Companion Planting Myths vs Evidence
Which companion planting claims hold up: Three Sisters support, nasturtium as a trap crop, flowering herbs for beneficial insects, and black walnut toxici...
Published by ThriveThyme
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Companion Planting Myths vs Evidence
Companion planting works when there is a physical, chemical, or biological reason: one plant holds up another, draws pests away, feeds beneficial insects, or poisons the soil around it. Charts built on plants that "like" each other have little behind them.
The Companion Planting Desk labels every pair as evidence-backed, some evidence, or traditional.
Backed by Evidence
- Physical support. In the Three Sisters, corn holds up pole beans and squash shades out weeds. See the Three Sisters planting guide.
- Trap crops. Nasturtium cut squash bug and cucumber beetle damage on squash in research trials, and cut cabbage worm damage on broccoli and cabbage. Spicy greens such as arugula and mustard draw flea beetles away from cabbage family crops. A trap crop only helps if you check it and remove pests.
- Beneficial insects. Small flowers on dill, cilantro, parsley, and sweet alyssum feed hover flies and parasitic wasps. Their young eat aphids and caterpillars. Let some herbs bloom.
- Allelopathy. Black walnut releases juglone, which stunts or kills tomatoes, peppers, potatoes, and blueberries. Sunflower residue slows germination of nearby seed.
- Shared disease. Tomatoes, potatoes, peppers, and eggplant carry verticillium wilt into the soil, which then kills strawberries and raspberries.
- Legume cover crops. Peas, beans, clover, and vetch fix nitrogen and cut fertilizer needs for the next crop. Grains such as rye and oats in the mix give vetch and peas something to climb.
Some Evidence, With Limits
- Basil with tomatoes. A few studies found fewer thrips. No good evidence it improves flavor.
- Thyme, sage, and onion with cabbage. An Iowa field trial found thyme and onion cut cabbage worm damage on broccoli. Sage and thyme cut diamondback moth in a greenhouse. Results vary.
- Marigolds. Some trials show less damage from squash bugs, cucumber beetles, and cabbage worms. For nematodes, marigold works only as a dense stand grown for 2 months or more in the same spot before the crop, then turned in.
Myths
- Marigolds repel every garden pest. They do not. Studies show marigolds do not deter Colorado potato beetle, and spider mites can build up on them and move to vegetables.
- A few marigolds in the row stop nematodes. Interplanting showed little or no effect in trials.
- Basil makes tomatoes taste better. No good evidence.
- Plants "love" or "hate" each other. Charts built on plant sympathies or crystal tests have no scientific basis.
- Beans feed the corn in the Three Sisters. Most bean nitrogen is not available to corn the first year. Corn still needs fertilizer.
How to Use a Companion Chart
- Treat chart pairings as ideas, not rules.
- Follow the evidence-backed conflicts every time: rotation, verticillium, black walnut, and soil pH.
- Plant a mix of flowers and herbs. Diversity helps more than any single pair.
- Give every crop its spacing and sun first.
See what to plant with tomatoes and plants that should not be planted together.
Sources: University of Minnesota Extension, Companion planting in home gardens; Mississippi State University Extension, Companion Planting: Myth or Truth?; University of Florida IFAS, Marigolds for Nematode Management; LSU AgCenter, Marigolds vs. nematodes; Cornell Cooperative Extension, Creating a Three Sisters Garden.