Companion Planting Herbs for Pest Control: What Really Works

Companion planting herbs for pest control: the chart

Herbs help with pest control in three ways. Scented herbs such as basil, thyme and sage make it harder for moths and thrips to find and lay eggs on a crop. Flowering dill, cilantro and fennel feed the wasps, hoverflies and lady beetles that eat pests. Nasturtiums pull aphids onto themselves as a trap crop. Penn State Extension sums up the first group plainly: herbs “don’t repel insect pests but rather confuse them.” The pairings below are the ones university extension sources connect to a named pest.

Herb	Plant it with	Pest affected	How it helps
Basil	Tomatoes	Thrips, tomato hornworm	Masks the plants from thrips and deters hornworm moths from laying eggs
Basil plus thyme	Tomatoes	Yellow-striped armyworm	Deters moths from laying eggs
Thyme	Cabbage, broccoli and other cole crops	Cabbage looper, imported cabbageworm	Interferes with egg-laying
Dill, sage, chamomile, hyssop	Cole crops	Imported cabbageworm	Deters egg-laying by the adult butterfly
Cilantro, dill, fennel	Lettuce and greens	Aphids	Flowers attract aphid predators
Cilantro, dill	Eggplant	Colorado potato beetle	Builds up predator numbers
Nasturtium	Broccoli, cabbage, kale and other brassicas	Aphids, flea beetles	Trap crop that the pests prefer

All of the herb rows come from Penn State’s review of the research. The nasturtium row comes from Utah State University Extension, which lists nasturtiums as a trap crop for aphids and flea beetles and radishes as a trap for flea beetles, root maggots and cabbage maggots.

Read the chart as a way to lower pest pressure. Expect fewer eggs and fewer aphids. Do not expect a pest-free bed. For pairings between vegetables, see the companion planting chart for the vegetable garden.

What herbs deter pests by confusing them

Many pest insects find a host plant by sight and by the volatile chemicals its leaves give off. Penn State describes how interplanted herbs get in the way. They visually confuse certain pests and limit their landings, they mask the crop’s chemical signal with their own, and they interfere with egg-laying, which cuts the size of the next generation.

Basil with tomatoes is the best-known case. It masks tomato plants from thrips and deters adult hornworm moths from laying eggs. Basil and thyme together deter yellow-striped armyworm moths. Utah State also lists tomatoes and basil as a pair for thrips. A hornworm that is already on the plant will keep eating, so hand-picking still matters. Our tomato hornworm guide covers that step.

Thyme appears twice in Penn State’s list. With cabbage, broccoli and other cole crops, it deters egg-laying by cabbageworms and cabbage loopers. Planted under tomatoes, it forms “a mulch-like carpet that deters egg-laying” by yellow-striped armyworm moths. Sage, chamomile, hyssop and dill are listed for cole crops as well, against imported cabbageworms. A single plant at the corner of a bed does little. These effects depend on the herb growing among the crop plants.

Does cilantro repel bugs?

Cilantro is better described as bait for helpful insects than as a repellent. Once it bolts, its small flowers feed adult hoverflies, lacewings, lady beetles and parasitic wasps, and those insects or their larvae go on to attack aphids and caterpillars. Penn State pairs cilantro, dill and fennel with lettuce and greens for aphid control, and cilantro and dill with eggplant to build predator numbers against Colorado potato beetles.

Field counts support the hoverfly part. In a 2017 to 2018 trial reported by University of New Hampshire Extension, alyssum, buckwheat, cilantro and dill had more hoverflies through the season than calendula, phacelia and fennel. The same report is careful about the next step, stating that more work is needed to show whether those hoverflies make a meaningful difference to aphids on the crop.

The practical point is that cilantro only helps after it flowers. Gardeners who pull it at the first sign of bolting remove the part that matters. If you want leaves and flowers both, sow a second patch for the kitchen and use the tips in how to grow cilantro without bolting on that one.

Does dill repel bugs?

Dill has a foot in both camps. On cole crops, Penn State lists it among the herbs that deter adult imported cabbageworms from laying eggs. In flower, it attracts lady beetles, which eat aphids, according to Utah State, and Penn State includes it with the herbs that bring in parasitic wasps for caterpillar control.

Flower size explains why dill, cilantro, fennel and caraway keep turning up on these lists. University of Wisconsin Extension points out that many of the natural enemies that benefit most from flowers are very tiny wasps, and the best flowers for them are also small. Its example is Queen Anne’s lace, a relative of dill with the same flat flower head: to a hoverfly less than 1/4 inch long, one head is hundreds of tiny nectar flowers. Wisconsin cites an organic market garden in Massachusetts where flowering fennel attracted 48 species of ichneumonid wasps and 8 species of predatory wasps.

What to plant with brassicas to deter pests

Cabbage, broccoli, kale and Brussels sprouts attract a predictable set of pests, and the herb options map onto them.

• Cabbage loopers and imported cabbageworms: thyme between plants, with sage, dill, chamomile or hyssop nearby.
• Aphids: flowering cilantro, dill or fennel within the bed to draw predators, and nasturtiums nearby as a trap crop.
• Flea beetles: nasturtiums or radishes as a trap crop.

A trap crop works by being more attractive to the pest than the vegetable, so it gathers insects by design. Look the trap plants over whenever you check the bed, and cut off growth that is heavily infested. Two insectary flowers need care around brassicas: the New Hampshire trial found that alyssum hosted flea beetles and calendula hosted imported cabbageworm.

Herbs lower egg numbers. They do not stop caterpillars that have already hatched, so pair them with the methods in how to get rid of cabbage worms, and use row covers when butterfly pressure is high.

How to plant herbs so they work

Placement decides whether any of this pays off. Penn State’s wording is to interplant: basil with tomatoes, thyme under tomato plants, calendula near collards. In a raised bed, that means herbs go between and beside the vegetables, with a few at the ends for flowers.

1. Pick the herb for one pest you see every year, using the chart above.
2. Interplant it with the crop it is meant to protect. Thyme can go right under tomato plants.
3. Let part of each dill, cilantro and fennel planting bolt and flower.
4. Resow dill and cilantro through the season. Wisconsin recommends sequential plantings so the flowers are available continuously, with bloom periods that overlap.
5. Inspect trap crops often and remove heavily infested growth.

Whether you start herbs from seed or buy starter plants depends on quantity. Dill and cilantro are cheap to sow in repeated batches, while a few thyme and sage transplants last for years.

Marigolds and other claims to treat with caution

Marigolds are the most repeated companion planting tip, and the research behind them concerns nematodes in the soil, not insects on leaves. University of Florida explains that marigold roots release alpha-terthienyl, a compound that stops root-knot nematode eggs from hatching. French marigolds (Tagetes patula) are effective against the widest range of nematodes.

The method differs from the usual advice, though. Florida’s answer to whether interplanting works is “probably not,” and it calls mixing marigolds with a susceptible crop very risky. Marigolds have to be grown as a solid cover crop in the same spot, planted at least two months before the vegetable, with plants less than 7 inches apart so weeds cannot host the nematodes in between.

Mint, catnip, chives and garlic are often credited with repelling a long list of insects. Utah State does describe garlic as a barrier crop against aphids and says onions, garlic and chives are known to repel common garden pests. Penn State’s review of the research does not include any of the four, so they are left out of the chart. Mint also spreads by runners and belongs in a pot of its own wherever you grow it.

Quick recap

• Basil with tomatoes, and thyme, sage or dill with cole crops, reduce egg-laying by moths and butterflies.
• Cilantro and dill help most in flower, when they feed hoverflies, lady beetles and tiny parasitic wasps.
• Nasturtiums and radishes work as trap crops and need regular checking.
• Interplant companions with the crop they protect and resow dill and cilantro for continuous bloom.
• French marigolds suppress nematodes only as a dense cover crop grown for two months or more ahead of the vegetable.