Wisconsin spring is in full swing in May and change is everywhere – budding, germination, decomposition, emergence, migration, fresh new growth, and warmth. Daylength, sun angle, and light intensity approach peak levels. Plant orders arrive and seedlings move out of the greenhouse. Volunteers and students are busy with outdoor work. Visitors enjoy the Arboretum in spring bloom and gather for the plant sale. May brings plants and people together.

Woodlands and woodland gardens are rich with understory flowers (as well as flowering trees) as leaves expand and the tree canopy closes, shading the forest floor. Prairies, savannas, and gardens that represent these plant communities green up rapidly in areas where prescribed fire was applied. Where fires did not clear dried vegetation, established perennials push through thatch. When seed dormancy breaks and moisture and light requirements are met, seeds germinate on bare or disturbed soil. Throughout the growing season, light (shade or sun) as well as soil type and moisture will help determine how successful different species will be.

Spring plant sales are held this month, and a wide assortment of native plant species is available. Plant selection is guided by questions and observations. How much full sun does the site receive? (“Full” sun is defined as 6 or more hours of full sun.) Many gardens have partial shade or edges with dappled light. Some are fully shaded with mature trees and a closed canopy. Is the soil well-drained or are there lower-lying areas that remain moist? Is there a suitable space for a rain garden? Select appropriate species that, growing together, have sequential and overlapping flowering, as found in regional plant communities.

The largest plant families in Wisconsin include the sunflower (Asteraceae), sedge (Cyperaceae), grass (Poaceae), rose (Rosaceae), pea (Fabaceae), figwort (Scrophulariaceae), and mint (Lamiaceae) families. Considering these families alone, suggestions for a shaded or semi-shaded garden include large-leaved aster (Eurybia macrophylla) with flowers held on stalks above the large basal leaves, Short’s aster (Symphyotrichum shortii) which blooms in August and seeds throughout the garden, and late-blooming zig-zag goldenrod (Solidago flexicaulis).

Sedges (Carex spp.) and grasses generally thrive in full sun, but a few species do well in shadier spots: small oak sedge (Carex pensylvanica) and larger Sprengell’s sedge (Carex sprengelii) bloom in April and May, and bottlebrush grass (Elymus hystrix) blooms in mid-summer. Both sedges can be planted densely as a ground cover or with other plants interspersed. Mints thriving in semi-shade include spring-flowering wood mints (Blephilia spp.) and summer-blooming giant hyssops (Agastache spp.). These mints spread by seed, not by rhizome. Woodlands and savannas can inspire other plant choices for shaded gardens.

Suggestions for full sun include spring- and early summer-blooming sunflower family members prairie coreopsis (Coreopsis palmata) and yellow coneflower (Ratibida pinnata), and legumes wild indigo (Baptisia spp.) and leadplant (Amorpha canescens), which are important pollinator resources. From the rose family, the small herbaceous plants wild strawberry (Fragaria virginiana) and prairie smoke (Geum triflorum), as well as shrubs and trees such as blackberries and raspberries (Rubus spp.) and cherries and plums (Prunus spp.), bloom early in the season. In the figwort family, penstemon species and Culver’s root (Veronicastrum virginicum) are in flower spring into summer. Many other species in the prairie flora are suitable for gardens in full sun.

Many of these plants (and more) for shade and sun will be available at the Friends of the Arboretum plant sale on May 17. We hope to see you there!

– Susan Carpenter, native plant garden curator