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You can let the radish form aYou can let the radish form a seed stalk with pods (at this point, the radish itself will not be tasty to eat), and leave them on the plant to dry as long as possible. If necessary, you can pick the maturing seed stalk and hang it up in a cool, dark, dry place to finish drying. The pods will turn brown. Once the pods are fully dry and brown, you can break open the pods (you might need to use a mallet on some of the pods), gather the seeds, and store them in a container in a cool, dark, dry place for up to 4 years. (Note that this method should work fine with an heirloom variety, but if your mother plant was a hybrid, the seeds may not produce offspring with the same features.)

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