Prepare Your Greenhouse for Winter

As winter’s frosty jaws bite deeper into the soft underbelly of autumn, now is the time to get your greenhouse sorted. No one wants tender plants to be savaged by the cold. Obviously.

Choose a relatively warm, dry day and get everything out of your greenhouse. Give all staging a wash down with soapy water including the glazing. Pay particular attention to the nooks and crannies or anywhere that bugs can overwinter. Let it all dry and then put your plants back in. Diseases will be reduced and light levels maximised.

If you can, put all your plants together on the staging in one half of the greenhouse. Choose the half away from the door. You can then put up a curtain of bubble plastic at this halfway point, in effect halving the length of your greenhouse. This will help in keeping heating costs down and your plants snug.

The rest of your glazing can be insulated using bubble plastic. It’s easily fixed to both timber and metal framed greenhouses. The gap of air between the plastic and the glazing, along with the bubbles in the plastic itself, all keep things warmer. A word of warning - if you use heaters in the greenhouse keep the plastic away from elements. Also, if you are using the half of the greenhouse with the vent, ensure it still opens. I’ll come onto that in a bit.

Doors are leaky. Close your greenhouse door and study the frame. I bet there’s a small gap somewhere. Draught excluder made for house doors is easily cut to size and effective at blocking air movement. Extra beading or thin strips of wood will cover up any small gaps caused when older greenhouses slump. Wood filler can be used for more permanent and larger gaps. These are small measures that will have a large impact.

Heating is vital if you want to grow more tender and exotic plants. A greenhouse will always be a couple of degrees warmer than the outside temperature (concrete floors act as radiators giving off heat and obviously the air warms up quickly on sunny days). But if it’s minus 5C outside chances are the inside of your greenhouse will be down to minus 3 or so. And that can be fatal to many plants. Heating is such a personal choice. Electric heaters are convenient, easily controlled and programmable. But you need electricity in your greenhouse. This can be expensive as qualified electricians are needed. You can get gas heaters and old-style paraffin heaters. You’ll need something and you’ll need it in tip top condition and, if it's paraffin, a reserve of fuel. You will run out on the coldest night.

Now, those vents. Just because it's cold outside doesn't mean your greenhouse doesn’t need ventilation. The temperature in a standard 6x8 greenhouse will rocket within a few hours, even on a sunny winter's day. Only a couple of weeks ago my own 6x8 boasted a 28C maximum temperature inside on what was a freezing day outside. Or, put another way, your automatic vents need to be working freely. It will create a more regulated and healthier environment for all your plants.

And what about that measurement of what’s happening temperature-wise in your greenhouse? A maximum-minimum thermometer is essential. It will tell you the hottest it got, how cold it plunged to and the current temperature. Perfect for ensuring all your insulation and heating is working well.

Get it clean, make it snug and your greenhouse will be a real asset in your garden this winter.


Article by Phil McCann