Answer: Your USDA Zone is determined by your average minimum winter temperature. According to my map, it looks like you are in zone 4, with an average annual minimum temperature of 20 degrees below zero. The is the zone system that most reference books and plant catalogs use.
Perhaps a more helpful zone map for you is the Sunset Zone map, which determines zones by a number of factors, including rainfall, elevation, etc. According the the Sunset Western Garden Book, you are approximately in zone 2.
Both zone systems are useful, just be certain that you know which system the book or nursery is using to classify plants.
Perennials for hot, dry conditions include: yarrow, gaillardia, bearded irises, evening primrose, Russian sage, potentilla, and coreopsis.
Ground covers include wooly yarrow (Achillea tomentosa), rockspray cotoneaster (C. microphyllus) and thyme
Hope this helps!
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