[Kevin S]

decided to plant onions etc from seed this year trying to keep up with the other allotmenteers... Any way its costing me about £10 a week to keep the frost at bay, This has got me thinking is their any 12 v heating mats you can get which could be wired up to a car battery and a few solar panels. Or is their any other alternative methods.

its a 8x12 greenhouse and using mains power is not an option.

Or is the consensus that starting early is just not really worth it if you are not showing the produce.

[DavidT]

First thing, insulate the greenhouse with large bubble insulation. This will not only make the greenhouse frost-free, but will also act as shading in the summer. Assuming we have one, that is.

[DD.]

Second thing - are you heating the whole greenhouse, when you only need heat part of it?

One trick is one of those small, cheap plastic greenhouses inside the main one.

[Nobbie]

The solar power thing is a no go as it couldn't store enough in a car battery even if the sun were bright enough at this time of year. I'd go with the small plastic greenhouse within the main one. Wilkos do one for about £12. I'll be using this with a small parrafin heater at the bottom on low within my 6*4 greenhouse when it comes this week

[DavidT]

I agree, a paraffin sump type heater is ideal for keeping a small area warm

[mumofstig]

In one of those little plastic greenhouses you'd only need something small like an old paraffin sump heater, if at all. Check your temperatures with min/max thermometer first.

edit ..Sorry David..just agreeing..oops

[DavidT]

Not a problem Mum, good to see you are alert.

[mumofstig]

I am a lert I am a lert ...that takes me back years

[NigelB]

Paraffin heaters... Yuk! 
Dirty things. Burning paraffin oil and eating up oxygen, replacing it with all kinds of fumes and gallons of water. There's nothing ideal about them at all.
Really. Don't bother wasting your time, or money. Certainly not for onions or any of the hardy veg.

Bubble-wrap or the cheap Wilkos greenhouses inside your glass greenhouse will be more than sufficient. Don't burn your money...... Give it to me instead. I'll find a much better use for it. 

[thearaig]

You could always try the old way. Make a hot bed

[Swing Swang]

Or indeed even wait a few more weeks