Quick answer: 46 native Illinois plants have a recorded bloom range that includes May.
46 species
- American basswood Tilia americana · 32 states
- American hazelnut Corylus americana · 16 states
- Black cherry Prunus serotina · 32 states
- Bur oak Quercus macrocarpa · 24 states
- Butterflyweed Asclepias tuberosa · 37 states
- Canada anemone Anemone canadensis · 22 states
- Canada wild rye Elymus canadensis · 18 states
- Canadian wild ginger Asarum canadense · 8 states
- Cardinal flower Lobelia cardinalis · 37 states
- Climbing prairie rose Rosa setigera · 8 states
- Common blue violet Viola sororia · 21 states
- Common hackberry Celtis occidentalis · 15 states
- Common ninebark Physocarpus opulifolius · 22 states
- Common winterberry Ilex verticillata · 10 states
- Coralberry Symphoricarpos orbiculatus · 20 states
- Downy serviceberry Amelanchier arborea · 34 states
- Dutchman's breeches Dicentra cucullaria · 9 states
- Eastern purple coneflower Echinacea purpurea · 22 states
- Eastern red cedar Juniperus virginiana · 36 states
- Eastern red columbine Aquilegia canadensis · 36 states
- Eastern redbud Cercis canadensis · 30 states
- Eastern white pine Pinus strobus · 19 states
- Foxglove beardtongue Penstemon digitalis · 31 states
- Golden alexanders Zizia aurea · 12 states
- Grayhead coneflower Ratibida pinnata · 12 states
- Jack-in-the-pulpit Arisaema triphyllum · 14 states
- Lanceleaf coreopsis Coreopsis lanceolata · 28 states
- Large-flowered bellwort Uvularia grandiflora · 8 states
- Lowbush blueberry Vaccinium angustifolium · 14 states
- Mayapple Podophyllum peltatum · 18 states
- Mexican hat Ratibida columnifera · 17 states
- Nannyberry Viburnum lentago · 23 states
- New Jersey tea Ceanothus americanus · 31 states
- Ohio buckeye Aesculus glabra · 12 states
- Partridgeberry Mitchella repens · 17 states
- Quaking aspen Populus tremuloides · 24 states
- Red-osier dogwood Cornus sericea · 22 states
- Shagbark hickory Carya ovata · 22 states
- Smooth Solomon's seal Polygonatum biflorum · 13 states
- Sundial lupine Lupinus perennis · 25 states
- Virginia bluebells Mertensia virginica · 14 states
- Virginia creeper Parthenocissus quinquefolia · 27 states
- White oak Quercus alba · 33 states
- White wild indigo Baptisia alba · 10 states
- Wild blue Phlox divaricata · 32 states
- Wild geranium Geranium maculatum · 11 states