetable Succession and Companion Planting; Conboy et al. 2019, PLOS ONE, French marigolds and glasshouse whiteflies; Ahmad et al. 2020, American Journal of Plant Sciences, Lamiaceae herbs and tomato; UF/IFAS, Marigolds for Nematode Management; Penn State Extension, Marigolds; Mississippi State Extension, Nematode Control in the Home Garden; Texas A&M, Vegetable Rotations, Successions and Intercropping; NC State Extension, Verticillium Wilt of Tomato and Eggplant; University of Nebraska-Lincoln Extension, Choose tomato cultivars wisely; UW-Madison Horticulture, Black Walnut Toxicity; Ask Extension, walnut trees (juglone); Ask Extension, Bulb Fennel Allelopathic?; WSU Skagit County Master Gardeners, Trap Cropping; UC Davis, Aphid Relief from Sweet Alyssum (1992); UNH Extension, Corn Earworm; Southern Living, plants to avoid with tomatoes (quoted only for the competitor claims named above).
FAQ
How long should I wait to plant tomatoes where potatoes grew? UConn says no more often than once in three years and UMN says three or four, while UNL recommends four to six years for a bed with a wilt history, three at minimum. If you till the whole garden, the spores spread through it, so UNL says rotation needs separated beds, or a container with fresh peat-based potting mix.
Can I plant tomatoes after a black walnut is cut down? Not right away. UW-Madison says leftover root pieces may keep releasing juglone for several years as they decay, and Ask Extension says toxicity may persist for years. The extension fix is a raised bed with a root barrier and new topsoil.
Which companion plant stops tomato hornworms? None has extension backing on tomatoes that I found. UConn’s controls are hand picking the caterpillars and tilling after harvest to destroy pupae in the soil; if a caterpillar has small white cocoons on its back, leave it, since those are parasitic wasps.
Does companion planting work in pots? One pot study found that a single basil per tomato helped shoot growth over four weeks, and that higher density hurt. It measured growth, not yield or flavor, in autoclaved soil. In a pot, root space and water are the limit.
