My late MIL who taught me a lot about gardening always said never plant them out until the beginning of June when its pretty certain there will be no more frosts. However my Mum usually puts hers out end of April beginning of May and risks the frosts/covers them over if we get any. We are all on the south coast. I've tended to leave mine until end of May but often find the garden centres have sold all the best stuff by then. I don't have a greenhouse or frame. Do you have a hard and fast rule or play it by ear/long range weather forecast?