The flower for May (at least in my world anyway) is Arrowleaf Balsamroot (Balsamorhiza sagittata). It is a member of Asteraceae or the sunflower family. It grows throughout the American west and is very prolific here in Eastern Washington. It just started to bloom and I have found that if I wait a few weeks the prints are much more vibrant and clear. And so we wait. I don't use the flowers (although sometime I should at least give it a whirl on paper), just the leaves. The scarf below was mordanted with alum and the print is anything from pale green to bright yellow. With an iron or rust pre-mordant it would come out olive green. It would also be possible to dip the yellow results in an iron post dip and achieve olive green also. As mentioned, the plant is very prolific here, but as with all plants in the wild, I approach harvesting with caution and common sense. I go where there is a lot of it and only pick one or two leaves off several plants over a large area; and harvest in a different area each time.
The pale green leaf down in the corner is the arrowleaf balsamroot, the bright orange is its malcontented neighbor spotted knapweed.