Marigold

Tagetes patula — the French marigold — has a reputation that runs ahead of the evidence. The claim that marigolds deter garden pests in general is mostly folklore. The thing that is genuinely documented is more specific: French marigolds planted densely and left in the ground through a full growing season can reduce root-knot nematode populations in the surrounding soil. That effect is real, but it requires commitment — a few plants tucked at the border of a bed will not do it. A full bed planted for the season, turned under at the end, can.

Marigolds are among the most forgiving annuals to start from seed. Sow indoors about 6 weeks before your last frost. Germination is fast — usually 5 to 7 days at 70°F. Transplant about 1 week after your last frost, once night temperatures are staying above 50°F. Space them 10 inches apart. They'll begin blooming in 50 to 60 days from transplant, and they'll continue until a killing frost. Direct sowing is also possible after last frost, though transplants bloom considerably earlier.

The main maintenance task is deadheading. French marigolds are compact enough that they don't need staking, but removing spent flowers extends the bloom period significantly. If the plant is allowed to set seed, it slows production. In a busy garden, deadheading every week or two is enough. The flowers are also edible — both the petals and whole flowers can go into salads or be used as a garnish — though the flavor is bitter and the scent strong.

Botrytis — gray mold — is the failure mode most likely to appear on marigolds, and it tends to show up during cool, wet periods. The affected flowers develop a gray, fuzzy coating and collapse. It's not fatal to the plant if caught early: remove infected flowers immediately, improve air circulation, and avoid watering late in the day. Spider mites can appear during hot, dry spells, causing the foliage to look stippled and faded. A strong water spray along the undersides of leaves can knock them back.

In a companion planting scheme, French marigolds do well planted at the edges of a tomato or pepper bed. The scent from the foliage can reduce thrips and whitefly pressure somewhat, though the effect is modest. The primary documented benefit comes from the roots — specifically the compound alpha-terthienyl, which marigold roots release into the soil and which suppresses certain nematode species. Plan to leave the plants in place all season rather than pulling them early.