Gardening with Native Plants: Silphium Summer

Tall sunflowers with yellow blooms in native plant garden

Compass plant blooming in the Native Plant Garden

During the past month, the Arboretum and the Madison area have experienced extreme or severe drought. Apparently unaffected by the dry conditions, the four Silphium species (prairie dock, compass plant, cup plant, and rosinweed) bloomed more conspicuously than usual in the prairie garden and Curtis Prairie. Because of the drought, many other prairie species did not grow as tall as usual and bloomed at shorter heights, with four-to-six-foot flowering Silphium stalks towering over them.

Compass-plant (Silphium laciniatum) was the first to bloom, beginning in late June. Most of its flower heads now hold mature seed and many seed heads have been harvested by birds and agile chipmunks. Rosinweed (S. integrifolium) and cup-plant (S. perfoliatum) bloomed next, with prairie dock’s (S. terebinthinaceum) tall but slender flower stalks extending last. In the dry mesic prairie area, near a cup-plant that we did not intentionally plant in the garden, we found a plant that appears to be a hybrid between cup-plant and rosinweed. While we have cut the flower heads from the cup-plant to prevent self-seeding, we would like to collect seeds from the possible hybrid. There will be no shortage of Silphium seeds this season.

Although there is variation from site to site within the garden, the tall warm season grasses are less prominent this year than usual. In our new dry prairie garden (planted in 2021 and 2022), the short warm season bunchgrasses are thriving. Purple love grass (Eragrostis spectabilis), side oats grama grass (Bouteloua curtipendula), prairie dropseed (Sporobolus heterolepis), and little bluestem (Schizochyrium scoparium) are full-sized and blooming. The cool season grass in that garden, Junegrass (Koeleria macrantha) did not set seed well this year, likely because of hot temperatures and drought conditions during and immediately after its bloom time in June.

Some species usually seen in late summer – for example, bottle gentian (Gentiana andrewsii) and prairie blazing star (Liatris pycnostachya) – did not grow or bloom in the garden this season. They grow well in mesic and wet mesic sites which were dry this season. In contrast, rough blazing star (Liatris aspera), which thrives in drier sites, is blooming well in the dry prairie. Joe-pye weed (Eutrochium maculatum) growing near the downspouts on the north side of the Visitor Center grew more than six feet tall, but the stand of Joe-pye weed in the rain garden only grew to four feet tall before it bloomed.

Joe-Pye weed
Joe-Pye weed

September is goldenrod and aster season. Following the earliest blooming species – flat-topped aster (Doellingeria umbellata) and elm-leaved goldenrod (Solidago ulmifolia) in the savanna garden – dozens of other aster and goldenrod species bloom this month. We are seeing some variation from typical bloom timing and plant size within these groups, perhaps due to the drought.

Although it is a native plant, we use hand weeding to discourage Canada goldenrod (Solidago canadensis) from spreading in garden areas. It originally arrives in the garden by seed. First year plants have a small root but send out horizontal rhizomes (underground stems) to produce plants nearby in the following year. Mature plants send out rhizomes in early August, prior to blooming. So their clonal growth is set before their flowers open, are pollinated, and produce seed. This year, we noticed that the mid-summer plants were not rooted down as tightly as usual at the base of each plant, and from dry soils we were able to consistently remove long rhizomes as well as the plants. One large patch of Canada goldenrod in the mesic prairie garden was infested by the goldenrod leaf beetle earlier this season. It was badly damaged when the beetles were larvae, and now months later, it has barely recovered and may bloom weakly, if at all.

Are you noticing drought or weather-related variation in plant size, bloom quantity, and seed production in your garden?

Postscript: After a summer of gardening, monitoring bumble bees, and learning, the student gardeners returned to school this month. I’m grateful for their keen interest in native plants, their curiosity about animals and birds in the garden, and their positive approach to our garden work. Thank you, Kiana and Emma!

—Susan Carpenter, native plant garden curator