|What perennials grow well in the mountains (elev 4200 ft)?|
|As far as I know, plants do not respond to altitude in particular. They do however have cold hardiness restrictions, typically expressed as winter hardiness zones.
If you are new to perennials, I would suggest you begin with some of the most reliable plants such as sedums, daylilies (Hemerocallis) and bulb lilies (Lilium), purple coneflower (Echinacea purpureum), black eyed Susan (Rudbeckia), Phlox paniculata "David", salvias such as "May Night" or "East Friesland" and, if you like them, ornamental grasses. Those would all do well in a sunny location (sun all day, or sun all afternoon, or sun for at least six hours including noon) with average or better soil and average soil moisture. For shade to part shade (morning sun or dappled light all day) you might try hostas (available in many leaf colors and patterns) as well as pulmonaria, ferns, Japanese anemones, Tricyrtis, violas, and if your soil is evenly moist, Astilbe.
Once you have experimented a bit, you will see which plants do best for you and can expand the selection accordingly. You might also want to look at a book or two about perennials, one I like is "Perennials for Dummies" by Marcia Tatroe, ISBN 0-7645-5030-6. It includes practical advice on analyzing the site and preparing the planting area, selecting the plants, planting the plants, and how to maintain them thereafter. It also has many illustrations and many suggestions for developing a pleasing arrangement based on a theme or on personal taste.
Your local county extension and professionally trained nursery personnel should be able to tell what zone you are in and provide suggestions about plants that do particularly well in your area -- tailored to suit your specific growing conditions and microclimate in your yard. The county extension should also be able to help you with some basic soil tests and interpreting the results with an eye on growing perennials. Soil preparation is very important for a successful perennial garden, so this should be one of the first things that you do.
Finally, the USDA plant hardiness zone map can be found at the following url
along with explanatory information to help you understand how to use the map in terms of analyzing your own microclimate and selecting plants.
Enjoy your flowers!