As in most things, I kind of fly by the seat of my pants on this, too. I have seen published last frost dates for my area as early as mid April and as late as mid-May. The frame of reference that most people (including me) seem to use is Mother's Day, or sometime around May 15. Even with that, I have had to cover things in late May for a late spring frost. It only happens about once a decade, but it does happen.
I have no qualms about planting out wintersown perennials early. I've done that as early as April 1st. Hardy annuals, too. But for tender ones started under lights or nursery-purchased ones May 15 is average for me. And before I do that I check the 10 day forecast and make sure there are no predictions for frost or near-frost conditions. That's after hardening off, which I usually start by March-April on warm spring days.
I honestly think I have more problems here with heat than with too cold. If I don't plant out early enough, seedlings struggle trying to establish in frying heat. And maybe drought too.
Karen
