This Saxifraga (or Micranthes) appeared as a "weed" in one of my pots. It is a rather strong grower (from next to nothing to 1' in 2 years), but what is it? The leaves are yellowish but that doesn't seem to bother it.
Quite an amazing plant, Trond! Did you ever figure out what it is?
I think it is a Micranthes. It is a bit like the American M. virginiensis actually. The nursery I often buy from does have this plant so it probably is a stray seedling in a pot I bought.