Korean Radish

At a glance

Tip for beginners: Sow on the 10-week fall date, cover the row with row cover the same day, and thin to one plant every 4 to 6 inches within two weeks — those three steps prevent most problems

Main challenge: Timing — spring sowings often bolt, and late-summer sowings need daily water in hot soil and row cover against flea beetles and root maggots

When to plant

• Sow, 10 weeks before your first fall frost.

What can go wrong

• Bolting before the root sizes up. A flower stalk shoots up and the root stays thin and woody. Most common in spring sowings, when cold nights are followed by lengthening days. Grow mu as a fall crop, and use bolt-resistant varieties for any spring trial.
• Pithy, spongy roots. The inside turns dry and spongy, with a hollow center. Caused by leaving roots too long in warm weather, drought stress, or long exposure to temperatures below about 30°F. Harvest at size and before a hard freeze.
• Cabbage maggot tunnels. Brown, winding tunnels in the root that often rot. Caused by larvae of the cabbage maggot fly, which lays eggs at the base of young plants. Row cover from the day of sowing is the most effective prevention.
• Flea beetle damage. Small round holes across the leaves, worst on seedlings in late summer. Heavy feeding slows growth and can kill young plants. Row cover prevents most damage.
• Forked, split, or stunted roots. Forking comes from stones, clods, or compacted soil; splitting from uneven watering, especially heavy rain after a dry spell; thin roots from crowding. Dig deeply, water evenly, and thin to 4 to 6 inches.
• Clubroot. Stunted, wilting plants with swollen, distorted roots. A long-lived soilborne disease of the cabbage family, worse in acidic, wet soil. Rotate all brassicas and do not follow napa cabbage or other brassicas in the same bed.

Growing guide

Korean radish, mu, sits between the small red radish and the long Japanese daikon. The roots are stout, often 6 to 8 inches long and 2 to 3 inches thick, white with green shoulders, denser and sweeter than most daikon. That firm flesh is what holds its crunch in kkakdugi and in the radish strips layered into kimchi. Like napa cabbage, mu is mainly a fall crop in the US. Sown in spring, it tends to meet lengthening days after cold nights and bolts before the root fills. Sown in late summer, it sizes up as the weather cools, which is also when the flavor turns sweet.

Sow directly about 10 weeks before your first fall frost. The taproot is the crop, so mu is not a candidate for transplanting. Loosen the soil 10 to 12 inches deep and pick out stones, since roots that hit an obstacle fork or twist. Korean types are shorter than daikon and cope better with heavier or rocky ground, but they still grow straighter in a raised or deeply dug bed. Sow seeds 1/2 inch deep, a few every 4 to 6 inches in rows about 18 inches apart. In hot late-summer soil, water daily until the seedlings are up.

Thinning is the step people put off. Within two weeks of emergence, cut extra seedlings at the soil line so one plant stands every 4 to 6 inches; crowded roots stay thin and woody. After that, mu asks for steady moisture, roughly an inch of water a week. A dry stretch followed by heavy rain can crack roots, and drought makes them hot and fibrous. Skip fresh manure and high-nitrogen fertilizer before sowing; they push lush tops at the expense of the root.

Late-summer seedlings face flea beetles, which pepper the leaves with small holes, and cabbage maggots, whose larvae tunnel into the root and leave brown, rotting trails. Row cover laid on the day of sowing keeps out both. Mu is in the cabbage family and can be infected by clubroot, so rotate it with napa cabbage and other brassicas rather than following them in the same bed. Bolt-resistant Korean varieties such as Summit and Alpine are sold for spring sowing, and a spring crop is workable in some gardens, but a stretch of cold weather after sowing can still trigger a seed stalk. Treat spring planting as a trial and fall as the main crop.

Start pulling when the shoulders reach 2 to 3 inches across, usually 55 to 70 days after sowing. Loosen the soil with a fork before lifting, since a long root snaps easily. Mature roots handle light frost, but long periods below about 30°F can leave the inside pithy, so harvest before a hard freeze. Cut the tops 2 inches above the crown. The leaves, mucheong, can be blanched and dried as siraegi. Topped roots stored near 32°F at high humidity typically keep 6 to 10 weeks.

Harvest and keep