The secret to growing a great radish is to plant when the weather is cool, to not plant too close together, and to harvest promptly before the roots get woody and bitter. Exceptionally loose soil promotes healthy larger radishes.
Plant in early spring or fall when the weather is cooler.
Radishes like very rich loose compost, ample fertilization, and water. Try planting radishes in groups of 3 to 4 seeds per hole and plant about 1/2 to 1 cm deep, spacing the holes 15-20 cm apart and harvest the largest radish in each group. Alternately plant in rows spacing seeds 8 to 12 cm apart and thinning to 4 seedlings per 30 cm.
The use of row covers helps prevent root maggots.