Radishes in the Winter Tunnel for Overwintering & Winter Harvest

Radishes work well in unheated (or slightly heated) hoophouses for both early spring and fall crops. They also can be grown through the winter in mild regions such as the South and West Coast U.S.

Protective Structure & Temperature Considerations

Winter radishes during trial evaluation at the Johnny's Research Farm.

Growers may use unheated or slightly heated high tunnels or low tunnels to extend the radish growing season.

It is critical that the crop is not subjected to significant periods of temperatures below 30°F/-1°C. Radishes exposed to low temperatures may look fine, but their interiors will be pithy or translucent due to cold damage, which ruins their eating quality.

Time the sowing of your crop so that it will be ready to harvest before temperatures in the tunnel drop below freezing. If you anticipate the arrival of a cold snap prior to harvest, you can add a protective layer of row cover (Ag-19 or equivalent weight), removing it when the temperatures in the tunnel rise again. Use row cover only when temperatures drop below freezing.

You can also use a layer of row cover (Ag-19 or equivalent weight) to promote optimal germination temperatures. Remove the row cover once roots begin forming; leaving row cover on will increase the risk of oval/elongated radish roots.

Growing in a slightly heated tunnel eliminates the risk of freeze damage.

Seeding Dates

While field-grown radishes mature in around 21 days, the time from seed to harvest for winter tunnel radishes is generally 5–6 weeks. The reason for the longer production time is that in spring and fall the temperatures are cooler and the number of daylight hours is fewer.

• For a fall planting, use our Planting Dates for Winter Harvest Chart to choose the best planting times for your region based on your “Persephone Period” (last day 10 hour day at your latitude). On our Zone 5b Research Farm, we have found that fall radishes sown in mid-September were ready harvest in approximately 4 weeks.
• For spring planting, we recommend planting after greenhouse temperatures have moderated and chances of a deep freeze have passed. On our Zone 5b Research Farm, we have found that early spring radishes sown in mid-March were ready to harvest in approximately 6 weeks.
• In mild regions where winter temperatures rarely dip below freezing, radishes may be grown in the tunnel all winter.

Seeding Rate

• Direct seed radish seeds 1.5” apart at a depth of 0.5”.
• Space rows approximately 3.5” apart.

At this seeding rate, you will be able to maximize valuable greenhouse space while still ensuring enough space to get a marketable crop. Radishes that are spaced too closely may fail to produce bulbs or produce elongated bulbs that are not marketable.

Irrigation

Irrigation for tunnel radishes can be tricky; the goal is to irrigate sufficiently for a good crop, but not overwater and cause cracked roots.

• First week after sowing: irrigate heavily for good stand establishment and taproot formation.
• Weeks 2–4: irrigate only if the soil is very dry or the plants look water stressed. The goal is for the radishes to grow a good taproot. Over-watering during this time can cause the roots to crack.
• Last week prior to harvest: if necessary, irrigate moderately to finish the crop. The goal is for the soil to be moist but not too wet. Overwatering can cause the roots to crack.

Variety Selection

'Rover', 'Donato', and 'Pepito' are our recommended varieties for red round radishes in the tunnel. If you are just getting started with growing red-round radishes in the tunnel, you may want to trial all three varieties to see which one works best in your conditions.

We recommend 'Rover' as our most reliable round-red radish for overwintering. It has performed best in both our spring and fall plantings. 'Donato' and 'Pepito' also performed well. If you are just getting started with growing red-round radishes in the tunnel, you may want to trial all three varieties to see which one works best in your conditions.

For specialty colors, we recommend 'Pearl' (white) and 'Bacchus' (purple). French breakfast types, including 'Nelson' are also suitable for winter growing.

Learn More

• Radishes  • Key Growing Information
• Winter Growing Guide  • Article
• Remarkable Roots: Radishes & Turnips  • Webinar