Asters — Color and Life in the Fall Garden
There are approximately 250 types of asters growing around the world. In 1994, Dr. Guy Nesom, a research botanist, attempted to reclassify the genus into a number of smaller units based on morphology (form and structure) and chromosomes. He concluded that none of the American so-called “asters” were closely related to Eurasian asters. The genus Aster is now restricted to plants from Eurasia. This revision makes it easier for us to identify native asters. If the botanical latin name of a plant includes the genera Aster, it is not native to the U.S.
The species native to North America that were formerly classified as being in the genus Aster have been divided into separate genera. Eastern North American genera include: Symphyotrichum, Eurybia, Ionactis, Seriocarpus, Doellingeria, Ampelaster and Oclemena. The majority of the former aster species belong to two genera, Symphyotrichum and Eurybia. The common name for all the above genera is still aster, and will be the common term used in this article, which focuses on North American native asters.
Virtually all of the native aster species are perennials with alternate leaves and often, prominent basal leaves. Most species have fibrous root systems; some species have rhizomes (underground storage stems), or stolons (above ground root stems), and can be aggressive spreaders. Some also spread by self-seeding. Plants range from less than 1’ to 7’ tall. While most species prefer full sun and dry-to-medium moisture, some species tolerate shade and wetter conditions. Habit tends to be leggy; cutting stems back by half by early summer will help promote bushiness, although flowering might be a little delayed. Many asters are browsed by deer and rabbits.
Flower heads are made up of 1) petal-like ray flowers that can vary in color from white to pink to blue to purple and 2) tubular disk flowers that are usually yellow. In some species, as many as 300 disk florets can be tightly packed into the circular flower head. The disk flowers are perfect (bisexual with male and female structures) and are fertile. In some species, the ray flowers are pistillate (female) flowers that can be pollinated and produce fruits, while in other species the ray flowers may be sterile.
LOCAL NATIVE ASTERS
As native plant enthusiasts, we can choose asters that are native to the U.S, or to the eastern part of the U.S., or to the mid-Atlantic region, or we can hone our selections even further. Choosing plants that are local to our specific area means that the plants should thrive in the soil, elevation, hydrology, and climate in which they’ve evolved. (Of course, the soil in many of our home landscapes has been disturbed.) Just as an illustration, I’ve selected asters through the Virginia Department of Conservation and Recreation Native Plant Finder. Once you get to the link, type in aster under “common name” and then look for the asters that are in the P (Piedmont) region.
Asters specific to the Piedmont region:
Chrysopsis mariana, Maryland golden aster, grows 1-2.5’ tall on stout, silky-woolly stems from a low rosette of basal leaves. It prefers partial or full sun, and low-to-medium moisture, although it tolerates some drought once established. It has an upright habit with fibrous roots and short stolons. The plant is crowned from August to October with clusters of yellow daisy-like flowers. Each flower has bright yellow rays surrounding a center disk of tiny yellow florets.
Doellingeria umbellata, flat-topped white aster, grows to 7’ tall. It prefers partial or full sun, and medium-to-high moisture. The upper leaf surface is medium to dark green, while the lower surface is pale green or whitish green. Flowers have white rays and yellow disk centers. Flat-topped aster is the primary host plant of the butterfly, Harris’ Checkerspot (Chlosyne harrisii). American tree sparrows feed on the seeds, while the ruffed grouse feeds on both the leaves and seeds; deer browse on the foliage as well.
Eurybia divaricata, white wood aster, grows to 3’ tall, prefers partial or full shade, and low-to-medium moisture. Distinctive leaves are heart-shaped, stalked, and coarsely toothed. Small but abundant flowers have white rays (infrequently pink or blue) and yellow-to-red center disks that are borne on dark burgundy, wiry stems. The flowers appear in flat-topped, terminal clusters in late summer to early fall. The plant has no serious insect or disease problems, although it is susceptible to powdery mildew. Aster wilt can also be an occasional problem, particularly if plants are grown in poorly-drained, clay soils. Good air circulation and some morning sun help reduce the incidence of foliar diseases. Propagate by division in spring.
Symphyotrichum concolor, eastern silvery aster, grows 2-3’ tall, and prefers full sun and low moisture. The oval leaves are downy-white or silvery, giving rise to the plant’s common name. Both the small leaves and the violet-blue flowers cling tightly to the long slender stems, giving a wand-like appearance. The plant blooms from September to October. It is browsed by deer.
Symphyotrichum cordifolium, blue wood or heart-leaved aster, grows 3-5’ tall and 2-3’ wide. It is easily grown in average, dry-to-moist, well-drained soils in full sun to part shade. It also tolerates clay and drought, but can be sensitive to poorly-drained soil and poor air circulation. Leaves are sharply-toothed, with the lower ones being heart-shaped, hence the common name. Bloom color is blue-violet to lavender, with yellow centers that mature to purple-red. There are no serious pests or diseases; it is sometimes browsed by deer.
Symphyotrichum pilosum, frost aster or hairy aster, has hairy stems that appear to be covered with thick frost. Leaves are also hairy. The plant is easily grown in moist, well-drained soils in full sun to part shade (more shade in southern climates), and can tolerate periodic flooding. It grows 2-4’ tall and wide. Flowers bloom from August to October. Each flower features white rays surrounded by a pale yellow center disk. Disk flowers turn reddish-purple with age. Stems may be pinched back in late spring to early summer to promote bushiness. Plants can spread aggressively by self-seeding. This plant is considered weedy in some areas of the U.S.
Asters are pollinated by long-tongued bees, short-tongued bees, butterflies, and moths. Asters provide nectar for many different butterflies and moths, and are important late-season nectar sources for monarch butterflies migrating from the north. According to the National Wildlife Federation Native Plant Finder (by zip code), asters in the zip code area 22901 are caterpillar hosts to 106 species of butterflies and moths. Check the link for your zip code. As Doug Tallamy teaches, using native plants in our suburban gardens creates a simplified vestige of the richly diverse ecosystem that once existed. Most insect herbivores can only eat plants with which they share an evolutionary history.
Foliar rust and powdery mildew are limiting factors to growing asters successfully. Both diseases are debilitating to plant health and disfiguring to leaves and plant habits. Rust diseases produce reddish, orange, or brown pustules on the undersides of leaves and along stems of the entire plant. Rust fungi rarely kill infected plants because the fungi need living plants to survive; however, rust infections diminish the ornamental display by reducing flower production and weakening the plants. Recommendations for reducing or eliminating foliar diseases include: choosing disease-resistant plants, improving air circulation by providing good spacing between plants, thinning out one-third of the stems, and minimizing overhead irrigation.
GARDEN TRIALS (EVALUATION STUDIES)
In 2003-2009, Richard Hawke, Plant Evaluation Manager of the Chicago Botanic Garden, conducted a comparative trial on 119 different asters including species, cultivars, and hybrids of native and nonnative asters. While the typical evaluation period for perennials is four years, the average evaluation period for the asters in this trial was six years. Performance ratings are based on flower production, plant health, habit quality (such as uprightness, compactness, “legginess”), and winter hardiness. Although this study is now about 10 years old, and the setting was not representative of Virginia growing conditions, the clear winners among fall bloomers were several American native species. Hawke’s best overall performer was aromatic aster (Symphyotrichum oblongifolium). This aster is native to parts of eastern and central U.S., including Virginia. With sky-blue flowers on stiff 1-3′ stems, it flowers prolifically into October, or even into November. The cultivar ‘Raydon’s Favorite’ was a top performer. Its billowy habit is well-suited to mass plantings and naturalizing.
Aromatic aster outperformed the familiar, often-recommended species, New England aster (Symphotrichum novae-angliae). The New England aster was more susceptible to powdery mildew than was aromatic aster, and its habit was leggier. The popular dwarf New England aster cultivar ‘Purple Dome’ often had browned-out leaves and bare lower stalks. The great virtue of New England aster and its cultivars is the greater range of colors, from purple through pink to white, while aromatic aster blooms only sky blue.
Heath aster (Symphyotrichum ericoides) is a bushy, somewhat compact plant with many-branched stems, typically growing 1-3′ tall. In late September or early October, this low plant is covered in with a fluffy carpet of flowers that can cloak the ground or cascade over a wall. Hawke’s favorite cultivar from the trial was ‘Snow Flurry’, which grows only 4-6” tall but up to 4’ wide, with a blanket of small white flowers. The cultivar is disease-free and tolerant of dry conditions.
Though most asters need full sun, one standout species in the trial did particularly well in dappled shade. White wood aster (Eurybia divaricata), an aster native to the Piedmont, is described above.
This article highlights just a few top performers from the trial. Refer to this link for a full description of the study details and the results published by the Chicago Botanic Garden in 2013.
MT. CUBA RESEARCH CENTER TRIALS
Aster trials were held from 2003-2006 at the Mt. Cuba Research Center in Hockessin, Delaware, about 30 minutes from the University of Delaware. In this study, 56 taxa of asters were considered. The results are similar to those at Chicago, with aromatic aster, S. oblongifolium ‘October Skies’ performing fourth best. The top performer was a smooth aster, S. laeve ‘Bluebird’, followed by a prairie aster, S. turbinellum, and a calico aster, S. lateriflorum ‘Lovely’. Two asters native to the Piedmont performed well: flat-topped white aster, Doellingera umbellata, and white wood aster, Eurybia divaricata ‘Raiche’. For a study description and full list of results, see this link.
I am going to discuss aromatic aster (Symphyotrichum oblongifolium) in more detail because it is native to parts of Virginia, was the top-performing aster in the Chicago Botanic Garden trials, performed well in the Mount Cuba trials, and is relatively unpalatable to deer, whereas many other asters are frequently browsed. Growing 1-3’ tall and wide, its rigid stems are much-branched from the base. Its oblong, toothless, blue-green leaves are fragrant when crushed. Both leaves and stems are sparsely covered with short hairs. Aromatic purple to lavender-blue flowers with yellow centers create dazzling purple mounds in the fall. The blooms make good cut flowers, and are attractive to butterflies. The plant is easily grown in average, dry-to-medium, well-drained soil in full sun, but it also does well in sandy or clay soils, and can withstand drought. It slowly colonizes by stolons, and regular thinning can help control its spread. The plant may open up if it gets too top heavy so it is a good idea to prune it back by no more than half by mid-June. Some support may be needed for taller plants since stems may tend to splay apart in autumn from the weight of the bloom. After it goes dormant, wait to cut back to the basal rosette in late fall, early winter.
Although many asters like drier conditions, swamp aster or purple-stemmed aster (Symphyotrichum puniceum) is a candidate for wet areas or for rain gardens, which is the reason it’s being highlighted in this article. Native to the eastern U.S., including Virginia, it is easily grown in average, moist-to-wet soil in full sun. The typical form has bristly, purplish stems and toothed, glossy, lance-shaped leaves. Bloom time is August-November with many light-violet or violet-blue rays surrounding yellow centers. The flowers are attractive to butterflies. This aster is a stout plant growing 4-8′ tall. It resembles a large and lighter-colored New England aster (S. novae-angleae). It easily self-seeds.
STRAIGHT SPECIES VS. CULTIVARS AS POLLINATOR PLANTS
In her studies on pollinator attractiveness at the University of Vermont, Dr. Annie White compared the New England aster, S. novae-angliae ‘Andenken an Alma Pötschke’ with the species.
The New England asters that I studied showed one of the largest differences that I saw between the native and the cultivar ‘Alma Potschke’—like 20 times more pollinators on the natives (straight species) than on the cultivars. That was one that really surprised me because the flowers are very similar morphologically, the same size, and they were blooming at exactly the same time. They just had a color difference. (The ‘Alma Potschke’ is a bright rose pink with yellow center disks.)
In a similar trial by Penn State, the straight species New England aster (S. novae-angliae) had three times more pollinator visits than the New England aster cultivar ‘Purple Dome.’
Many different species of native asters add color to the fall garden, and prolong the availability of pollen and nectar late into the season. Although many asters prefer dry-to-medium moisture conditions in full sun, there are options for dry, shady spots, or for wet areas of the landscape as well. There are also plenty of choices in height, habit, and color. Rust and powdery mildew can mar an aster’s beauty, so be sure to look for asters that are more resistant to these diseases. Add this low-maintenance native plant to your garden, and fall will be even more beautiful. Your late season garden will also be buzzing and whirring with life!
Bringing Nature Home, Douglas W. Tallamy, 2009.
“Flower Power: Cultivars vs. Straight Species,” Ann Lawson, The Humane Pollinator, https://www.humanegardener.com/flower-power-a-qa-with-annie-white/
Native Plant Finder, Virginia Department of Conservation and Recreation, https://www.dcr.virginia.gov/natural-heritage/native-plants-finder
Missouri Botanical Garden, Plant Finder, https://www.missouribotanicalgarden.org/plantfinder/plantfindersearch.aspx
“Native Asters Shine in the Garden,” Chicago Botanic Garden, Beth Botts, https://www.chicagobotanic.org/plantinfo/smart_gardener/native_asters_shine_garden
“A Comparative Study of Cultivated Asters,” Richard Hawke, Chicago Botanic Garden, Plant Evaluation Notes ISSUE 36, 2013,
“Asters for the Mid-Atlantic Region,” Mount Cuba Research Center,
“Overview of Chances to Asteraceae and Aster Species Found in Northern Virginia to Upper Shenandoah Park,” Prince William Wildflower Society, https://vnps.org/princewilliamwildflowersociety/botanizing-with-marion/overview-of-changes-to-asteraceae-and-aster-species-found-in-northern-virginia-to-upper-shenandoah-national-park/
Lady Bird Johnson Wildflower Center, https://www.wildflower.org/plants/result.php?id_plant=SYPUP
“Symphyotrichum cordifolium (Blue Wood or Heart-leaved Aster),” Master Gardeners of Northern Virginia, https://mgnv.org/plants/perennials/symphiotrichum-cordifolium-blue-wood-or-heart-leaved-aster/
Feature Photo: Aromatic aster (Symphyotrichum oblongifolium), Dr. Thomas G. Barnes, U.S. Fish and Wildlife Service, Public Domain