Companion planting charts promise that basil protects tomatoes and marigolds keep pests away. Washington State University Extension’s 2023 peer-reviewed review found that most popular charts lack credible science. Here is what holds up for containers.
The four rules that really decide it
- Same water. Basil, mint, parsley and cilantro want steady moisture; rosemary, thyme and oregano want to dry out between waterings. Do not mix the two groups.
- Same sun. A plant that needs 6–8 hours and one happy with 4 can share only if the tall one shades the short one, not the other way round.
- Same season. Cool-season and warm-season plants overlap for only a few weeks; they follow each other in a pot better than they share it.
- Enough room. Every extra plant means less water and fertilizer for each, and crowding raises the risk of leaf disease.
Pairs that work, for practical reasons
- Lettuce + radish: radishes are out within a month, before the lettuce needs the room.
- Lettuce + green onions: both shallow-rooted, cool-season and fond of steady moisture.
- Basil + sweet pepper: basil grows beside and partly under the taller pepper.
- Lettuce in the shade of a tomato: the shade delays bolting in early summer.
Popular, but unproven
- Basil keeps pests off tomatoes: no garden-scale study behind it; the flavor claim is folklore. Harmless to try, in the pot next door.
- Scented plants repel pests: field research has not confirmed it. What does confuse pests is a mix of different green plants.
- Marigolds: the effect on root nematodes comes from growing them as a crop and working them into the soil, not from having one nearby.
Keep apart
- Mint and anything: it spreads by underground stems and takes over the pot.
- Tomato and bush beans in one pot: beans need full sun and the tomato will shade them.
Sources
Every fact in this guide comes from the publications below. None of them reviewed or endorsed Balcony Bounty.
- Washington State University Extension. EM128E: Gardening with Companion Plants (L. Chalker-Scott, 2023, peer reviewed). rex.libraries.wsu.edu/esploro/outputs/report/Gardening-with-companion-…
- University of Illinois Extension. Companion planting: Combining plants for a healthy, well-balanced garden (2025). extension.illinois.edu/blogs/good-growing/2025-03-07-companion-plantin…
- University of Maryland Extension. Vegetable Succession and Companion Planting. extension.umd.edu/resource/vegetable-succession-and-companion-planting
- University of Maryland Extension. Growing Vegetables in Containers and Salad Tables. extension.umd.edu/resource/growing-vegetables-containers-and-salad-tab…
- University of New Hampshire Extension. Growing Herbs in Containers. extension.unh.edu/blog/2018/05/growing-herbs-containers
- Penn State Extension. Herbs Make Good Plant Partners and Companions. extension.psu.edu/herbs-make-good-plant-partners-and-companions
- University of Arizona Cooperative Extension, Yavapai County. Backyard Gardener #254: Companion Planting (2024). www.extension.arizona.edu/sites/default/files/2024-10/CompanionPlantin…
- Utah State University Extension. Mint in the Garden. extension.usu.edu/yardandgarden/research/mint-in-the-garden
- University of Maryland Extension. Growing Beans in a Home Garden. extension.umd.edu/resource/beans
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