Quick answer: 80 native Wisconsin plants have a recorded bloom range that includes May.
80 species
- American basswood Tilia americana · 32 states
- American bittersweet Celastrus scandens · 11 states
- American bladdernut Staphylea trifolia · 15 states
- American hazelnut Corylus americana · 16 states
- American hornbeam Carpinus caroliniana · 29 states
- American plum Prunus americana · 25 states
- Bearberry Arctostaphylos uva-ursi · 16 states
- Black cherry Prunus serotina · 32 states
- Black walnut Juglans nigra · 33 states
- Bunchberry Cornus canadensis · 7 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
- Chokecherry Prunus virginiana · 35 states
- Cinnamon fern Osmunda cinnamomea · 13 states
- Climbing prairie rose Rosa setigera · 8 states
- Cockspur hawthorn Crataegus crus-galli · 4 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
- Creeping juniper Juniperus horizontalis · 10 states
- Downy hawthorn Crataegus mollis · 7 states
- Downy serviceberry Amelanchier arborea · 34 states
- Dutchman's breeches Dicentra cucullaria · 9 states
- Eastern hop-hornbeam Ostrya virginiana · 18 states
- Eastern leatherwood Dirca palustris · 5 states
- Eastern purple coneflower Echinacea purpurea · 22 states
- Eastern red cedar Juniperus virginiana · 36 states
- Eastern red columbine Aquilegia canadensis · 36 states
- Eastern white pine Pinus strobus · 19 states
- Foxglove beardtongue Penstemon digitalis · 31 states
- Golden alexanders Zizia aurea · 12 states
- Gray alder Alnus incana · 12 states
- Gray dogwood Cornus racemosa · 8 states
- Grayhead coneflower Ratibida pinnata · 12 states
- Green ash Fraxinus pennsylvanica · 23 states
- Heart-leaved meadow parsnip Zizia aptera · 8 states
- Highbush cranberry Viburnum opulus var. americanum · 8 states
- Indigo bush Amorpha fruticosa · 16 states
- Jack-in-the-pulpit Arisaema triphyllum · 14 states
- Lanceleaf coreopsis Coreopsis lanceolata · 28 states
- Large-flowered bellwort Uvularia grandiflora · 8 states
- Low prickly pear Opuntia humifusa · 6 states
- Lowbush blueberry Vaccinium angustifolium · 14 states
- Marsh marigold Caltha palustris · 8 states
- Mayapple Podophyllum peltatum · 18 states
- Mexican hat Ratibida columnifera · 17 states
- Nannyberry Viburnum lentago · 23 states
- New Jersey tea Ceanothus americanus · 31 states
- Northern red oak Quercus rubra · 24 states
- Old man's whiskers Geum triflorum · 8 states
- Pagoda dogwood Cornus alternifolia · 25 states
- Partridgeberry Mitchella repens · 17 states
- Prairie crabapple Malus ioensis · 8 states
- Quaking aspen Populus tremuloides · 24 states
- Red baneberry Actaea rubra · 22 states
- Red pine Pinus resinosa · 13 states
- Red-osier dogwood Cornus sericea · 22 states
- Richardson's alumroot Heuchera richardsonii · 8 states
- Shagbark hickory Carya ovata · 22 states
- Smooth Solomon's seal Polygonatum biflorum · 13 states
- Smooth sumac Rhus glabra · 34 states
- Sugar maple Acer saccharum · 28 states
- Sundial lupine Lupinus perennis · 25 states
- Swamp white oak Quercus bicolor · 9 states
- Virginia bluebells Mertensia virginica · 14 states
- Virginia creeper Parthenocissus quinquefolia · 27 states
- Virginia iris Iris virginica · 9 states
- Virginia strawberry Fragaria virginiana · 8 states
- White ash Fraxinus americana · 31 states
- White baneberry Actaea pachypoda · 16 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