Most companion planting advice is rooted in tradition. Some of it is well-founded. Some is not. University of Minnesota Extension has reviewed the research behind the most commonly recommended combinations — and the evidence is stronger in some areas than others. Here is what it actually supports.
Quick Answer
Tomatoes: basil and marigolds have the strongest research support — both reduce pest populations. Bush beans fix nitrogen that feeds heavy-feeding tomatoes. Avoid brassicas, fennel, potatoes, and corn near tomatoes — all create pest, disease, or nutrient competition problems.
Beans: most vegetables benefit from being near beans because beans fix atmospheric nitrogen into the soil. The main exceptions are alliums — onions, garlic, leeks, and chives all inhibit bean growth and should not be planted alongside them.
Brassicas: nasturtiums, thyme, and onions have research-backed evidence for reducing cabbage pest damage. Arugula, mustard, and napa cabbage work as trap crops for flea beetles — drawing them away from main brassica crops.
According to University of Minnesota Extension’s companion planting guide, companion plants work through three main mechanisms: emitting odours that repel or mask crop scents from pests; providing habitat for beneficial predatory insects; and using visual diversity to confuse insects that rely on leaf shape or colour to locate their host plants. Not all companion planting claims meet these mechanisms with evidence — but several do. The Iowa State brassica studies and basil-tomato trials are among the most consistently replicated findings in home garden research.
Companion Plants for Tomatoes
Basil is the most evidence-backed tomato companion. UMN Extension cites multiple studies showing that basil reduces thrip populations on tomatoes in both field and greenhouse conditions, and that intercropping with basil may promote tomato growth. The mechanism is scent masking — basil’s volatile compounds disguise tomato plants from insects that use smell to locate their hosts. Marigolds provide a complementary effect, reducing whitefly populations when planted alongside tomatoes. Bush beans planted between tomato rows fix nitrogen that heavy-feeding tomatoes deplete from the soil, and improve air circulation around the base of plants — reducing the fungal disease risk that is a persistent problem in dense tomato plantings. Carrots and radishes can be tucked into gaps, making use of space without competing significantly for resources.
Companion Plants for Brassicas
An Iowa State University study cited by UMN Extension found that thyme, onion, and nasturtium planted together reduced cabbage looper and imported cabbageworm damage in broccoli. A second study found that marigold, onion, and nasturtium achieved the same result in cabbage. Sage and thyme have been shown to reduce diamondback moth populations on Brussels sprouts in greenhouse conditions. For flea beetles specifically — a significant brassica pest — UMN Extension notes that spicier brassica relatives including arugula, mustard, rapeseed, and napa cabbage act as effective trap crops, drawing beetles away from the main planting. Research also shows that diverse compositions of three or more companion species outperform single trap crops in reducing flea beetle damage.
How Beans Benefit the Whole Garden
All beans fix atmospheric nitrogen through a symbiotic relationship with Rhizobium bacteria in their root nodules. West Coast Seeds’ companion planting guide notes that beans grow productively alongside brassicas, carrots, celery, chard, cucumbers, peas, potatoes, radishes, and strawberries. The consistent exception: alliums — onions, garlic, leeks, and chives — inhibit bean growth and should not be planted nearby. Pole beans and beetroot also stunt each other’s development.
| Crop | Best Companions | Mechanism | Avoid |
|---|---|---|---|
| Tomatoes | Basil, marigolds, bush beans, carrots, radishes | Basil and marigolds reduce pest populations (research-backed); beans fix nitrogen; carrots and radishes use space efficiently | Brassicas, fennel, potatoes, corn — all cause pest, disease, or nutrient competition |
| Brassicas | Nasturtiums, thyme, onions, marigolds, arugula (trap crop) | Iowa study: thyme + onion + nasturtium reduced cabbage looper and cabbageworm; arugula draws flea beetles away | Tomatoes, peppers, potatoes — different pH preference; also attract competing pests |
| Beans (all) | Most vegetables — brassicas, carrots, cucumbers, peas, potatoes, strawberries | Nitrogen fixation benefits all heavy feeders; beans are broadly compatible | Onions, garlic, leeks, chives — inhibit bean growth; beetroot stunts pole beans |
What to Do
- Plant basil between tomato plants at 30cm spacing— this is the single most research-supported tomato companion combination; plant at the same time as tomato transplants go in
- Add a row of marigolds around the tomato bed perimeter— French marigolds (Tagetes patula) have the strongest evidence for whitefly reduction; single-flowered varieties are more accessible to beneficial insects
- Interplant nasturtiums at the edge of brassica beds— combined with thyme and onions, Iowa research found meaningful reductions in the two most damaging brassica caterpillar pests
- Use arugula or mustard as a sacrificial trap crop for flea beetles— sow a row alongside brassicas; flea beetles prefer these and concentrate there rather than on your main crop
- Grow bush beans between rows of tomatoes or brassicas— nitrogen fixation improves the soil for the whole bed; beans also suppress weeds in the gaps between larger plants
- Keep onions and garlic away from beans— this is one of the most consistently documented incompatibilities in companion planting research; alliums inhibit bean root development
Common Mistakes
- Planting tomatoes next to brassicas— both are heavy feeders that compete for the same nutrients; brassicas also attract aphids and cabbage worms that can spread to tomatoes
- Treating all companion planting as equally proven— UMN Extension is clear that research supports some combinations and not others; many claimed combinations have no documented mechanism and should not be relied upon as the primary pest management strategy
- Planting fennel in the vegetable garden— fennel releases allelopathic compounds that inhibit the growth of almost all neighbouring plants including tomatoes, beans, and most herbs
At Planting Time
- Plant basil between tomatoes as transplants go in
- Sow nasturtiums at brassica bed edges — they grow quickly
- Sow arugula or mustard as flea beetle trap crop alongside brassicas
- Keep onions and garlic well away from bean rows
Through the Season
- Add marigolds around tomatoes once established
- Check trap crops first — flea beetles should concentrate there
- Let bush beans grow on undisturbed — roots fix nitrogen continuously
- Remove nasturtiums if aphid colonies become too large to manage