Plants That Should Not Be Planted Together
Keep tomatoes and potatoes apart, keep tomatoes away from corn, keep berries out of old tomato and potato beds, keep sensitive plants away from black waln...
Published by ThriveThyme
companion planting bad companions plant disease black walnut garden planning
Some outbound seed-house links may be affiliate links.
Plants That Should Not Be Planted Together
The bad pairings with real reasons behind them come down to shared disease, shared pests, toxic roots, and soil needs. Most other "never plant together" lists are folklore.
Check any pair on the Companion Planting Desk, or run every crop in My Garden at once.
Real Reasons to Keep Plants Apart
- Tomatoes and potatoes. Same family. They share late blight, early blight, and Colorado potato beetle. Keep them in separate beds and rotate both.
- Tomatoes and corn. Corn earworm and tomato fruitworm are the same insect. Corn can build up moths that move to tomatoes.
- Strawberries or raspberries after tomatoes, potatoes, peppers, or eggplant. Those crops host verticillium wilt, a soil fungus that kills berries. Extension guides say not to plant berries where those crops grew in the last 3 to 5 years.
- Black walnut and sensitive plants. Walnut roots, hulls, and leaves release juglone. Tomatoes, peppers, potatoes, eggplant, cabbage, asparagus, blueberries, and apples are sensitive. Keep them 50 to 60 feet from a mature tree, or grow them in raised beds lined to stop roots. Beans, beets, carrots, corn, melons, onions, and squash tolerate it.
- Blueberries and most vegetables. Blueberries need acid soil, about pH 4.5 to 5.5. Most vegetables want near-neutral soil, so one side will struggle. Grow blueberries in their own bed.
- Cucumbers and melons in one tight patch. Same family. Both draw cucumber beetles, which spread bacterial wilt. Rotation matters more than spacing.
- Sunflowers next to seed beds. Sunflower hulls and residue slow germination of nearby seed, and tall plants shade short crops. Plant them on the north side and keep hulls out.
- Mint in an open bed. Mint spreads by runners and takes over. Grow it in a pot.
Common Advice With Little Testing
These show up on most charts. They may be fine to follow, but do not worry if you break them.
- Onions or garlic stunt beans and peas.
- Tomatoes and cabbage family crops dislike each other.
- Fennel stunts everything near it. Research is thin. It does grow big, so give it its own spot.
- Carrots and dill should be apart. They share the carrot family and some pests, and big dill shades carrots, but there is little direct testing.
- Lettuce dislikes parsley or celery.
A Better Rule Than Any Chart
Plants compete most when they want the same space, light, and water at the same time. Give every crop its spacing and sun first. Then use the real conflicts above, and rotate plant families. See the Crop Rotation desk.
See also what to plant with tomatoes and companion planting myths vs evidence.
Sources: University of Wisconsin Extension, Black Walnut Toxicity; Penn State Extension, Allelopathy in the Home Garden; University of Illinois Extension, Growing Strawberries; University of Minnesota Extension, Companion planting in home gardens.