Updated: Dec 8, 2021
Saving radish seeds is a fantastic way to increase your self-dependency. They are very easy to grow, both as a harvest crop and for seeds. The best part is the speed. Most radishes can be eaten 30 days after you sow seeds. It is especially convenient that they are a dry saving seed, and can be immediately sown once saved.
For crop, you should sow your seeds 4-6 inches apart in rows 2 feet apart, although they can be multi-sown in clumps of 3-4 seeds per hole, just space clumps of radishes no less than 6-8" apart. When saving for seed, space them 1 foot apart, the plants will get bigger than you can imagine. It takes 6 months to get seed from radishes, depending on the weather. So, you may be able to re-sow the new seeds in the fall, for a winter crop.
The varieties can be either or annual or biennial. You need at least 5 to get viable seeds. That's 5 of the same variety. It takes over 800 feet distancing to keep them from crossing. If you are saving them to use for many years, then save seed from 40 plants. If you are preserving it long term, for it's genes, save seeds from at least 80 plants.