Companion plantings that are said to repel pests, such as basil with tomato, alliums with carrot and marigolds with beans, have uneven evidence, so plant them as a hypothesis, compare against a plain row, and keep barriers and scouting in place until your own results show the pairing works. The pairings with the best support are flowers that feed natural enemies and mixed plantings that make the host plant harder to find.
A pest-repelling combination is a planting of two or more species chosen so that one reduces pest damage on another.
The Carrier’s Table, With Evidence Grades
The carrier lists seven combinations. Treat the “effect” column as a claim to test.
| Combination | Claimed effect | How well supported |
|---|---|---|
| Tomato with basil | Repels aphids and whiteflies | No field trial is cited in this corpus; a hypothesis |
| Brassicas with thyme, sage, rosemary | Confuses cabbage moth | Mixed plantings can reduce moth egg-laying, but the herb scent is unproven as the cause |
| Carrot with onion or leek | Confuses carrot fly | Some trials found fewer carrot fly eggs, others did not; spacing and density matter |
| Beans with marigold | Roots suppress nematodes | True only in a narrow sense (below) |
| Squash with nasturtium | Trap crop for aphids | A trap only if the trap plants are destroyed (Trap Cropping Strategies) |
| Corn with squash | Shades weeds, shelters ground beetles | Plausible, and testable by counting weeds and beetles |
| Alyssum borders | Attract hoverflies | Well supported: shallow flowers feed hoverflies and parasitic wasps |
For the general picture see Pest-Confusing Polycultures, which finds that diversity lowers some pests in many trials but not all, and that flowers for natural enemies are better supported than repellent herbs. The Phase 2 guides give the annual-crop versions (Pest Repellent Pairs, Companion Planting and Rotation).
Marigolds And Nematodes Are A Rotation Effect
Marigolds (Tagetes species) have been shown to reduce root-knot and root-lesion nematodes, but mainly when grown as a dense full-season crop and dug in before the next crop, and the effect depends on the marigold species and on the nematode (Hooks and colleagues, Applied Soil Ecology 46, 2010; recalled, check before quoting). A few marigolds beside a row of beans are not the same treatment.
Method: Test A Pairing Honestly
Step 1: Pick one pairing and one pest. For example, chives between carrot rows against carrot fly.
Step 2: Plant two matching rows. One with the companion and one without, at the same spacing, in the same soil and light.
Step 3: Scout both the same way and on the same dates (Monitoring and Identification).
Step 4: Weigh or count at harvest. Compare undamaged roots, fruit or leaves.
Step 5: Repeat for several seasons. Pest numbers vary widely year to year, and one season shows little.
Step 6: Keep the pairing only if it shows a difference. Keep the physical protection, such as row cover, until it does.
Practical Rules
- Mix plants that are different in shape and smell so the crop is not a solid block of its host.
- Do not let the companion shade or crowd the crop; competition costs yield.
- Strong-smelling herbs are cheap and give food and flowers anyway, so the downside of trying them is small. The downside of dropping protection on an unproven pairing is the harvest.
Common Mistakes
- Believing a pairing because it is in many books. Repetition is not evidence.
- Replacing a barrier with a herb. Do both until you have data.
- Overplanting the companion. It competes with the crop for water.
- Not recording. Without a log next year’s gardener starts from folklore.
What’s Next
- Learn next: Trap Cropping Strategies — plants that pull pests away
- Combine with: Beneficial Insect Attractors — the better-supported route