Radishes are a fast growing, cool weather crop with brightly coloured roots packed with iron and potassium. As they are so quick to grow, radishes are often used in the garden as a row marker for slower germinating crops. Growing Tips: Sow seed directly outside as soon as the soil can be worked in early spring. For a continual harvest, re-seed every 7 to 10 days throughout the growing season, planting about 12 to 15 seeds per foot, 6 mm ( ¼”) deep in rows spaced 30cm (12”) apart. Thin the seedlings to 4cm (1.5”) .