Its good to keep pushing the boundaries a bit though; received wisdom isn't always so wise. Maybe the climate is changing - or maybe the plants are more adaptable than we knew? I remember that when I was young keen gardeners carefully dug up dahlia tubers every autumn and wintered them over in the shed. I'm too lazy - and not only do my dahlias survive the winter perfectly well in the ground but they self seed all over the place giving me amazing new colours and forms. Who'd have thought it?
I'm the same here too Margaret,i leave my Dahlia's in the ground in winter and they dont seem to mind at all and we get -9 frost here.
Talking of pushing the boundaries,one seed line i grow for Koanga is the Nutty celery which had been grown in Northland for over a hundred years,my first year growing it i planted about 50 plants and by the end of the following winter i was down to one plant still and only just alive, i was covering it at night with woollen blankets,i managed got seed from that one which went onto another sowing,the second generation after exposing the block of plants to most of the winters frosts ithen had to covered the last of about 6 plants still living till spring.I'm up to five generations now and i'm not covering any plants at all but i still will lose about half,so its a far more cold tolerant line than it was and i would imagine that it should continual to improve.