I too was surprised at the cold comment because "common hackberries" grow here in Zone 4 and are rated as cold tolerant to zones 2 and 3. So I did some Googling and there are at least two other Celtis species that are grown in the milder areas and which would not be hardy here. So, as noted, we aren't all necessarily talking about the same species. Having said that, I do remember one spring that had a very late freeze which defoliated some hackberries growing near here so although well and truly hardy in winter they certainly didn't like cold once leafed out. But they regrew the leaves quickly afterwards and all was good after that.
