Radishes prefer the cool temperatures of early spring and late summer. Long rooted types may tolerate heat and long days better. Sow seeds in cool to warm soil (18-26C) 0.5 inch deep in rows 8-12 inches apart. Thin to 2 inches apart once germinated. Radishes are best grown quickly with plenty of water and harvested promptly when ready as they do not keep well in soil. Fertile soil, cool temperatures and consistent moisture will yield the best crops. Sow in succession for an extended harvest.