Quick answer: 29 native Colorado plants have a recorded bloom range that includes May.
29 species
- Alderleaf mountain mahogany Cercocarpus montanus · 9 states
- American plum Prunus americana · 25 states
- Apache plume Fallugia paradoxa · 6 states
- Banana yucca Yucca baccata · 6 states
- Bearberry Arctostaphylos uva-ursi · 16 states
- Buffalograss Bouteloua dactyloides · 9 states
- Butterflyweed Asclepias tuberosa · 37 states
- Chokecherry Prunus virginiana · 35 states
- Common elderberry Sambucus nigra ssp. canadensis · 16 states
- Common hackberry Celtis occidentalis · 15 states
- Common ninebark Physocarpus opulifolius · 22 states
- Common yarrow Achillea millefolium · 31 states
- Creeping Oregon grape Mahonia repens · 11 states
- Creeping juniper Juniperus horizontalis · 10 states
- Gambel oak Quercus gambelii · 6 states
- Gray alder Alnus incana · 12 states
- Green ash Fraxinus pennsylvanica · 23 states
- Highbush cranberry Viburnum opulus var. americanum · 8 states
- Indian blanket Gaillardia pulchella · 18 states
- Indigo bush Amorpha fruticosa · 16 states
- Mexican hat Ratibida columnifera · 17 states
- Quaking aspen Populus tremuloides · 24 states
- Red baneberry Actaea rubra · 22 states
- Red-osier dogwood Cornus sericea · 22 states
- Rocky Mountain bee plant Peritoma serrulata · 22 states
- Rocky Mountain penstemon Penstemon strictus · 4 states
- Silver buffaloberry Shepherdia argentea · 11 states
- Smooth sumac Rhus glabra · 34 states
- Spotted beebalm Monarda punctata · 14 states