They do say it is going to be a bad winter this time - so it may be worth taking some precautions. You might consider using bubble wrap (It is sold from a roll) to act as secondary double glazing. Whilst it will cost a few pounds, you can reuse it for several years. It will help keep the heat in and the cold out - it will act just like double glazing in the house.

However if the greenhouse gets very cold there will be nothing to keep in. So you might also consider a small parafin heater - available from garden centres. Whilst I do not use one myself, I gather they are much cheaper than electricity. But you do have to monitor it. A max/min thermometer would also be a good investment.

Best of luck

Everything Peter LS says works for me. But it depends a bit how tender your seedlings are.

I use a paraffin heater which is fine as long as I am at home to light it. I have had losses in years when there has been a sharp frost when I've been away on holiday. Perhaps you've got a helpful neighbour?

If you only light it on nights when frost is forecast, paraffin is incredibly cheap. About £10 last winter.

Your other consideration is how much space you really need to keep frost free. A simple cold frame within an average domestic sized greenhouse may only be about a 20th of the volume of the greenhouse itself, and consequently need only ~100watts to heat it, instead of ~2000watts to heat the whole greenhouse. So don't heat more than necessary, or protect seedlings that are hardy anyway. Electronic cable-less max/min thermometers are excellent to monitor the temperature, if your house is within say 50-60' of the greenhouse.

Brian

If you are planning to heat the whole greenhouse or even half the greenhouse then a quality thermostat (probably not the one on the heater unless it is an expensive heater) will pay for itself very quickly in lower bills. Other than that, bubble wrap is the insulator of choice.

Many thanks for all the helpful hints and suggestions. I am seriously thinking that as I am reshaping a garden bed close to the green house maybe I should lay a cable to have electricity into it.

But I will be buying some bubble wrap this week-end and installing it. Once again, thanks for all the helpful suggestions.

Hope I'm not too late with a tip regarding bubble wrap. Try
and be sure to buy the polythene with small bubbles and
a layer of polythene on both sides of the bubbles. I invested in electric cable to my greenhouse and also bought a Parwin thermostatic heater some 15 years ago, this has to be one of the best you can buy, easy to maintain and very
economical.

I've seen large bubble wrap for £4 per metre about 150 cms wide. But you can get small bubble wrap in a roll from Focus at £15 for 30 metres, 75 cms wide (ie 50p per metre).

I think even plain polythene will do 80% or 90% of the job. It's the gap between the glass and polythene that insulates, same as in double glazing in the house.

All my seedlings are still outside, protected only from most of the rain. Its been down to about -4C here.

I'm in Cornwall and I don't bother to either insulate or heat my greenhouses. I do have lots of seedling and some very tender plants in there too.

I found that when I used bubble wrap I lost more seedlings to botrytis than to the cold, and a parafing heater only makes the problem worse for me as they throw out large amounts of water.

I have found that the best way for me is to drape the plants and trays with fleece at night if it threatens to be very cold, and in the last three years I haven't lost anything at all.