Posted: 11/04/2013 at 23:22
Half hardy actually means they can't cope with forsts and should be hardened off by day but brought under cover at night till teh frosts are over in mid May for most of mainland Britain. They will die with the first autumn frosts.
Hardy used to mean it could withstand frost but there are, of course, degrees of cold so the RHS has recently revised its hardiness definitions to take account of this. Se ehere and click on "hardiness ratings" in the text for a full list - http://www.rhs.org.uk/News/Hardiness-rating-system-improved
Tender plants need to be kept indoors, in conservatories or in heated greenhouses as they can't stand cold, let alone frost. Most houseplants are in this category but can go out for some fresh air in summer.