Answer: The most important aspects of saving seed include harvesting the mature seed from your best plants, then store it cool and dry. Be certain the seeds are dry, then place them in a closed container in the refrigerator to keep them fresh.
Ideally you would save seed from open pollinated varieties. Seed saved from hybrid plants may or may not be viable and the resulting plants may or may not be similar to the parent you took the seed from.
You should still be able to grow radishes and greens such as lettuce and spinach from seed as a fall crop. It is a bit late now but next year you could consider growing peas, cole crops such as cabbage or bok choy, as well as beets to harvest in the fall. Fall crops usually take about ten days longer from seed to harvest due to the shortening days.
Depending on whether your greenhouse is heated or not, and how warm you keep it, you may want to experiment and see how the above do for you.
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