How to grow french marigolds

French marigold is a compact, bushy annual grown for continuous color from spring into fall, provided spent blooms are removed. Plants typically reach 6 to 12 inches tall and 6 to 18 inches wide, staying low enough to edge a vegetable bed without shading neighboring crops. The foliage carries a sharp, resinous scent, and that same chemistry, not the flower color, is why gardeners plant it near vegetables: the roots release a compound tied to suppressing root-knot nematodes in soil.

The hardest part of growing French marigold for nematode control is timing. University of Florida IFAS Extension research shows that tucking marigolds directly among susceptible vegetables, the approach most gardeners try first, does not reduce nematode damage and can even put the vegetable at greater risk, since root-knot nematodes still find and reproduce on its roots regardless. Effective suppression instead requires a dedicated planting of marigolds grown alone in the same ground for at least two months before the vegetable goes in, because the nematicidal compound alpha-terthienyl comes only from living roots. A gardener who tucks a few seedlings among transplanted tomatoes, expecting season-long protection, gets ornamental flowers and no measurable nematode relief.