To begin with, I’ve always been curious about how any given cultivar receives its hardiness zone rating. For example, if you purchase a Zone 5 rose, does that mean it will survive “most” Chicago winters? Is that 1 or 2 days of extremes, or several weeks? And how much die back? 10%? Or will it die back to the ground? Does the rating come from the breeder? From the ARS? General consensus?
Anyway, it certainly seems to me that hardiness ratings are a soft, rather than hard science. So this year, I decided I’d record my comparative findings for % die-back at the end of this winter before pruning. I have about 100 different cultivars, almost all planted in the spring of 2020 when I moved to a new house on the Ohio river, right on the USDA 6a/6b borderline, moderated to some degree by close proximity to the river. For the winters of 2020/21, and 2021/22 we had actual low temps of +9 and +5F temperatures respectively, and little or no die back even to such obviously zone-pushed roses as Lady Banks.
This year, however, we experienced a low of -6F, followed by nearly 2 full days below zero. Now, most roses are demonstrating various degrees of damage from mild to severe, so I thought this might be of some use to those who breed for hardiness (although there might be few roses here that any sane person is using in a breeding program😉)
All roses here receive approximately 6” of wood mulch in the fall, and no other protection. The percent of die back recorded is just a rough and ready assessment of the amount of green cane remaining above the mulch level. I prune all roses except once-bloomers in late winter.