Radishes prefer cool weather. Direct sow seeds outdoors 5/20 – 8/1. Plant seeds ½” deep, in rows spaced 18” apart. Sow 6 seeds per foot and thin to 4-6” apart. Days to germination: 3 - 6 (longer in cool soil). Keep seeds and plants consistently moist. Use crop rotation to avoid diseases (all brassicas). If necessary, cover with row cover fabric to protect against flea beetles and cabbage root maggots.
Harvest radishes when they reach full size. If hot, do not leave roots in the ground too long or they will become pithy. If temperatures are cool, radishes can remain in the ground longer. Use a garden fork to loosen the soil before pulling up. This type of radish stores well; it is very spicy and best eaten pickled or cooked.
Biennial radishes need exposure to cold temperatures in order to flower and produce seed so plan to grow them to be mature in the fall. Radishes are insect pollinated and are cross pollinated – they will cross with all other radishes that are blooming nearby. Isolation distance: ½ mile. Minimum population size: 10-50. Dig radishes and identify best roots for replanting for seed (clip off leaves before replanting). Space 2 - 3 feet apart. Stake plants. Clip seed pods when seeds are fully ripe. Lay out to dry.