The other day I was at a friend’s house and I was admiring the Mediterranean spurge (Euphorbia characias subsp. wulfenii) in their front yard. It was in full bloom and it stole the show from all the other plants in its vicinity.
It turns out that their plant is a seedling courtesy of the plant in their neighbor’s yard:
That plant, in turn, is a volunteer from seed that came from further down the street.
That should answer the question whether Mediterranean spurge self-sows. I’m not sure I’d call it invasive (quite a few people do), but it certainly gets around.
Euphorbia characias subsp. wulfenii is hardy to zone 7 and can grow 5 ft tall and 4 ft wide. It prefers full sun and isn’t particular about soil as long as there is good drainage. It its native habitat (Greece and Turkey) it grows on rocky hillsides. In cultivation, I think it looks best as a solitary specimen.