This depends on the species. Some seeds, especially from Alpine plants, do not survive dry storage and need to be stored moist in a plastic bag of damp sand or vermiculite, or sown immediately. Some seeds can be dry-stored but lose viability quickly; such seeds will last longer if refrigerated and are best if sown within six months of harvest. Some seeds have been known to survive dry storage for thousands of years. Once again, you can post questions about specific seeds to the Growing From Seed forum.
The seed of *most* flowering plants stays viable longest if dried before being stored. Once seed has ripened and at least begun to dry on the parent plant, it should be harvested and placed in a cool, dry, shaded location with good air circulation to complete the process slowly. Be patient; some seeds may need up to six weeks to dry thoroughly. Too much heat can cause seeds to dry too quickly, dessicate, and die.
Once the seeds have ceased any changes in appearance (some seeds will darken, shrivel, etc. during the drying process), they can be safely stored for later use.
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