Do Brussels Sprouts Need Frost Protection?

Brussels sprouts are one of the hardiest vegetables you can grow. Established plants can withstand temperatures as low as -10°C and are one of the few crops that genuinely improve after a frost — the cold converts some of the starch in the sprouts to sugar, improving their sweetness and reducing the bitterness that makes some people dislike them. For most of the growing season, frost is not a problem and no protection is needed. However, there are specific stages at which frost can cause damage that is worth guarding against.

When frost can harm Brussels sprouts

Young seedlings and recently transplanted plants are much less frost-tolerant than established mature plants. Frost on small seedlings in April or May can kill them or set them back significantly. If planting out in spring, protect young plants with horticultural fleece on cold nights until they are well established — once they have been in the ground for several weeks and are growing strongly, they can generally be left without protection.

Established plants in autumn and winter

Established Brussels sprout plants — those that have been growing since early summer — are remarkably frost-hardy and do not need protection except in the most extreme weather. Light frosts (down to about -5°C) are welcome as they sweeten the sprouts. More severe extended frosts (below -10°C for several days) can damage even established plants, causing the outer leaves to brown and the sprouts to deteriorate. In areas with very harsh winters, a covering of horticultural fleece over the plants during the most extreme cold spells is a sensible precaution for crops you intend to harvest later in winter.

Frozen sprouts — picking and handling

Do not pick frozen Brussels sprouts — wait until they have thawed naturally before harvesting. Picking while frozen causes the cells to break down and the sprouts to become mushy. Check the plants in the morning after a frosty night and leave harvesting until later in the day when the sprouts have fully defrosted. Sprouts that have been frozen and thawed repeatedly may eventually start to deteriorate, so if a prolonged hard frost is forecast, harvest a supply before it arrives.

Hardening off transplants

The key moment requiring frost awareness is when transplanting seedlings raised indoors or under cover. Harden them off over a week to ten days before planting out — place them outside in a sheltered spot during the day and bring in at night, gradually increasing their exposure to outdoor conditions. Do not plant out when a hard frost is forecast for the same night.