Endless varieties of forms and colors.
A great place to see Peonies is the Adelman Peony Gardens, located about a mile east from exit 263 on I-5 in Oregon (near Salem). The peonies start blooming locally in late April, but really peak in mid to late May in most years.
The Skagit Valley tulip festival usually runs through most of April. The fields are located in the area between Mt. Vernon and LaConner Washington.
Wherever you go, get there early to avoid the crowds on sunny weekends (or go during the week!)
The Woodburn, OR tulip fest usually runs during the...
Dahlias are a popular flower that have been bred into a myriad of shapes and colors. The largest grower in the United States is Swan Island Dahlias. They're located in Canby Oregon, just north of Salem, or about a 1 hour drive south from Portland. Check their web site for detailed directions and more...
A great place to see irises is the display garden at Schreiner's. It's located in Oregon, just off I-5 at exit 263. It's best to go in the early morning for best photographic conditions, and to escape the crowds. Weekdays can be similarly rewarding.
The Flower Fields (yes, that's the official name) are located in Carlsbad, California. They're right off of I-5, and easy to find. Try to visit during the week. This is one of the most populated areas of the US, and the crowds can get pretty insane on the weekends.