Spearmint

Spearmint does real work near brassicas — cabbage, broccoli — because its volatile oils interfere with the host-finding of aphids and cabbage moths. Those insects locate their target plants largely by scent, and spearmint's strong terpenoids muddy the signal. It won't eliminate pest pressure, but it reliably reduces the number of insects that land and stay. Carrots benefit for a similar reason: carrot flies (Psila rosae) are put off by the spearmint scent masking the compounds carrots emit. Pair spearmint near tomatoes and you get a similar above-ground disruption effect, though the evidence there is softer than for the brassica pairing.

Marigolds — French marigolds specifically — are worth growing alongside spearmint because they address a different problem layer. Spearmint handles above-ground pest confusion; French marigolds produce thiophenes in their roots that suppress root-knot nematodes in the surrounding soil. The two together cover more ground than either one does alone. One practical note: spearmint spreads aggressively by underground runners, so any low-growing companion — lettuce, radishes — needs a physical barrier or a sunken container between them, or the mint will crowd them out within a single season.

Parsley and chamomile are listed as poor companions mostly because of competitive overlap — same moisture needs, similar growth habit, and neither one holds its own against mint's spreading. Rue (Ruta graveolens) is a harder problem: it produces allelopathic compounds that actively suppress neighboring plants, and mint planted within 18-24 inches of rue tends to show reduced vigor. Keep at least 2 feet of separation, or just don't plant them in the same bed.