They will spread! They are everywhere here, including some grown commercially for their flowers. We had all the ones in our garden cut down as they make such a mess - seed pods everwhere and suckers coming up in the paths. Happy to see photos of the flowers though!.
I wonder if I could grow a mimosa tree up here in Lancashire Hazel? Perhaps it's too cold but I do like this soft & fuzzy shrub. Looks lovely with the blue sky behind. There again just reading @laroque description of them do I want one?
@happypat One other problem we encountered was that the leaves are evergreen and not designed to shed snow. So when, once every five years, we get a snowfall, then the snow builds up and eventually the branches break off or even the trunk splits. Grow it as a shrub maybe, in a raised bed to prevent the suckers spreading?
Looks like one of our many Australian wattle trees. And @laroque is correct in saying they are appropriate for snow areas - plus they have a short life as well :)
It is the same, I believe!