Plant spring radish seeds directly in the garden starting 4-6 weeks before average last spring frost. Optimum soil temperature for seed germination is 50-65 degrees. Since they mature so quickly, spring radishes are often planted with carrots and parsnips, between slowly growing cole crops, or between small tomato and pepper plants. Plant every 2-3 weeks for a continued harvest until about 4 weeks after average last frost. Can be sown again in early fall. Plant winter radish varieties 6-8 weeks before average first frost.