rockm
Spuds Moyogi
I'm not convinced as to winter hardiness for this species in a container below zone 7. I'll have to see how it goes.During that winter when it got down to -19° it was in February and it was about 10 days of sub freezing temperatures where we got into the negatives several times. They handled it well at that nursery. I'm convinced they can handle zone 5 winters if established in the ground.
Aaron
Typically, you subtract a species' hardiness zone for a containerized bonsai, so what's hardy to say Zone 6 or 7, is hardy to zone 7 or 8 in a bonsai pot. That's a rough estimate. Can depend greatly on the species. FWIW, I've had smaller containerized Texas live oaks survive in our winters here for years, but they all lost ALL their limbs every winter, even with protection. They're native to north Texas and hardy to Zone 7. Northern Va. is zone 7. Dallas. Tex. is zone 7a to 8b. Not a huge difference. I'm on the edge of their cold tolerance.
I've seen Grey Oak hardy listed as both hardy to Zone 7 and Zone 6. Given its distribution from Durango, Mex. up to (but apparently not including) Flagstaff, Ariz, I'm assuming northern collected trees are de facto more hardy than southern collected. That's been my experience with species like bald cypress where Florida collected trees aren't winter hardy here, but Louisiana collected are tough as nails in the cold here.
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