Quick answer: 24 native Washington plants have a recorded bloom range that includes May.
24 species
- Bearberry Arctostaphylos uva-ursi · 16 states
- Bog Labrador tea Ledum groenlandicum · 4 states
- Bog laurel Kalmia polifolia · 5 states
- Broadleaf cattail Typha latifolia · 11 states
- Bunchberry Cornus canadensis · 7 states
- California bay laurel Umbellularia californica · 3 states
- Canada wild rye Elymus canadensis · 18 states
- Checker lily Fritillaria affinis · 3 states
- Chokecherry Prunus virginiana · 35 states
- Common camas Camassia quamash · 3 states
- Creeping Oregon grape Mahonia repens · 11 states
- Douglas iris Iris douglasiana · 3 states
- Marsh marigold Caltha palustris · 8 states
- Pacific madrone Arbutus menziesii · 3 states
- Pacific trillium Trillium ovatum · 3 states
- Prairie Junegrass Koeleria macrantha · 15 states
- Quaking aspen Populus tremuloides · 24 states
- Red-osier dogwood Cornus sericea · 22 states
- Rocky Mountain bee plant Peritoma serrulata · 22 states
- Rocky Mountain iris Iris missouriensis · 7 states
- Smooth sumac Rhus glabra · 34 states
- Tall Oregon grape Mahonia aquifolium · 3 states
- Western azalea Rhododendron occidentale · 3 states
- Winterfat Krascheninnikovia lanata · 6 states