In winter the greenhouse need not be put to sleep. It’s a useful convalescent home for any tender plants that you may need to overwinter and even in an unheated space they will be protected from frost. Bubble wrap or horticultural fleece can also help to provide a protective micro-climate around these tender species.
Cut and come again lettuce, spinach, herb and salad leaves can also be successfully maintained in an unheated greenhouse to maintain a steady supply of fresh produce without the need for a dash to the supermarket.
If you decide to heat the greenhouse most plants and crops will be happy with a steady temperature of 2°C. Pelargoniums and less hardy species will require a higher average temperature of 5 – 7°C. There is no need to super heat the space and waste energy. If you do use additional heating then remember to ventilate the space as increased humidity can be problematic. Use a thermometer that allows you to record maximum and minimum readings and keep a close eye on the weather for predicted temperatures and frosts.
Propagation can be undertaken in a warmer greenhouse or you could use a heated propagator to give plants an early start without wasting energy on the whole greenhouse space. Smaller propagators are available for windowsill use if you don’t have the luxury of a greenhouse.