Dormancy is a temporary suspension of visible growth of any plant structure containing a meristem*. The term "dormant" is not restricted to deciduous plants but applies also to evergreen and semi-evergreen plants, which can retain some, or all, of their foliage while having dormant buds. All daylilies, regardless of foliage habit, are capable of cold temperature dormancy in the technical sense where it gets cold enough to suspend growth.
* Reference: Lang et al, “Endo-, Para-, and Ecodormancy: Physiological Terminology and Classification for Dormancy Research”, HortScience, Vol 22(3), June 1987.
Also see: Deciduous.