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Insulating the greenhouse

I am behind the curve. In fact so behind it am I that I’ve lost sight of it altogether and am ambling along on some vague trajectory of my own devising that has no bearing whatsoever on what I ought to be doing at this time of year.

It’s been a busy autumn, but that’s not the reason I haven’t got around to putting up the bubblewrap insulation yet. I am also terminally lazy: nope, that’s not why either.

It’s just that it’s hard to get motivated to tuck up the greenhouse for the winter when…. well…. you can’t really see the point.

You see, here in the south-west of England it’s been quite difficult to believe winter has actually started yet. We got a bit excited the other day, when the temperatures fell to a heady zero degrees and there was a definite white dust on the grass. But that was it. And that’s the coldest it’s been since the winter before last.

You hard nuts over in the south-east where the clouds occasionally part to reveal a minus-five inducing bit of moonlight, or you even harder nuts upwards of the Watford Gap where we softy southerners never venture without GPS, thermals and a hot water bottle – no doubt you’ve been chipping ice off the cars every morning with a pickaxe. Don’t take a blind bit of notice of what I say in this post as you should have insulated the greenhouse at least a month ago, and if you didn’t your lah-di-dah bananas and lemongrass and all that fancy muck will be so much frozen black mush by now.

But my bananas – including the notoriously tender Ensete ventricosa I got free from a jumble sale when they couldn’t sell it even for 50p – are not only not frosted, but actually still growing. I probably shouldn’t be surprised: daytime temperatures are averaging about 10-12°C and even at night time it’s struggling to dip below 7°C. And it’s December. Nearly January, really.

That’s all about to change though, according to the the Met Office.....

All of which is a long-winded way of saying: don’t take any notice of the calendars. They’ll tell you to sow in March, and pull up the maincrops in October, and insulate your greenhouse in November; but what’s true for Carlisle is not true for Cornwall and even if it is, you should still look up into the sky and think about how cold (and/or wet) your nose is before you do a thing. React to the weather happening right now, over the top of your garden, day by day, not the gardening books: it is the only sure indication of when to do anything.

So this week I’ve been unrolling my carefully-stored pieces of bubblewrap polythene, snipped to shape over the years to fit my greenhouse (I also mark which bits went on which part of the greenhouse with black permanent marker otherwise it’s like doing a jigsaw puzzle with particularly unwieldy pieces). I fix them to the frame with green plastic insulator clips so they’re held a little off the glass, as the theory is they trap warm air between plastic and glass, increasing the insulation potential.

Once I’ve moved the tender plants inside (mainly the scented-leaf pelargonium collection, overwintering nicotianas, a grapefruit tree and my two baby banana plants) that should be enough protection, as you can expect an extra 2-3°C above ambient (i.e. the air temperature outside) from the insulation and that’s generally as much as you need to be frost-free here in Somerset.

In the south-east, where it regularly dropped to a heady minus 5 at night, I used to heat my greenhouse to just above zero as well as insulating – I’m told bubblewrap saves around a quarter on your heating bills. If you do live up north and you’re looking at minus 10 or so that saving really mounts up, so insulation becomes even more worth the trouble. Or you could just stop being soft enough to grow bananas, I suppose.

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