CALIFORNIA NATIVES HAVE a reputation for being intolerant of extreme cold. This truism is so universally acknowledged that most rock gardeners east of the Sierra shun plants from that state. And so it was with me in Colorado for much of my rock gardening career. Accidentally at first, and then with greater and greater determination, I have found some spectacular rock garden plants that not only survive but thrive in the severe continental climate of the Front Range of Colorado. I have a hunch that many of these (and many more) will prove not just hardy but tough across much of North America.