Start marigolds from seeds indoors in spring, six to eight weeks before your region's last frost date. Or, if you don't mind later blooms, start them after the last frost date. Sow the seeds directly in the ground, covering them with about 1/4-inch of soil. Thin (gently pull out extra seedlings) until they're 8 to 18 inches apart, depending on the variety (check the seed packet).
Alternatively, buy healthy, green plants with no signs of wilting or disease. Plants should be stocky with plenty of leaves. It's actually a plus if they don't have any flower on them so that they'll divert their early energy into root development rather than flowering.
Plant marigold seedlings - either those you've started yourself or those you've purchased - outside in spring after your region's last frost date. They need full sun and rich, well-drained soil.
Position seedlings 8 to 18 inches apart, depending on the variety. Pinch off any blooms at planting time to redirect the plants' energy to their roots for long-term health and better flowering.
Mulch to prevent weeds and conserve moisture.
Fertilize marigolds every four to six weeks, if desired. Organic gardeners may want to simply work in plenty of compost at planting time instead.
Pinch or snap off spent blooms to prolong flowering.
Pull up and discard plants in autumn after frost blackens the leaves.