Elizabeth wrote:

Try a search with "Poncirus trifoliata Flying Dragon" and see if
that's what you're looking for. Commonly used as a dwarfing
rootstock for citrus, it also is a stunningly thorny ornamental
and is naturalized in some areas. Hardy to below 0, it is well
suited for many climates. Hope this helps,

That's it!

I must have seen it in its winter state... I don't recall the leaves or
fruit at all.

Thanks!

--Yan