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Companion planting

Which plants help or hinder each other, graded by how good the evidence is. Most popular pairings rest on tradition, not trials.

Pairings
38
Tested in a study
8
Extension guidance
5
Tradition only
25
Last reviewed
Some plants seem to help each other grow. I show which pairings scientists actually tested.
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Figure 1. Which plants help or hinder each other, and how strong the evidence is. Read across a row; each pairing appears in both its row and its column.
  • Helps
  • Trap crop
  • Disputed
  • No clear effect
  • Keep apart
  • Ringed: tested in a study
  • Small and faint: traditional only

Traditional pairings come from a UC Master Gardener chart that traces to the UC Davis Edible Landscaping Handbook and ATTRA. The chart gives no reasons, and we found no trial of these pairs; treat them as garden lore.

  1. Tomatoes

    Helps
    Marigold (tested in a study), Carrots (traditional pairing), Onions (traditional pairing), Garlic (traditional pairing), Cucumbers (traditional pairing), Nasturtium (traditional pairing)
    Disputed
    Basil (tested in a study)
    Keep apart
    Potatoes (extension guidance), Fennel (traditional pairing)

  2. Cucumbers

    Helps
    Tomatoes (traditional pairing), Beans (traditional pairing), Peas (traditional pairing), Lettuce (traditional pairing)
    Trap crop
    Blue Hubbard squash (tested in a study)
    No clear effect
    Marigold (tested in a study)
    Keep apart
    Potatoes (traditional pairing)

  3. Zucchini

    Helps
    Nasturtium (traditional pairing)
    Trap crop
    Blue Hubbard squash (tested in a study)
    Disputed
    Marigold (tested in a study)
    Keep apart
    Potatoes (traditional pairing)

  4. Beans

    Helps
    Cucumbers (traditional pairing), Potatoes (traditional pairing), Peas (traditional pairing)
    Keep apart
    Onions (traditional pairing), Garlic (traditional pairing)

  5. Peas

    Helps
    Beans (traditional pairing), Carrots (traditional pairing), Cucumbers (traditional pairing)
    Keep apart
    Onions (traditional pairing), Garlic (traditional pairing), Potatoes (traditional pairing)

  6. Lettuce

    Helps
    Cilantro (extension guidance), Dill (extension guidance), Fennel (extension guidance), Carrots (extension guidance), Cucumbers (traditional pairing), Onions (traditional pairing), Garlic (traditional pairing)

  7. Carrots

    Helps
    Onions (tested in a study), Lettuce (extension guidance), Tomatoes (traditional pairing), Peas (traditional pairing), Garlic (traditional pairing)
    No clear effect
    Marigold (tested in a study)
    Keep apart
    Dill (traditional pairing)

  8. Onions

    Helps
    Carrots (tested in a study), Tomatoes (traditional pairing), Lettuce (traditional pairing)
    Keep apart
    Beans (traditional pairing), Peas (traditional pairing)

  9. Garlic

    Helps
    Tomatoes (traditional pairing), Lettuce (traditional pairing), Carrots (traditional pairing)
    Keep apart
    Beans (traditional pairing), Peas (traditional pairing)

  10. Potatoes

    Helps
    Marigold (traditional pairing), Beans (traditional pairing)
    Keep apart
    Tomatoes (extension guidance), Zucchini (traditional pairing), Cucumbers (traditional pairing), Peas (traditional pairing)

  11. Basil

    Disputed
    Tomatoes (tested in a study)

  12. Marigold

    Helps
    Tomatoes (tested in a study), Potatoes (traditional pairing)
    Disputed
    Zucchini (tested in a study)
    No clear effect
    Carrots (tested in a study), Cucumbers (tested in a study)

  13. Nasturtium

    Helps
    Tomatoes (traditional pairing), Zucchini (traditional pairing)

  14. Blue Hubbard squash

    Trap crop
    Cucumbers (tested in a study), Zucchini (tested in a study)

  15. Dill

    Helps
    Lettuce (extension guidance)
    Keep apart
    Carrots (traditional pairing)

  16. Cilantro

    Helps
    Lettuce (extension guidance)

  17. Fennel

    Helps
    Lettuce (extension guidance)
    Keep apart
    Tomatoes (traditional pairing)

Reading the evidence

Tested in a study means a field or controlled trial looked at this pair and this effect. Even then the result often holds only under conditions: young onions, a sprayed trap border, marigolds grown months ahead as a cover crop. Each pair's page lists them.

Extension guidance means a university extension publication recommends it without citing a trial. Traditional means it appears on companion charts that go back to gardening tradition. Washington State University's review of the subject finds that many of these claims do not hold up when tested.

Printable companion chart · How we grade evidence