Assuming it's not a low energy bulb, but an old fashioned incandescent, it may well do, but in practice, Pelargoniums withstand up to ten degrees of frost - what they can't cope with is the combination of cold and wet, so if you keep them dry, the temperature dropping overnight to -4 deg C won't be an issue, provided the pot they're in is big enough not to freeze, and dry enough. They might be at greater risk if the temperature never went above zero during the day for longer than a week or so.

Hi, it will also depend on the size of your greenhouse, I have an 8x12, and I certainly wouldn't expect a light bulb to keep that frost free, and also the outside temperature, Derek.

A greenhouse does not work in winter. It relies on the greenhouse effect where short wave radiation goes through glass and warms things inside, and the warmth re-radiated is long wave and is trapped by the glass.

Unfortunately in winter we lack one vital ingredient for the greenhouse effect...... sunlight of any strength :roll: Throughout the whole UK, solar altitude remains below 20º from November thru January. During this time you may be lucky to have 4 hours solar gain and 20 hours heat loss. Also those bright sunny days are often preceded or followed by radiation freezes, therefore, to me, insulation is the most important consideration, over light transmittance.

So insulation is key. A lightbulb is only really going to keep the immediate area above freezing, assuming a normal winter. If it gets well below -8 or less then it will have no effect. Try an electric heater with a thermostat.

thanks

Kildermorie has explained it rather well, that a greenhouse is probably the most inefficiently insulated structure regarding heat retention. A light bulb would probably be the worst thing to use to try and heat a greenhouse as most of the energy output is in the form of light with just some of the energy turning into heat, so all that light energy would be just wasted.

If you have no other place than your greenhouse to keep your geraniums, and it's paramount that you need to heat it, then I would suggest one of those little paraffin heaters or even a low wattage electric heater. It will probably only raise the temperature by a degree or two, but this might be sufficient to keep it frost free if frost is forecast and the temperature doesn't fall too low.

You could also try taping some insulating material to the inside of the glass, like sheets of bubble wrap or thin polystyrene sheeting to insulate the inside and retain heat.

I don't know if this helps, but it might be of interest - I overwinter my pelargoniums on the balcony outside, but I pull them right up against the house wall, under a sort of parapet to try to keep the rain off, first having cut them all back and stuck them altogether in one large pot. If I lose any at all, it's always the ones nearest the outside, which still catch a bit of rain and so succumb to fungal infection. The only year it didn't work was a couple of years ago, when we had that really, really cold winter with lots of snow - lost the lot then, but I also lost dahlias stored in the garage, so it must have been pretty cold.

Hi, I've just finished putting heat sheet insulation up in my greenhouse, like small bubble, bubble wrap only a posh name, which I hope will cut the cost of using my 6kw propane greenhouse heater, the price of propane is ridiculous these days, I only need to keep it about 41deg but it has a lot of power if needed, Derek.