The RHS Encyclopedia does not mention S. purpurea ssp purpurea.
Neither did the OP and neither did I.
I have never known the RHS Encyclopedia to be as far wrong as this!


They probably don't mention the other 4 or 5 subspecies either, but
nonetheless, S. purpurea is found in northern New England and into Canada where
temperatures dip well below -10F. They should correct the entry as it is
misleading. The northern subspecies is not easy to cultivate and requires a
cold winter dormancy. The southern subspecies range north to New Jersey and
will also experience temperatures below 0F. The largest populations are near
Mobile, Alabama where temperatures at this time of year easily go below 20F. I
doubt that they will prove effective in capturing slugs.