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The exact way you dry seeds depends on the type of plant they come from, but the key aspect of seed drying is controlling the temperature and humidity around the drying area. You'll also need to consider the type of plant you're harvesting from, since the seeds from different plants need to be harvested at different times. If you're very concerned about getting the exact same plant for the next year, then you'll have to be very careful to avoid cross-pollination.
The two main methods of seed drying are wet processing, which is used for plants that produce seeds in fruit or another wet medium; and dry processing, which is used for plants that produce dry seeds, like those that produce pods.
When the seeds are completely dry, they are ready for storage. You should remove any extra chaff or leftover dirt by gently brushing the seeds with your fingers or by sifting them through a fine sieve — even if an individual seed is too large to go through, small bits of material will. Store them in paper envelopes labeled with the type of seed they store. Whether you're wet or dry processing, it's important to control the environment around the drying area. Temperatures between 40° and 85°F (4.5° to 30°C) are best.
Different seeds are harvested at different times. Most fruit seeds can be extracted after ripening, but before rotting. Squash, cucumber, and pumpkin should be left on vine until after the first frost. The seeds can then be separated from the pulp and dried at room temperature. Pod plants and seed heads should be left to dry on the vine, and the seeds should be gathered before dispersion. Biennial crops, primarily made up of root plants, do not produce seed at the end of the growing season; instead, the roots should be dug up during the fall and stored at a temperature between 32° and 45°F (0° to 7°C) through the winter.
If you're very concerned about getting the exact same type of plant for the next year, then you'll need to make sure that you only harvest from specific ones. For the plant to be the same as its parent, it can only be pollinated from plants of the same variety. For airborne pollinated crops, no other varieties can be within 1 mile (1.6 km). Insect pollinated plants must be at least 0.25 mile (0.4 km) away from other varieties. Self-pollinated plants have no risk of cross-pollination.