I have just planted some young plants up into pot from seed trays they are in my semi heated greenhouse at home and wondered if they would be OK to go into my unheated polytunnel on my allotment or will they suffer with the cold any advise would be great
They should be allright - as they have been outside they should be hardened off and they are all pretty hardy anyway.
If you have just potted them on they wont be strong enough to take any cold. No way is your tunnel going to keep them above 40deg, the roots need to be kept warm i would leave them in your GH till this cold spell breaks, even then you will have to watch the night time temp.
I put a couple of plants outside under cloches and they have long since disappeared. My windowsills are full. Not even warm enough during the day to start hardening them off.
I have got some int the cold GH with a parasene heater underneith. Its curtained of to keep the warmth in and so far so good
Cabages and Cauli,s
(http://i142.photobucket.com/albums/r111/stumpinsci/Picture006_zps709012b2.jpg)
I'm potting up in the GH at the moment... last night they were in the hotbed, but they should be OK in the general GH now as long as it doesn't get too cold... big greenhouses do hold the warmth better than small ones so YMMV... I always grow brassicas in pots despite the old-timers muttering about it and I generally get good results... remember to put a little bit of extra lime in it...