8 Companion Plants for Broccoli That Cut Aphids and Deter Cabbage Worms
Iowa State research found thyme best protects broccoli against cabbage worms. Plant these 8 companions and avoid the 4 that make pest pressure worse.
A 2006 Iowa State University study tested multiple companion plants alongside broccoli and found that thyme ‘might have been its best protector’ against cabbage loopers and imported cabbageworm — ranking ahead of marigolds, onions, and nasturtiums. That finding captures something important about growing broccoli with companions: the research exists, specific plants genuinely help, and the popular guides rarely cite the actual data.
Broccoli’s main pest threats — imported cabbage worm (Pieris rapae), cabbage loopers, cabbage aphids, and flea beetles — locate their host plants primarily by scent. All brassicas emit glucosinolate compounds that act as chemical beacons for these insects. The right companion plants disrupt that scent signal, support the natural enemies that hunt those pests, or draw insects away from the main crop altogether. For a broader look at how vegetables pair in the garden, see our Vegetable Companion Planting Guide.
Below: the 8 companions with the strongest evidence, a quick-reference table with evidence ratings, a bed layout for a standard 4×8 raised bed, and 4 plants that actively work against broccoli. If you’re still getting your broccoli crop established, our Broccoli Growing Guide covers planting, spacing, and harvest timing first.
Why Companion Planting Works for Broccoli
Three documented mechanisms explain most of what companion plants actually do:
- VOC masking. Aromatic plants release terpenes — thymol, carvacrol, allicin — that interfere with the olfactory cues pest insects use to locate brassica hosts. Research published in PMC confirmed that 86.67% of Aphidius colemani parasitoid wasps responded to the specific volatile profile of aphid-infested brassica plants. When that profile is diluted by surrounding aromatic plants, pests find it harder to lock onto their target.
- Beneficial insect habitat. Nectar-rich flowers sustain parasitoid wasps and hover flies throughout the broccoli growing season. These predators locate and attack pests more efficiently when they have a reliable carbohydrate source nearby. Without flowering companions, they search farther afield and spend less time in your bed.
- Trap cropping. Some companion plants are more attractive to target pests than broccoli itself. A nasturtium row at the garden’s edge pulls aphids and cabbage moth adults toward it — concentrating damage away from the main crop, if positioned correctly.
Illinois Extension (2025) is candid about the limits: ‘There hasn’t been a lot of research done, especially on a garden scale, on companion planting.’ The best-documented effects come from specific field trials, and that’s where this guide starts.
The 8 Best Companion Plants for Broccoli
1. Sweet Alyssum
Research by Micaela Colley and John Luna at Oregon State University on a commercial broccoli farm near Corvallis found that parasitism of cabbage aphids by parasitoid wasps was nearly doubled in broccoli plots interplanted with sweet alyssum, compared to pure broccoli stands. More hover fly adults and their eggs also appeared in alyssum plots.
The mechanism: alyssum’s small white flowers provide nectar for Aphidius species and other parasitoid wasps throughout the growing season. These wasps use the volatile chemical signals from aphid-infested brassicas to locate their targets; a reliable nectar source keeps them active in the area longer.
Honest limitation: USDA ARS researcher Dr. Eric Brennan notes that controlling cabbage aphids on broccoli is harder than on lettuce because aphids hide in the flower head’s crevices. The OSU study found doubled parasitism rates but no statistically significant reduction in actual aphid numbers. Alyssum builds the biological control infrastructure in your garden — the full benefit may take more than one season to become obvious.
Plant sweet alyssum between broccoli plants and along the bed border. Direct seed after last frost or start transplants 4 weeks early. Allow it to bloom continuously; it self-seeds freely and often returns the following year.
2. Thyme
The Iowa State study found that for broccoli specifically, thyme ‘might have been its best protector’ against cabbage loopers and imported cabbageworm — no other companion tested matched it for broccoli performance in that trial.
The probable mechanism: monoterpenes in thyme — primarily thymol and carvacrol — are thought to interfere with the olfactory receptors that adult Pieris rapae butterflies use to identify suitable brassica hosts for egg-laying. Fewer eggs laid means fewer caterpillars hatching on your heads.
Thyme is perennial in USDA zones 5 and warmer, so a single planting creates a permanent companion bed feature. Plant it as a low border around the broccoli patch or interplanted directly between heads. Let thyme flower rather than trimming it entirely — the small blooms attract parasitoid wasps that attack cabbage worm eggs.
3. Nasturtium (With a Spacing Warning)
The Iowa State study listed nasturtiums among the companions that reduced cabbage looper and imported cabbageworm damage in broccoli. They function as a trap crop: aphids and adult cabbage moths are drawn to nasturtiums’ vivid blooms and peppery foliage ahead of the lower-profile broccoli plant.
Critical spacing rule: Aphids that build up on nasturtium foliage can overflow onto adjacent broccoli when populations spike — especially in warm weather. Plant nasturtiums at least 18–24 inches from your broccoli plants, or better still, place them as a perimeter row outside the main bed. Check nasturtiums weekly; if a dense aphid colony forms, knock it off with a strong jet of water or remove the plant entirely before the colony migrates inward.
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Nasturtium leaves and flowers are edible — peppery and slightly tangy — which makes them a productive dual-purpose plant even in low-pest years.
4. Onions, Garlic, and Chives
The Iowa State study listed onion among the companions that reduced cabbage looper and imported cabbageworm damage in broccoli. The mechanism goes beyond simple scent masking.
When alliums grow, their roots and decaying leaves release sulfur-based compounds — allicin, diallyl disulfide, and related molecules — that deter aphids and the cabbage root fly (Delia radicum), which lays eggs at the base of brassica stems and causes root damage in young plants. These sulfur compounds also attract ground beetles, which prey on soil-dwelling pest larvae.
Garlic planted between broccoli heads shares convenient timing — garlic is harvested before broccoli reaches full canopy size, so there’s no long-term shade competition. Chives make a low, non-shading border. Space alliums through the bed rather than grouping them, to distribute their deterrent effect more broadly.
5. Dill
Dill doesn’t repel broccoli pests directly. What it does is build the biological control force that fights them.
When dill bolts and flowers, its flat-topped umbel blooms provide nectar and pollen for parasitoid wasps — including Cotesia glomerata, a braconid wasp that parasitizes imported cabbage worm larvae — and for Trichogramma egg parasitoids and hover flies. The open structure of dill’s umbels is specifically accessible to smaller wasp species that can’t reach nectar in deep tubular flowers.
Let at least a few dill plants go to seed rather than harvesting all of them. A dill plant allowed to flower at the bed’s edge does more for pest control than a dozen plants harvested before bloom. One practical note: keep dill away from fennel — they cross-pollinate readily, reducing quality in both.
6. Marigolds
Marigolds appeared in the Iowa State study among the companions that helped reduce cabbage worm damage in broccoli. Their primary documented role is attracting and sustaining hover flies, whose larvae feed on aphid eggs and young nymphs throughout the season.
Honest caveat on flea beetles: UMN Extension specifically notes that marigolds are widely promoted to repel flea beetles from brassicas, but states ‘there is little research to support this.’ Expect hover fly support — not flea beetle control.
French marigolds (Tagetes patula) are preferable to African marigolds for interplanting — they stay smaller, won’t shade broccoli leaves, and flower more continuously. Let them bloom fully rather than constantly deadheading; open flowers attract the hover flies.
7. Lettuce or Spinach
Lettuce and spinach are the most practical broccoli companions — not for pest control, but for space efficiency and microclimate benefits.
Their shallow root systems occupy the top 4–6 inches of soil while broccoli roots reach 12–18 inches down, so there’s no real competition for water or nutrients at the same depth. Broccoli’s large outer leaves shade the soil surface, which prevents lettuce and spinach from bolting in spring warmth and stabilizes soil moisture around the broccoli stems.
In return, the low-growing greens act as a living mulch — suppressing weeds and reducing soil splash from rain, which lowers the risk of fungal spores reaching lower broccoli leaves. Direct-sow seeds between broccoli transplants at planting time. Harvest the greens before broccoli reaches full canopy width.
8. Chamomile
Chamomile’s flat, open daisy flowers are excellent all-season habitat for ground beetles, parasitoid wasps, and hover flies. The shallow flower structure is specifically accessible to short-tongued beneficial insects — including many wasp species that can’t reach nectar in deeper tubular blooms.
German chamomile (Matricaria chamomilla) is the self-seeding annual; plant it once and it returns reliably. Roman chamomile (Chamaemelum nobile) is perennial in zones 4 and above and spreads to form a low mat useful as a border.
Honest caveat on flavor: The claim that chamomile ‘improves broccoli’s flavor’ circulates widely in companion planting guides. Like the tomato-basil flavor improvement claim — disproven by a 3-year West Virginia University double-blind taste trial — it has no experimental backing. Plant chamomile for its documented beneficial insect role, not for flavor effects.
Companion Plant Quick Reference
| Companion | Primary Benefit | Key Mechanism | Evidence Level |
|---|---|---|---|
| Sweet alyssum | Aphid parasitoids | Nectar for wasps and hover flies | Field trial (OSU) |
| Thyme | Cabbage worm deterrence | Monoterpene VOC masking | Field trial (Iowa State) |
| Nasturtium | Trap crop | Aphid and moth magnet — keep 18+ in. away | Field trial (Iowa State) |
| Onion / Garlic / Chives | Root fly and aphid deterrence | Sulfur compounds (allicin) | Field trial (Iowa State) |
| Dill | Parasitoid wasp habitat | Umbel nectar when in flower | Consensus + parasitoid research |
| Marigolds | Hover fly attractor | Abundant nectar throughout season | Field trial (Iowa State) |
| Lettuce / Spinach | Living mulch, space efficiency | Root depth compatibility | Practitioner consensus |
| Chamomile | Broad beneficial insect habitat | Shallow flower, accessible nectar | Practitioner consensus |
How to Design Your Companion Bed
Density and timing matter as much as plant selection. Dr. Michael Bomford’s research, cited by Illinois Extension, emphasizes that companion planting success depends on ‘crop density, ratio, and relative planting times’ — not simply which plants you combine.
For a 4×8 raised bed with 4–6 broccoli plants:
- Plant thyme or chives as a perimeter border at 6-inch spacing along all edges
- Place sweet alyssum between every 2–3 broccoli plants in the bed rows
- Set nasturtiums outside the bed perimeter, at least 18 inches from the nearest broccoli plant
- Alternate marigolds and dill in open corners and at the bed ends — let both flower fully
- Fill remaining gaps with lettuce or spinach seeds at transplant time
The key variable is timing: alyssum, marigolds, and dill must be in bloom when broccoli aphids and cabbage worm adults are active. Start companion transplants indoors 4–6 weeks before your last frost date, alongside your broccoli seedlings. For zone-by-zone planting windows for cool-season crops, our year-round planting guide covers the full calendar. You can also review how broccoli compares to its closest relatives in our broccoli vs. cauliflower guide, which affects how you rotate your brassica beds.
4 Plants to Keep Away from Broccoli
Fennel
Fennel is the most reliably harmful companion in the vegetable garden. Its roots and decaying leaves release anethole and fenchone — compounds that interfere with seed germination and root development across a wide range of vegetable species. Research on fennel allelopathy documents these chemicals reducing root elongation in sensitive plants within 72 hours of exposure, with soil effects that can persist for more than one growing season after the plant is removed.
If you grow fennel for culinary use, keep it in a container at least 3 feet from any vegetable bed. Never add fennel material to compost that goes back on vegetable beds. Fennel does attract parasitoid wasps when in flower — but that benefit doesn’t outweigh the root damage risk when planted in proximity.
Other Brassicas (Cabbage, Cauliflower, Kale, Brussels Sprouts)
Grouping broccoli with other brassicas multiplies pest pressure. Cabbage aphids, imported cabbage worm adults, and flea beetles are drawn to the combined glucosinolate signature of multiple brassica species — a larger, more reliably detectable signal than a single plant. Brassica family members also share soil diseases, particularly clubroot (Plasmodiophora brassicae), which spreads rapidly when the same family stays in one area.
Keep broccoli in rotation — at least 2–3 years before any brassica family member returns to the same bed. If you do grow broccoli and cauliflower in the same season, place them in separate beds with a different crop family between them.
Tomatoes and Other Nightshades
Tomatoes, peppers, and eggplants are heavy nitrogen feeders. Broccoli also needs significant nitrogen to develop dense heads. Planted together, they compete directly for the same nutrient pool and one will underperform. There’s also a pest-management ecosystem mismatch: the beneficial insects most effective against hornworms — the primary nightshade pest — are different from those that target brassica aphids and cabbage worm. Mixing the two families in a small bed dilutes both pest-control systems without strengthening either.
Strawberries
Strawberries are classified as heavy feeders, and their spreading runners can block airflow around broccoli stems, raising humidity at ground level and increasing the risk of fungal disease. Multiple sources report that broccoli grown alongside strawberries may show slower growth and smaller head development, though controlled studies are limited. Strawberries also prefer a slightly lower soil pH than broccoli, creating a secondary soil chemistry conflict when both are amended in the same bed. Keep them in separate, dedicated areas.
Frequently Asked Questions
Can I plant broccoli and cauliflower in the same bed?
Better to separate them. They share the full brassica pest lineup — imported cabbage worm, aphids, cabbage loopers, flea beetles — and grouping them doubles the scent target that attracts these pests. Stagger them in different beds and rotate each annually.
How close should aromatic companions be to broccoli?
Within 12–18 inches for VOC concentrations to be effective. Trap crops like nasturtiums should be farther — at least 18–24 inches — so aphid colonies don’t overflow onto broccoli. Beneficial insect plants (sweet alyssum, dill, marigolds) can sit anywhere in the bed because their effect works through the insect population, not through close-range scent.
Will companion planting eliminate pests entirely?
No. Even in the Iowa State study, companions reduced pest damage rather than eliminating it. Think of companions as reducing pest pressure and building a more resilient garden ecosystem. For heavy infestations, hand-picking caterpillars and using row covers remain the most reliable controls — especially at transplant time when seedlings are most vulnerable.
When should I plant broccoli companions?
Transplant broccoli seedlings 4–6 weeks before your last frost. Plant thyme, garlic, and onions at the same time. Start sweet alyssum, marigolds, and nasturtiums indoors 4–6 weeks before last frost, or direct-sow after last frost. Dill can be direct-sown as soon as soil is workable. Time all flowering companions to be in bloom while broccoli is actively growing — not after the heads have been cut.
Does chamomile really improve broccoli’s flavor?
This claim appears in many companion planting guides but has no experimental backing. There is no documented mechanism by which a neighboring plant alters the flavor profile of a fully formed vegetable head. Plant chamomile for its proven role as a beneficial insect habitat — that benefit is well-supported.
Sources
- Companion Planting in Home Gardens — University of Minnesota Extension
- Growing Broccoli in Home Gardens — University of Minnesota Extension
- Companion Planting: Combining Plants for a Healthy, Well-Balanced Garden — Illinois Extension, 2025
- Response of Aphid Parasitoids to Volatile Organic Compounds from Undamaged and Infested Brassica oleracea with Myzus persicae — PMC
- Flower Power: Attracting Natural Enemies of Pests to Your Field and Garden — USDA ARS
- Riesselman LB (2006). Companion Planting: A Method for Sustainable Pest Control. Iowa State Research Farm Progress Reports. Cited via UMN Extension companion planting guide.
- Colley M, Luna J. Alyssum attracts predator insects into broccoli fields. Oregon State University research, Stahlbush Island Farm, Corvallis OR. Cited via OSU Newsroom (news.oregonstate.edu).
