The Pacific Northwest region typically includes the states of Oregon, Washington as well as Northern California. From a climate standpoint, in relation to gardening, the Pacific Northwest region has been extended by the National Gardening Association to also include southern British Columbia of Canada.
According to this map of the Pacific Northwest, the region lies in USDA Plant Hardiness Zones 7 through 9 and in AHS Heat Zones 1 through 6. Stretching over a long expanse of territory, the Pacific Northwest climate is influenced by the Pacific Ocean and the Cascade Mountains. Generally speaking, this region enjoys 2 main distinct seasons characterized by mild but very wet winters, cool but very dry summers.
The frost free growing season covers approximately 200 days with the last frost dates ranging from mid-April and the first frost dates starting about mid November. Gardeners in this region are able to grow more perennial varieties than gardeners in any other parts of North America. In fact, this region is famous for its abundant production of tulips, daffodils, dahlias, garden lilies and many more perennial plants. Some plants grown as annual plants in other parts of the country (e.g. Iceland poppies or carnations), will happily overwinter in the Pacific Northwest and re-bloom the following year. An Eden for plants!
However, carefully selecting plants suited to the Pacific Northwest climate will be a key step to success. Plants will have to be drought tolerant in order to cope with the summer months with little or no rain. They will also have to survive the regular cold rainfall in winter and should be resistant to fungal diseases (as a result of the high humidity).