Start with the caveat
Linda Chalker-Scott, a Washington State University Extension urban horticulturist, has called popular companion planting lore "a horticultural zombie: it never dies". The University of Arizona Cooperative Extension quotes her recommendation that "traditional 'companion plant' charts have entertainment, not scientific, value". Mississippi State University Extension takes a middle line: some companion principles have been studied scientifically, while others "lack any scientific proof that they work".
What the sources do agree on is the mechanisms that can make neighbors matter: physical support and shade, root systems that do not compete, trap crops that pull pests away, flowers that feed beneficial insects, and mixed plantings that make it harder for pests to find a whole crop.
The chart
Evidence labels: Supported = the source lists it as a principle that works or cites studies; Some studies = limited or specific trial results; Anecdotal = the source itself calls it anecdotal or unproven; Weak / disputed = the source says the benefit is unlikely.
| Crop | Companion | Claimed effect | What the source says | Evidence |
|---|---|---|---|---|
| Tomato | Basil + marigold | Fewer thrips | Mississippi State: a few studies showed reduced thrips in greenhouse and field tomatoes | Some studies |
| Cabbage | Marigold, nasturtium, onion | Fewer cabbage loopers and imported cabbageworms | Mississippi State: one study found these plants helped reduce both pests | Some studies |
| Cabbage | Mustard (intercropped) | Trap crop for cabbageworms | WSU Extension lists mustard as a cabbageworm trap; trap crops must be checked and cleared often | Supported (as trap crop) |
| Cabbage family | Radish (intercropped) | Trap for flea beetles and root/cabbage maggots | WSU Extension trap-cropping guide | Supported (as trap crop) |
| Cabbage, green beans | Nasturtium, planted away from them | Draws aphids off the crop | UVM Extension: nasturtiums are "a magnet for aphids", so place them at a distance; WSU lists them for aphids and flea beetles | Supported (as trap crop) |
| Carrot | Lettuce | Less root competition | Chalker-Scott: pair deep taproots such as carrots with shallow roots such as lettuce | Supported (principle) |
| Tall trellised crops | Radish, turnip, lettuce underneath | Uses space under vines | Virginia Cooperative Extension: these can grow beneath vertical crops; put sun-lovers on the south side | Supported (spacing) |
| Any vegetable | Marigold (general pest deterrent) | Repels insects | Mississippi State: benefits are "blurry"; marigolds may not give the protection credited to them | Weak / disputed |
| Vegetable plot | French or African marigold | Kills root-feeding nematodes | Mississippi State: only after spent plants are tilled into soil (one study); UVM: "some scientific evidence" but "not proven" | Some studies |
| Tomato | Zinnia | Attracts predators; may deter hornworms, cucumber beetles | UVM Extension lists it as a "may" effect; tall zinnias can shade crops | Anecdotal |
| Tomato, carrot, cabbage, beans | Calendula | Attracts predatory insects | UVM: mostly beneficials, but may also draw aphids and whiteflies; repellent claims anecdotal | Anecdotal |
| Leafy greens | Chervil | Lures or deters slugs | WSU lists chervil to lure slugs; UVM says it "may" deter them | Anecdotal |
| Tomato | Basil | Repels whiteflies | Denver Urban Gardens' youth-garden list repeats this claim; no study is cited | Anecdotal |
| Corn | Beans + squash ("Three Sisters") | Nitrogen, support, living mulch | Arizona: a historical practice, "not necessarily" mutual liking; Chalker-Scott: of historic and cultural interest but "unlikely to benefit plant productivity or soil quality" | Weak / disputed |
| Most vegetables | Sunflower nearby | Harm: neighbors "do not do well" | Arizona: attributes it to allelopathy, which it says is documented | Some studies (negative) |
| Any bed | Mixed heights, leaf shapes, colors | Confuses some insects | Mississippi State lists it among principles that work; Chalker-Scott: spacing same-species plants apart limits infestation | Supported |
| Any bed | Legumes as cover or under-crop | Lower fertilizer needs | Mississippi State principle; Arizona notes beans supply only "small amounts" of nitrogen | Supported |
| Any bed | Cosmos, dill, parsley, coriander, borage | Feed beneficial insects | UVM: cosmos attracts lacewings, parasitic wasps, tachinid flies and hoverflies; mint and borage spread aggressively | Supported (insectary) |
- Main crop row (cabbage)
- Trap crop at the edge (mustard or radish)
- Nasturtium placed away from the crop to draw aphids
- Insectary flowers (cosmos, dill) feeding predators
- Shallow-rooted lettuce between deep-rooted carrots
Using companions sensibly
- Annuals act fast. Chalker-Scott advises annual companions when immediate benefits are wanted; perennials take a year or two to establish.
- Trap crops need upkeep. WSU warns that if traps are not checked and cleared, "the insect pests will continue to reproduce and multiply". Plant traps before the main crop flowers and restart them every 2 to 3 weeks for longer control.
- Mind side effects. UVM notes that a trap crop can attract a pest that was not previously a problem, and that tall zinnias and cosmos can shade sun-loving vegetables.
- Contain spreaders. UVM suggests containers for mint and borage.
Spacing still matters more than pairing; see the square foot spacing table.
Sources
- Mississippi State University Extension — Companion Planting: Myth or Truth?
- University of Arizona Cooperative Extension — Companion Planting (PDF)
- The Garden Professors (Linda Chalker-Scott, WSU) — Weeding out the pseudoscience in companion planting
- Washington State University Extension — Trap Cropping (PDF)
- University of Vermont Extension — Creating a Biodiverse Vegetable Garden (PDF)
- Virginia Cooperative Extension — Vertical Gardening Using Trellises, Stakes, and Cages (HORT-189NP)
- Denver Urban Gardens — Plants for Youth Gardens (PDF)
All sources were opened and checked in October 2026. Rules and figures change; confirm with the original before acting.