Marigolds begin to die back in late fall after night temperatures begin to drop below 40 degrees Fahrenheit in areas where marigolds grow as perennials. Cut back the marigolds to the ground with clean shears after they die back completely. Remove the trimmings from the bed so they don't harbor pests over winter. If the marigolds don't survive winter in your area, pull them up after they begin to decline in fall. Floridata: Tagetes Spp.