A greenhouse really makes a difference to what you can grow in cooler climates, and not just in summer. In winter it’s invaluable for keeping crops in good condition that would otherwise suffer outdoors. Not only does it keep plants a few degrees warmer, it also protects them from being battered by wind, rain and snow – invaluable for seedlings in spring.
Even if you’re growing crops outdoors over winter, it makes sense to bring on a few under cover too. For instance I’ve found that growing cabbages in my greenhouse over winter means they’re ready a couple of weeks before the outdoor ones in spring – plus they serve as insurance to offset any losses due to weather or pigeons.
If you’re worried that your unheated greenhouse is just too cold for your overwintering plants, here are a few cost-effective ways to make it extra snug.
Insulating Your Greenhouse
Before insulating your greenhouse, the first thing to do is block draughts. Replace any broken panes, seal gaps in the frame, and make sure that doors and vents fit securely.
Lining a greenhouse with an inner layer of plastic – essentially creating double glazing – will seal off air gaps and reduce the rate at which heat escapes. Good old bubble wrap is the greenhouse grower’s go-to cheap insulation material.
Bigger bubbles are better, as they allow more light in and provide better insulation. Purpose-made horticultural bubble wrap has large bubbles and is also UV-stabilised, so it should last longer than the stuff used to pack your latest online purchase. Having said that, if you’ve got a lot of that lying around why not save a few dollars and use it? When it degrades you can send it away for recycling knowing you’ve extended its useful life and helped reduce its impact on the environment.
Attach your bubble wrap to the inside of an aluminium greenhouse’s frame using greenhouse clips, or for a wooden frame, use drawing pins or a staple gun. Don’t forget to insulate the roof, and make sure to leave the bubble wrap hanging loose across the door so you can get out again! If you have a large greenhouse but you’re only using part of it for overwintering plants, you can section an area off to clad in bubble wrap. Make a screen divide out of bubble wrap or plastic.
Only use insulation if you feel your plants won’t make it through the winter without it because it will reduce light transmission slightly, and light is precious in the darkest depths of winter!
It can be worth insulating the inside of the South side of your greenhouse using a roll of inexpensive thermal insulation foil. This material has bubble plastic sandwiched between two layers of foil, and it will reflect both heat and light back into the greenhouse.
Don’t forget the insulating properties of snow. If you experience heavy snowfall, don’t be too quick to dig it all away from your greenhouse (unless it’s in danger of damaging the structure, of course). There’s a good reason it’s often referred to as a ‘blanket’ of snow! You may need to knock it off the sun-facing side of the roof to let more light in.
Insulating Your Greenhouse at Night
Any heat that has built up during the day can be trapped in the greenhouse overnight using a wide range of materials. Purpose-made thermal screens and blinds are expensive, but you can easily make your own night-time insulation.
Double or triple layers of horticultural fleece laid directly on top of plants work well. Old blankets and other thick, heavy materials can be suspended on hoops or canes to prevent them from flattening your crops. And thermal insulation foil can be fixed to the inside of the greenhouse roof to help retain heat for longer.
Of course the downside to using materials that aren’t transparent is that you need to remove them promptly to let the light in during the day, but they can be worth it on the coldest nights to give your crops the best chance of survival.
Insulating Plants in Your Greenhouse
Cold frames and cloches aren’t just for outdoors – they can also be placed inside the greenhouse to provide an extra layer of protection for your plants.
Alternatively, recycle plastic bottles as mini cloches. Cut them in half and pop each half over individual plants. Any other clear plastic container can be recycled for the same purpose, or use that old faithful, a layer of bubble wrap.
You’ll need to remove cold frames and cloches on mild, sunny days to prevent overheating. Replace them before nightfall to help build up some warmth once again.
Plants in pots are especially vulnerable to having their roots freeze in the cold, and some pots may crack if exposed to very low temperatures. Wrap pots in bubble wrap, or place them in old potting soil sacks and stuff them with scrunched-up newspaper, straw or dry bracken. You can even insulate trays of seedlings in old polystyrene fish boxes, which are often given away free by fishmongers.
Do you have any more tips for ways to keep your greenhouse cosy in winter? If so, we’d love to hear about them – please share them in the comments section below.