Potato + Marigold

French marigolds (Tagetes patula) are one of the more evidence-backed companion plants in vegetable gardening, specifically for nematode suppression. Their roots release thiophenes, compounds toxic to root-knot nematodes (Meloidogyne spp.) that tunnel into potato tubers and cause knobby, unmarketable roots. The effect is soil-specific: nematode pressure tends to be highest in warm, sandy soils in the Southeast and Southwest. In heavy clay or cold northern soils where nematode populations are low, the benefit is minimal.

Timing matters. Marigolds need 4 to 6 weeks of growth before potatoes go in the ground; planting them simultaneously does little. Space marigolds at 12 inches throughout the potato bed rather than just at the border. After the season, till the marigold debris into the soil rather than removing it. Marigolds do not provide meaningful pest control against Colorado potato beetle or late blight, so they work as a nematode management tool, not a general protective companion.