Marigolds are one of the easiest flowers to grow from seed, and one of the most prolific seed producers. Spent flower heads become brown seedpods, filled with long, black, tufted seeds. Two to three pods will often contain enough seed for a 3-foot-by-10-foot bed of naturalized blooms. Collect the pods before they reopen, or they will be consumed by goldfinches or blown away by the wind.
Look through the blooms in your marigold bed for flowers that have dropped their petals and begun to close. Watch these until they no longer have any color remaining at their tips. If you do not have your own marigolds, ask other gardeners in your neighborhood if they will trade seeds, seedlings or cuttings of other plants for some of their marigold seeds, seedlings or plants.
Pluck the swollen seed pods one or two days after they turn brown. Take only those pods that break away with a touch. Leave any pods that do not break away on the plant for another day or two.
Pop the pods open by pressing them between your thumb and forefinger to reveal the long, black seeds.
Shake the seeds onto a paper towel. Sort out any malformed or small seeds. Keep only the longest, shiniest, blackest seeds, as these will germinate sooner.
Store the seeds from three to five pods in envelopes in a cool, dry place. Each envelope will have enough seed to sow a 3-foot-by-20-foot flower bed if all the seeds germinate.