Posted: 02/10/2013 at 11:54
I've got a wooden 'growhouse' very similar to this one
http://www.greenfingers.com/superstore/product.asp?dept_id=200360&pf_id=LS3336D got it from Notcutts and it's quite sturdy. I over-winter plugs and cuttings of hardy perennials in it, and sweet pea seedlings. I have the lid propped open quite a bit most of the day and only shutting it on frosty nights.
When it was very very cold last winter I popped a layer of the 'big bubble' bubblewrap around things that I thought were a little more tender at night time, again removing it during the day.
I would've thought a garage would be too dark for plugs and cuttings.