Gardening with Native Plants: Savanna Highlights

Savanna plants blooming in the Native Plant Garden

Savanna plants blooming in the Native Plant Garden

Mid-summer heat or hint of fall? Droughty weeks or monsoon-like rains? During August in Wisconsin, all are possible. Seasonal gardening tasks include harvesting seeds from plants that bloomed early in the season, weeding, edging, and removing invasive species among tall late-summer species. We water new trees and shrubs when rainfall measures less than 1 inch a week. Established native plantings need no watering.

Besides being pleasant places to explore during August, savannas offer inspiration and plant choices for native plant gardening. These sites feature open-grown trees with spreading branches, partial shade at the ground layer, and understory plants that are easy to grow. In home gardens with large trees, or where slopes and buildings create partially shaded or moist microclimates, there will be suitable conditions to grow native species found in savannas.

Cool season grasses that thrive in light shade, Canada rye, bottlebrush grass, and silky wild rye (all Elymus species), disperse seed in August. These seeds germinate readily, without a dormant period. In a mature planting, they will re-seed at edges or in disturbed open areas, but not persist when other plants are dense.

Many summer-blooming species in savannas have pale flowers that contrast with the foliage and are easily seen in the shade. White flowers are prevalent in our bur oak savanna garden understory. Nodding wild onion (Allium cernuum), starry campion (Silene stellata), culver’s-root (Veronicastrum virginicum), flat-topped aster, (Doellingeria umbellata), and flowering spurge (Euphorbia corollata) can do well in semi-shade or sunny areas with mesic soil conditions. Many of these species have good wildlife value for insects (pollinators and as food for caterpillars) and other animals. By the end of August, some of these will be producing seed, which you can collect to enhance your plantings in new areas.

Among the aster species blooming in August are two with heart shaped basal leaves and narrow leaves on the part of the stem that bears flowers: sky-blue aster (Symphyotrichum oolentangiense) and arrow-leaved aster (Symphyotrichum urophyllum). Sky-blue aster has rough-surfaced leaves and pale blue flowers. Arrow-leaved aster has smooth leaves with winged petioles, and flower color ranging from white to blue/purple. Mesic soils in open areas or light shade provide good growing conditions for both species.

Spend time this month in the dappled shade of savannas for inspiration for your own garden or shady restoration.

Looking ahead, our Native Gardening Conference on September 17 offers resources, tips, and ideas for your garden or small restoration. Author and garden designer Benjamin Vogt will present the keynote “A New Garden Ethic.” Registration for the conference is open through September 7. See you there!

—Susan Carpenter, Arboretum native plant gardener