I have a Pagodatree, Styphnolobium japonicum (syn. Sophora japonica) that has died, sort of. Evidently, it was grafted on a Kowhai understock, I'm guessing Sophora microphylla. This is stupid-unlikely, but it appears so. I air-layered a sucker last year and was feeling pretty happy with myself because it over-wintered in my greenhouse and was evergreen, until the scion didn't leaf out this spring. It was a crumby spring, very wet and very long, after a very long and deep winter. I lost other marginal woody plants, too, but the tree had struggled since I put it in, ~2015, losing some of the top each winter and growing new, but less, each summer. It is a solid zone 4 to poorly-in-zone-7 tree, and I'm a solid 6b so it should have been happy. I suspect it came with some disease, but will never really know. I have a distant cousin Laburnum anagyroides Golden Chain Tree that also has struggled with disease and/or depth of winter marginal hardiness. Anyway, I'm getting a lot of suckers from the base but I don't have any idea of where the growing zones of this (or any other) Kowhai are, and probably wouldn't be able to match it zone-for-zone with the USA, anyway. It appears that the NZ zone 8a matches to USA 8a of middle Georgia. I need someone who knows my zone, Detroit, and the zones in New Zealand to tell me whether or not this understock might be able to grow here, or just winter-kill every year until it wears-out.
No chance at all with Kowhai in Michigan! I'd suspect your Pagoda Tree failed because the rootstock froze, as Pagoda Tree on its own roots should be hardy in MI. New Zealand zones are more like coastal California, mild all year round, without Georgia's summer heat.
