Growing Brassicas in Winter — Frost Tolerance Guide

Brassicas are among the most frost-tolerant vegetables you can grow, making them the backbone of the winter kitchen garden. Many varieties not only survive frost but actually improve in flavour after a cold spell — frost converts starches to sugars, making sprouts, kale, and parsnips noticeably sweeter after the first cold nights of autumn.

Which brassicas are hardiest

Kale is the hardiest of all — most varieties tolerate temperatures down to -10°C or below without damage. Brussels sprouts are similarly tough and are traditionally harvested after the first frosts, which improves their flavour. Winter cabbage varieties like 'January King' and 'Savoy' are bred specifically for cold-weather growing. Purple sprouting broccoli withstands moderate frosts. Calabrese (summer broccoli) is less hardy and will be damaged by hard frosts.

• Kale: very hardy, tolerates -10°C and below.
• Brussels sprouts: hardy, flavour improves after frost.
• Winter cabbage: hardy depending on variety.
• Purple sprouting broccoli: moderately hardy.
• Calabrese: not frost-hardy, harvest before frosts.

When brassicas need protection

Even the hardiest brassicas can benefit from protection in specific situations. Prolonged severe frosts below -8 to -10°C can damage even kale. Young transplants before they are fully established are more vulnerable than mature plants. In areas with extreme or prolonged cold, fleece or cloches extended the harvest season and protect during the coldest spells. Polytunnel or cold frame protection allows growing of Oriental brassicas like pak choi and mizuna through winter in conditions that would otherwise destroy them.

Protecting mid-hardy brassicas

Purple sprouting broccoli, winter cauliflower, and Romanesco varieties are hardier than summer types but can be damaged by prolonged hard frosts, particularly on the developing flower heads. Fleece draped over the plants during forecast hard frost periods provides useful protection without needing a permanent structure. Remove the fleece during mild spells to allow air circulation and prevent disease.

Extending the harvest

With the right variety selection, you can harvest brassicas through the winter and into early spring. January King and other winter cabbages provide harvests from December to February. Purple sprouting broccoli produces its main crop in February and March when little else is available. Kale can be harvested continuously from autumn through winter. Planning a sequence of brassica varieties gives you fresh vegetables for six months of the year from one crop family.