Companion Plants for Broccoli: Trap Crops and Wasp Food

Feed the wasps, and give the butterfly a decoy

The insects that control cabbage worms are tiny parasitic wasps, and adult wasps live on nectar. They cannot use deep flowers, so the flat, open flower heads of umbrella-shaped herbs are what keep them in your garden.

Trap cropping is the other half. Nasturtium is a favorite egg-laying target for cabbage whites, so planted at the bed edge it pulls eggs away from the broccoli and shows you when pressure starts.

• Dill and cilantro left to flower: flat flower heads feed the parasitic wasps that kill cabbage worms.
• Nasturtium at the edge: a trap crop the cabbage white prefers over your broccoli.
• Beets: no competition, different rooting depth, and they take the same cool season.
• Onions and garlic: take almost no room and pull no brassica pests.
• Thyme and rosemary: aromatic ground cover along the edge, with some evidence they make it harder for the butterfly to find the crop.

What to keep away

Tomatoes and broccoli are both heavy feeders on the same nitrogen, and broccoli is a cool-season crop while tomatoes want heat. They rarely make sense in the same space at the same time.

Strawberries are worth keeping apart too. They compete at the surface and are prone to some of the same soil problems in a crowded bed.

Bulb fennel slows the growth of nearby vegetables, which puts it on this list as it is on every other.

Why brassica rotation is not optional

Clubroot is the reason. It is a soil organism that swells and deforms brassica roots until the plant wilts and dies, and its resting spores survive in soil for about 7 years. There is no cure once a bed has it.

The brassica family is bigger than people expect: broccoli, cabbage, cauliflower, kale, Brussels sprouts, kohlrabi, collards, mustard greens, bok choy, turnips, radishes, and arugula. All of them host clubroot, and all of them host the cabbage root maggot.

Treat the entire family as one rotation group and give a bed 3 years minimum before any of them returns. That includes the radishes you tucked in as a quick filler.

• Do not follow broccoli with any brassica, and that includes radishes and turnips.
• Give the bed 3 years minimum before the family returns.
• Clubroot spores survive about 7 years and cannot be treated.

What matters more than any pairing

Broccoli bolts into small, loose, bitter heads when it gets hot, so timing beats every companion decision. Get it in early enough to head up before real heat, or grow it as a fall crop, which is easier in most of the country because it matures into cooling weather.

After that, a floating row cover put on at transplant is worth more than every trap crop combined. The butterfly cannot lay eggs on what it cannot reach.

The free planner sets spring and fall dates from your own frost dates so the heads form in cool weather instead of bolting.