hey man heres something simple to use to keep it warm...manure...keep the plants in pots and pack the ground over with manure...it produces some good heat...i think this would be your best bet as far as keeping them alive goes...i'm not sure how far north this works but in tn it keeps plants alive....if you wanted to not keeps them in pots you would need to mix the soil with manure maybe a foot and a half deep...this would keep it warm...although i'm sure you dont have to worry much about it now b/c its almost time to go outdoors everywhere...but next winter season remember this...its what we use in hothouses