How to Grow Radishes

Cool weather and fast timing

Radishes want fertile, well-drained, deep sandy soils rich in organic matter, and above all they want cool weather. Sow the spring crop 2 to 3 weeks before the last frost, once soil hits about 40°F; seeds germinate best at 55 to 75°F. For fall, start in mid-to-late August to avoid summer heat.

Plant seed half an inch deep and thin to 1 to 2 inches between plants in rows 10 to 12 inches apart. Spring types mature in 25 to 45 days depending on variety, so succession-sow a short row every week or two rather than one big planting.

The when-to-plant-radishes calculator localizes the spring and fall windows to your frost dates.

Why radishes go wrong

All leaves and no bulb usually traces to one of three things: sowing too thickly without thinning, temperatures climbing above 80°F, or too little sunlight. Radishes are a race against heat, and a crowded, hot, shaded row bolts to top growth instead of forming roots.

A hot, pungent radish is one that grew too slowly, got heat-stressed, or was left in the ground too old. The fix is fast, cool growth and prompt harvest.

Cracked or pithy roots come from moisture swings — a heavy watering after a dry spell will split mature roots, and old radishes split on their own. Frequent, uniform water is the prevention.

Harvest

Pull radishes when the roots reach full size, generally within that 25-to-45-day window; there is no benefit to leaving them, and every extra day risks pithiness and heat.

Because they are so quick, radishes make an excellent row marker for slow crops like carrots or parsnips — they come up fast and are harvested before the slow crop needs the space.

The when-to-harvest-radishes calculator gives the days-to-maturity readout for your variety on its own.

Sources

Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.

• Utah State University Extension — Radishes in the Garden — Radishes prefer fertile, well-drained, deep sandy soils rich in organic matter; plant 2-3 weeks before the last frost when soil reaches 40°F (germinating best at 55-75°F) and again mid-to-late August for fall; sow seeds 1/2 inch deep and thin to 1-2 inches apart in rows 10-12 inches apart; roots mature 25-45 days from seeding depending on variety; water frequently and uniformly because moisture fluctuations cause root cracking; all-tops/bolting is caused by planting too thickly without thinning, temperatures over 80°F, or insufficient sunlight; radishes that grow too slowly, are heat stressed, or are very old are 'hot'; old radishes split and a heavy watering after a dry period causes mature roots to split; harvest when roots reach full size.

Keep going