Radish Growing Information Raphanus sativus Direct Seed: Small round and French breakfast types can be sown as soon as soil can be worked until 1 month before heavy frost, or in tunnels to extend the season. Watermelon, daikon, and longer maturing specialty varieties are best grown for fall harvest in most regions, sown mid-late summer. Soil temperature for germination is 45-70° F. Spacing: 1” apart in rows that are 8-12" apart for round and French breakfast types. 4-6" in rows that are 18” apart for daikon, watermelon and longer maturing specialty types. Growth needs: Radishes grow best in well-drained, loose soil. Row cover can be used to control pests and for frost control. Radishes are a cool-season crop, so make sure to choose an adaptable variety for warm-weather planting. Hot temperatures can cause roots to be pithy. French breakfast varieties are especially susceptible to pithiness. Daikon, French Breakfast, Watermelon and other longer maturing specialty types grow best for cool weather harvest. Harvest: When of desired size. Fast-growing varieties do not hold well in the field and will become pithy quickly when allowed to grow oversized. Storage: Daikon, watermelon and black Spanish types can be stored topped in perforated bags in a cooler for months.