Willow-leaf oak is closely related to emory oak, but the leaves are generally shinier, slimmer, much longer, and brighter green. Branch tips are generally less robust than emory oak. There is overlap in range of the two species in Sonora, but generally as one heads south Q. emoryi fades out as Q. viminea becomes more common. in areas of overlap they usually grow at different elevations with emory oak at the lower end of the oak zone, often withor just above blue oak. Quercus viminea often grows in the mid to upper oak woodland zone and sometimes into the pine/oak woodland.
Other differences that help to distinguish it from emory oak where they overlap: the growth form of willow-leaf oak is often different with ascending, less angular branching; emory oak (less in large specimens) generally have one main trunk with very stiff, thick, angular branches coming off more or less parallel to the ground with hard angles whereas willow-leaf has a much more flowing, upward angling branching pattern; when the leaves dry on the plant emory oaks are generally a tan to brownish or slightly rusty color, where willow-leaf are a fairly bright red.
The only record in Arizona is in the Patagonia Mountains from the early 1900's until May 2013 when Tom Van Devender collected specimens also in the Patagonias. Quercus viminea is fairly common in some parts of the Sierra Pinito and other northern Sonora Sky Islands. It is interesting that they have not been found in any other Arizona Sky Islands.