First blooms usually start appearing around mid to late April, and continuing until the hot (well, hot for Oregon) weather arrives sometime in early July. To catch a nice selection of wildflowers, I would probably suggest sometime in early to mid May. The exception to this is the higher elevations around Mt. Hood, where the lupine usually hit peak in mid to late June. It’s all dependent on weather conditions, and how long it stays cold. I think the wildflowers in the Columbia River Gorge are really spectacular in the first couple of weeks of May, when the bleeding hearts are blooming, along with a variety of other things.