Answer: Allow the flowers to mature fully on the plant so that they brown and dry. When quite dry the seeds inside should be ripe. Pick the flower heads and place them in a dry location to dry some more. You can break them open to free the seeds to help them dry better. When completely dry, store in a jar or other air tight container in the refrigerator or other cool dry location until spring. (Check for condensation -- if there is any moisture the seeds will rot.) If you grew hybrid zinnias, keep in mind that the seeds may or may not be viable and the offfspring probably will not exactly resemble the parent plant.
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