Answer: You may not be successful in press-drying whole sunflower heads because of the tremendous amount of moisture they contain, not only in the tissues, but in the nuts that are encased in hard shells. To dry the heads so you can harvest the seeds, wait until the petals wilt and the backs of the heads turn from green to tan. Then cut from the plant and hang upside down until thoroughly dry. Rub two heads together to extract the seeds and then lay them out to dry until they're really crispy-dry. To preserve a sunflower for dried flower arrangements, you may want to try using silica gel (available in craft stores), placing a layer on the bottom of a large container, laying your sunflower on top and then sprinkling more silica gel on top. The chemical should draw out all the moisture, but you may have to replace it with fresh silica sometime during the process. To kill insect eggs, treat your dried flowers to a week-long stay in a sealed plastic bag with a mothball or two. When you remove the flowers, place them in an airy place to evaporate the mothball odor. Then store until you're ready to use them in arrangements. Commercially dried sunflower heads are dried at very low heat; the heat treatment kills any insect eggs that might be on or in the head.
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