I doubt that hardiness is the issue---my very large Anemonopsis colony back in Ontario certainly did fine with lows to -35C or colder and with often, no snow cover. Usually frozen rock solid.
BUT my plants *always* struggled in the hot, dry and humid summers, even though they grew in the best woodland conditions I was able to provide (good, rich soil, moist but well drained, etc. etc.) I had to water them almost non-stop during the summer to keep the foliage from becoming crunchy and even retreating underground, even though they were in high shade. By flowering time, if I was lucky, I could count on 30-50% of the plants flowering---only the ones that had not been weakened by all the heat and humidity that never abated, even at night. Even my succulents had problems in that climate as their CAM mechanism (which needed some relief from the heat at night) could never get it.
I brought a large colony of them with me when I moved here to Nova Scotia---where they have thrived from the minute they got planted. In the beginning this was not even in proper soil---they were literally just heeled in the existing sandy soil as I was just building new woodland gardens from bare lawn. Soil here is very acidic--back in Ontario was very alkaline. As I have begun to improve the soil conditions to rich, humousy, I have actually not noticed much change in their health, which is interesting.
They are also now in a maritime climate, with plenty of natural moisture in the air and cool summers. Even when it is hot during the day, it always cools down at night. The level of heat and humidity is nothing compared to Ontario. What I find the most difficult is figuring out how to grow them to best effect in the garden---as I find them very difficult to place right aesthetically.
I would have guessed that being in Washington State, your weather, at least, might be similar to that here.
