You might think a greenhouse or cold frame are only useful in spring when you’re starting off seeds or in summer when you want to grow heat-loving crops such as tomatoes, but they’re really important in winter too. Both can be employed to provide protection for plants that would otherwise struggle or even die if left outside in the cold and wet of autumn and winter.

Succulents

I have a collection of tender succulents including aeoniums, kalanchoe and crassula which spend the summer outside, but come autumn they need to be brought under cover. The timing of this changes each year. Some years a mild autumn means they’ll still be outside in November, whereas other years predictions of an early frost will mean they could be tucked up in September.

I move mine onto a sunny window ledge in my potting shed where they’ll stay until it gets too cold, even in the shed. I have a thermometer which monitors the minimum temperature and if that gets close to 5C then I’ll move the succulents onto a windowsill in my house. If you plan to keep them in a greenhouse all through winter you’ll need to provide extra heat and look at insulating the greenhouse.

I also have some hardy succulents in pots, some sempervivums, which are happy enough in very extreme cold but persistent wet weather can cause them to rot. For this reason I like to move these plants into my cold frame around mid to late autumn where they’ll be happy enough being kept dry until spring.

Borderline hardy

A greenhouse or cold frame can be used to overwinter plants such as tender salvias and tender agapanthus (the evergreen ones). These are plants I grow in pots so it’s an easy job to tidy them up, removing any dead or yellowing leaves and any old flowering stems before moving them to their winter storage.

If it gets particularly cold and you don’t have a heater you can protect plants by covering them with a few layers of horticultural fleece or hessian, or use cloches that you’d put over plants in the garden for extra insulation.

Mediterranean herbs

While many herbs from the Mediterranean are cold tolerant they don’t like to sit in soggy soil, so I move any I have growing in pots into the cold frame where they’ll be much happier.

Agastache

Giant hyssop, with its fragrant foliage and flower spikes that are loved by bees, is one of my favourite garden plants. In my old garden I tried growing it in the border but it never survived from one year to the next. I don’t think it liked sitting in cold, wet soil over winter, so in this garden I’ve tried a different strategy. I grow it in pots and overwinter the plants under cover. The pots they spend the summer in are pretty heavy so I’m not able to move them, instead I repot the plants into lighter plastic containers, then I move them into the cold frame. It’s best to leave the stems in place over winter as this can protect the crown of the plant from the cold, so I just trim the stems so that they fit into the cold frame.

Ventilation

The air in a greenhouse, potting shed or cold frame can become quite stagnant if there’s no air flow which can cause fungal diseases to form on plants. Of course, leaving doors and windows open means you let in the cold, so it’s a tricky balancing act to keep plants warm while making sure that they don’t rot.

On days where the temperature is above 5 degrees open vents or windows a little and lift the lights on a cold frame. It’s best to do this on a morning for an hour or so then close them again so that the temperature inside can rise again before it gets dark. Breezy days are a good time to ventilate as there’s good air movement.

Terracotta Pots

Clay pots are prone to cracking over winter particularly if they have got wet and are then exposed to a frost. For pots that are small enough to lift I move these into the cold frame – you could also use a greenhouse – so that they have some protection.

Top Tips

• Try not to cram plants together as this means there’s less air flowing around them, and be vigilant for signs of grey fluffy mould. Remove infected plants and increase air flow.
• If you’re purchasing a new greenhouse or cold frame see if you can position one against a house wall, as it and the plants inside will benefit from the radiated heat.
• During cold spells sheets of hessian are really useful for providing extra insulation – wrap pots with them to protect roots or cover the glass on your cold frame on bitterly cold nights.