Culture: Radishes are not all that fussy to grow. Sow summer varieties in a sunny site as soon as the ground is workable in the spring - winter radishes are generally sown in mid-summer. Sow seed every two weeks 12 mm (1/2") deep and about 2.5 cm (1") apart in rows about 30 cm (12") apart. Rapid growth is essential for tasty, tender roots so keep the soil evenly moist during both the germination and growth stages. Harvest in 3-4 weeks when the roots have reached the size of a large marble.