French and signet marigolds can be planted anytime through midsummer, but the tall American marigolds are best planted right away in the spring (after danger of frost has passed) because they are slower to mature.
Sow seeds directly into the garden once the soil is warm in the spring. You can start seeds indoors, but they germinate so easily outside that there’s really no advantage. Marigolds sprout within days in warm weather and plants bloom in about 8 weeks.
CHOOSING AND PREPARING A PLANTING SITE
Marigolds thrive in full sunshine and can often withstand very hot summers.
If planted in shade and cool, moist areas, they are prone to powdery mildew and won’t bloom well.
Though they grow in almost any soil, marigolds do best in moderately fertile, well-drained soil.
Prepare the soil by digging down about 6 inches to loosen it. Remove stones.
HOW TO PLANT MARIGOLDS
Optional: Add some slow-release (granular) fertilizer in the planting hole. A 5-10-5 works fine.
Moisten the soil, then sow seeds 1 inch apart and no more than 1 inch deep.
While still small, thin the seedlings. Space French and signet types 8 to 10 inches apart. Larger American varieties should be at least 10 to 12 inches apart.
If planting transplants, thoroughly water each plant after planting in the garden.
If planting in containers, use a soil-based potting mix. Either mix in slow-acting granular fertilizer at planting time or plan to water with diluted liquid fertilizer periodically. Take care to space properly; marigolds grown in containers can become crowded.