In Southern Cal you might see them as a streetside tree that somebody planted, especially in older neighborhoods. They don't seem to need any special care, other than the care any other citrus would need. Mainly keeping freezing temperatures off the plant works wonders, although they do take some light frost, I think anything lower than 28 degrees will harm them.

Other than that, they produce jillions of fruit and make very tasty marmalade!

I have never started one from seed, but that method could result in a long juvenile period where you might wait 5 or 10 years for the plant to mature so it can make fruit. Grafted plants are the way to go. If you had a greenhouse to protect from freezing temps, you could easily grow them in the PNW.

My neighbor a few miles down the road just the other day gave me a half a bucket of navel oranges, raised right in his greenhouse! They are ripe, and you cannot tell the difference between them and storebought navel oranges.

Maybe I will get him a kumquat tree too!