Companion planting — growing certain plants together to influence pest activity — is one of the oldest agricultural practices. Records of companion planting go back thousands of years. The modern version combines traditional knowledge with ecological research on plant volatiles, beneficial insect habitat, and pest-plant interactions.
The evidence base is uneven. Some companion planting practices are well supported by research. Others are based on tradition and anecdotal reports with limited or conflicting scientific data.
How Companion Planting Works (in Theory)
Several mechanisms may explain companion planting effects on pests:
Volatile repellency: Some plants release volatile organic compounds that deter specific pests. Aromatic herbs like basil, mint, and rosemary are rich in volatile compounds that certain insects avoid.
Trap cropping: Sacrificial plants attract pests away from the primary crop. Nasturtiums attracting aphids away from vegetables is the classic example.
Habitat for beneficial insects: Flowering plants provide nectar and pollen for parasitic wasps, hoverflies, lacewings, and other predators that feed on garden pests. This is the best-supported companion planting mechanism.
Masking: Strong-scented plants may make it harder for pests to locate their host plants by scent. This is sometimes called the “confusion effect.”
Physical barriers: Taller or denser companion plants can physically shield vulnerable plants or disrupt pest movement.
Well-Supported Combinations
Insectary Plantings (Beneficial Insect Habitat)
The most well-documented companion planting strategy is planting flowers that attract beneficial insects. USDA research and multiple university extension programs confirm that insectary plantings increase populations of natural pest enemies.
Effective insectary plants include:
- Alyssum — attracts hoverflies, whose larvae consume large numbers of aphids. A study from Oregon State University found that alyssum planted between lettuce rows reduced aphid populations significantly
- Dill, fennel, and other umbellifers — attract parasitic wasps that attack caterpillars, aphids, and other pests
- Yarrow — attracts ladybugs, lacewings, and predatory wasps
- Marigolds (see below for additional effects)
- Sunflowers — attract predatory insects and serve as trap crops for stink bugs
- Buckwheat — provides nectar for parasitic wasps and other beneficials
Marigolds
Marigolds are perhaps the most studied companion plant:
- French marigolds (Tagetes patula) suppress root-knot nematodes in the soil. This effect is well documented in research from multiple institutions. The mechanism involves thiopene compounds released from marigold roots
- Marigolds are sometimes reported to repel whiteflies. A 2019 study from Newcastle University found that French marigolds planted alongside tomatoes reduced whitefly numbers, with the effect attributed to the volatile compound limonene
- The common claim that marigolds repel all garden pests broadly is overstated — the documented effects are specific to certain pest-plant combinations
Basil and Tomatoes
The classic companion planting pair. Some research supports the combination:
- A study published in the Journal of Chemical Ecology found that basil volatiles can repel thrips and may reduce aphid colonization on neighboring plants
- Some homeowners report fewer tomato hornworms when basil is interplanted, though controlled studies on this specific interaction are limited
- At minimum, basil does not compete with tomatoes for resources and attracts pollinators
Nasturtiums as Trap Crops
Nasturtiums attract aphids, cabbage white butterflies, and whiteflies, potentially drawing them away from vegetable crops. Research from the Royal Horticultural Society and other institutions has found mixed results:
- Nasturtiums do attract aphids reliably
- Whether this actually reduces aphid numbers on adjacent crops depends on the situation — if the nasturtium trap crop is too close or too small, it can increase overall aphid populations in the area
- Effective trap cropping may require removing and destroying the infested trap crop periodically
Moderately Supported Combinations
Garlic and Onions (Alliums)
Alliums planted near roses, carrots, and brassicas are traditionally said to repel aphids, carrot flies, and cabbage pests. The sulfur compounds in alliums do have documented repellent properties against some insects. However:
- Interplanting garlic among roses has limited formal research supporting aphid reduction
- Some studies on carrot fly have shown modest reduction when onions and carrots are interplanted
- The effect may be more about scent masking than direct repellency
Mint Near Brassicas
Mint is reported to deter cabbage moths and flea beetles. Mint’s strong volatile compounds are plausible as repellents. Practical limitation: mint is aggressive and invasive. Growing it in pots near brassicas is more manageable than planting it directly in the garden bed.
Rosemary and Sage
These Mediterranean herbs are reported to deter cabbage moths, carrot flies, and bean beetles. The aromatic compounds are documented insect repellents in extracted form. Whether the growing plants release sufficient volatiles to affect pest behavior in open-air garden conditions is less certain.
Weakly Supported or Debunked Combinations
Citronella Grass for Mosquitoes
While citronella oil is a documented mosquito repellent, growing citronella grass in the garden does not release enough volatile compounds to repel mosquitoes from an area. The oils must be extracted and applied. Multiple university extension services note this distinction.
Lavender for General Pest Control
Lavender is frequently described as a pest-repelling plant. While lavender oil has some documented repellent properties, the plant itself growing in a garden has not been shown to meaningfully reduce pest populations in adjacent areas. It does attract pollinators, which is beneficial.
Chrysanthemums for All Pests
Chrysanthemums contain pyrethrin, a natural insecticide. However, the pyrethrin is in the flowers and must be extracted and concentrated to have insecticidal effect. Growing chrysanthemums near other plants does not produce a meaningful pest control effect.
Practical Approaches
For gardeners interested in companion planting for pest management:
Focus on insectary plantings. Attracting beneficial insects has the strongest evidence base and provides broad-spectrum pest management. Border the garden with alyssum, yarrow, dill, and other flowering plants.
Use trap crops strategically. Nasturtiums, sunflowers, and sacrificial plantings can be useful if managed actively — monitor trap crops and remove heavily infested plants before pests spread.
Plant diverse gardens. Monocultures are more vulnerable to pest outbreaks. Mixed plantings create habitat complexity that supports natural predator populations and makes it harder for specialized pests to find their host plants.
Maintain realistic expectations. Companion planting is a supplementary strategy. It reduces pest pressure at the margins but does not replace other management practices — row covers, hand picking, biological controls, and proper plant care.
Do not expect plants to function as insecticides. A basil plant next to tomatoes may have a modest effect on certain pests; it will not prevent all insect damage. Companion planting works best as part of a diverse, well-managed garden ecosystem.
For serious infestations, consult a licensed pest control professional.