One of the things you have to be aware of is that desiccation can create some problems with lots of plants. If the soil freezes solid and there's no form of protection for the buds on the upper part of the crown and you get a really windy winter, those buds can lose enough moisture to burn right off. Yes, hops are pretty resilient but it can happen. Even if you don't have access to a suitable mulch, some of the surrounding soil can be mounded up to help them make it through a windy winter . . . or hope you get some good snow cover. It's kind of a long shot but it never hurts to err on the safe side!