
Old field goldenrod (Photo: Susan Carpenter)
Will August bring mid-summer heat or a transition to fall? Dry weeks or more rainy days? Gardening tasks include harvesting seeds from plants that bloomed earlier in the season, weeding, monitoring bees, removing non-native and even aggressive native plants among many tall late-summer species. With early season warmth and ample rains this year, the garden is taller and filled with more flowers than ever before.
Besides being pleasant places to explore during August, savannas offer inspiration and practical ideas for native plant gardening. These sites feature stately trees, partial shade at the ground layer, and showy late-season plants that are easy to grow. In home gardens with large trees, or where topography and buildings create partially shaded or moist microclimates, there will be suitable conditions for native savanna understory species.
Cool season grasses that thrive in light shade – Canada rye (Elymus canadensis), bottlebrush grass (Elymus hystrix), and silky wild rye (Elymus villosus) – begin seed dispersal this month. The seeds germinate readily, without a dormant period, allowing the grasses to spread where seeds land on bare or disturbed soil. These species grow vegetatively during spring and fall.

Members of the sunflower family, Asteraceae, bloom in savannas this month. Elm-leaved goldenrod (Solidago ulmifolia) and old field goldenrod (Solidago nemoralis) can grow in a range of open to shaded sites. The former is up to four feet tall with ovate leaves while the latter grows up to thirty inches tall and has elongated grey green leaves. Both have arching stems of bright yellow flowers.

Among the aster species blooming in August are two with heart-shaped basal leaves and narrow leaves on the flowering stem – sky-blue aster (Symphyotrichum oolentangiense) and arrow-leaved aster (Symphyotrichum urophyllum). These species have rough-surfaced leaves and smooth leaves with winged petioles, respectively. Mesic soils in open areas or light shade provide good growing conditions for both species. Their blue flowers produce wind-blown seeds; they do not spread by rhizomes. In full bloom this month, flat-topped aster (Doellingeria umbellata) spreads by rhizome as well as seed. It is perennial and grows up to three feet tall, with white flower heads.
Members of the mint family, giant purple hyssop (Agastache scrophulariifolia) and anise hyssop (Agastache foeniculum), bloom in light shade, reaching four to six feet in height. These species are valuable nectar sources for butterflies, bees, hummingbirds, and other pollinators. We often observe recently emerged bumble bee future queens (gynes) foraging on these hyssops, perennials that spread by seed, not rhizomes.
Enjoy flowers and gardening in the savanna’s shade as the end of summer draws closer.
—Susan Carpenter, native plant garden curator