Blimey it’s chilly. Inside and out: life got a bit hectic last autumn and I didn’t get around to my usual bubblewrap-insulation-and-greenhouse-heater routine. So my greenhouse – usually a cosy refuge at this time of year – is distinctly less than welcoming at the moment.
However: the decision not to heat the greenhouse this winter, if a little unintentional, has been enlightening. Normally I would have the heater on 24/7 when the weather is like this: I don’t heat my greenhouse to tropical temperatures but I do like to keep it somewhere around the 5°C mark. When it’s -5°C outside, as it was last night, that would mean having to lift the temperature by a whole 10°C above ambient – loading my electricity bill to groaning point and playing who knows what havoc with the environment.
I’ve always felt mildly guilty about heating the greenhouse. As well as being positively profligate with resources I normally shepherd carefully – that is, electricity and warmth – it is very expensive and makes something of a mockery of my pretensions to thriftiness. After all, when your overwintered chillies cost you at least £50 to keep alive in a frosty winter you could probably buy gold-plated ones for less.
Failing to heat my greenhouse, though, has been an eye-opener. Just look at my lemons! (No smutty jokes at the back, please). The scented-leaf geraniums have fared well too, and the lemon verbena.
Most of the tender herbs and edibles which I move into the greenhouse over winter to protect them from frost can survive down to a few degrees below. Lemons, for example, can tolerate -5°C; geraniums (pelargoniums), lemon verbena and French tarragon to about -1°C. The secret is to keep them dry. Soggy compost freezes at anything below zero, wrecking delicate root systems, while dry compost, though cold, will not freeze so does no damage.
So I haven’t watered my lemon tree, or the geraniums, since I brought them indoors in early November. They’re fine. So is the grapefruit, and the lemon verbena, and even the Nerine sarniensis which is the only thing in here which isn’t edible but I can’t bear to evict it as it’s so lovely when it flowers. The overwintering chilli (an Aji type, one of the more hardy) has succumbed, so I’d guess that very heat-loving Mexican types with fleshy, tender stems freeze at zero.
But for most, just bringing them into a greenhouse without heating it has been enough. The glass alone raises temperatures by about 5°C, after all (and much more on a sunny day, though that heat is lost by nightfall). So if you take last night, the coldest here for several years at about -5°C, inside the greenhouse it will still have been only just at freezing. Not enough to do any damage. Line the greenhouse with bubblewrap or – I’m told but haven’t tried myself – cardboard, or wrap plants individually in horticultural fleece, hessian with straw tucked underneath, or more bubblewrap – and you can raise that by a few degrees further, potentially keeping even quite tender plants frost-free without the need for heating.
Other little tricks to try include keeping a pond in the greenhouse to act as a heat sink, absorbing the sun’s heat by day and releasing it by night; and of course hotbeds, which is too big a subject to tackle here but the most natural greenhouse heater you’ll ever have.
But I think my days of artificially heating a greenhouse are over. I’m sure the environment will thank me one day. My bank account certainly will.