A cold polytunnel provides the perfect growing conditions over winter for a range of winter brassicas. Although they are hardy and can be grown outside in the open, the added protection from wind and heavy rain helps to keep the plants in good condition and cropping for longer. Kale especially grows very well under cover and the secret is to pick leaves little and often to keep them growing through until spring. By growing two or three different types of kale you’ll be able to have a steady supply of these nutritious vegetables from October to March. Winter cabbages, Brussels sprouts and leafy cabbage grown for greens also do well and can be harvested and cleared in time for spring crops.