Radishes are fast-maturing, cool-season vegetables, often ready to harvest in under a month. Timing is paramount because radishes quickly bolt (send up a flower stalk) if exposed to heat. This premature flowering causes the root to stop swelling and become woody, pithy, and unpleasantly bitter or overly spicy.
Optimal Spring Planting Schedule
The first planting should catch the narrow window of cool, pre-summer weather. Begin sowing seeds four to six weeks before the average date of the last expected spring frost. This early start allows roots to develop fully before warm weather arrives. The physical condition of the soil is the best indicator for the earliest planting date. Radish seeds germinate in soil temperatures as low as 40°F but emerge fastest between 55°F and 75°F. Monitoring soil temperature ensures reliable sprouting under preferred cool conditions (ideally 50°F to 65°F).
Succession Planting for Continuous Harvest
Once the initial planting is established, use succession planting to ensure a steady supply of fresh radishes. This involves sowing small batches of seeds every seven to ten days following the first planting. This staggered schedule prevents a single massive harvest followed by a long gap. Continue sowing only as long as the weather remains cool. Stop planting when average daytime temperatures consistently exceed 70°F to 80°F, as heat exposure triggers bolting and ruins root quality.
Timing for Fall and Winter Varieties
The second major planting window opens as the intense heat of summer subsides. Fall crops often develop a milder flavor due to decreasing temperatures and shorter daylight hours.
Spring-Type Radishes
For quicker, smaller spring-type radishes, sowing should resume four to six weeks before the average first expected fall frost date.
Winter Radishes
For larger, slower-maturing varieties, known as winter radishes, the planting date must be earlier to allow for extended development time. Varieties like Daikon or Black Spanish, which take 60 to 70 days to mature, should be planted eight to ten weeks before the first fall frost. This timing ensures roots reach full size just as the weather turns cold, and some varieties improve with a light frost.
Essential Environmental Conditions
Successful radish cultivation requires specific environmental conditions. The soil must be loose, well-draining, and free of large stones or debris; compacted or rocky soil leads to misshapen roots. Incorporating organic matter prior to planting helps create the friable texture necessary for rapid, uniform root expansion. Radishes require a minimum of six hours of full sun exposure daily to fuel their fast growth. Consistent moisture is also required, particularly during the root-swelling phase. Irregular watering causes the radish to become woody, develop cracks, or acquire a sharp, hot flavor.