Here in Zone 6 upper south, we can have 70s in Feb and freezes in May.
Yellowhorn (Xanthoceros) has never yielded for me here, as the blooms
usually freeze out. If you are getting nuts on a (semi) regular basis,
where are you located? How do they taste?
As for absolute winter minimums during dormancy, Dirr (Manual of Woody
Landscape Plants) notes a report of yellowhorn growth in Nebraska with -25
to -30 F lows.
Richard Moyer
Zone 6, East Tenn
Anna Kiwi harvest just starting