Aster adoration

Autumn is the time of asters. In days suffused with the brilliant tones of senescing leaves, asters finally show their true colors in gardens, both cultivated and natural, along roadsides, and in native places. Like clockwork, their starry flowers in rich hues of blue, purple, pink, or white burst forth to mark the change of seasons.

A ubiquitous nature often saddles asters with the reputation of looking too wild, but their natural beauty and garden merit cannot be overlooked. Asters blend beautifully with a variety of grasses and perennials in a quintessentially autumnal tapestry.

The Chicago Botanic Garden evaluated 119 asters between 2003 and 2009. The garden conducted full-sun and partial-shade trials. The goal was to identify outstanding asters for upper Midwestern gardens.

Seven asters received five-star excellent ratings for their overall performance, including Aster tataricus ‘Jindai,’ Eurybia divaricata, E. divaricata ‘Eastern Star,’ Symphyotrichum ericoides ‘Snow Flurry,’ S. lateriflorum, S. lateriflorum ‘Lady in Black,’ and S. oblongifolium ‘Raydon’s Favorite.’

Top-rated asters displayed consistently strong habits, superior flower production, excellent disease resistance, and full winter hardiness throughout a six-year evaluation term. Additionally, 19 asters received four-star good ratings for their strong performances.

The asters were evaluated on four general criteria: 1) cultural adaptability to the soil and environmental conditions of the test site; 2) disease and pest problems; 3) winter hardiness or survivability; and 4) ornamental qualities associated with flowers, foliage, and plant habit. Final performance ratings are based on flower production, plant health, habit quality, and winter hardiness.

Top-rated asters
Aster tataricus ‘Jindai’ is somewhat of an enigma among garden asters. Large, coarse basal leaves, nearly 2 feet long, are more reminiscent of horseradish than of a typical aster. Rhizomatous stems create vigorous clumps that make ‘Jindai’ a good massing plant. Flower stems begin to grow in August, eventually reaching up to 4 feet tall before flowers open in late September.

The lovely violet-blue and yellow flowers continue blooming well into late November or early December in Chicago. ‘Jindai’ is 2 feet shorter than the species.

White wood aster, Eurybia divaricata, is one of the few asters that grow well in shady gardens, although morning sun encourages more flower production. The smallish white flowers with yellow centers, produced in profusion from early August to early November, are borne on dark burgundy, wiry stems. Its mounded bushy habit was more upright than ‘Eastern Star.’ Differences between the species and ‘Eastern Star’ were minor but noticeable in side-by-side comparisons.

‘Eastern Star’ featured larger flowers, darker green leaves, and a slightly shorter stature. The arching dusky stems were more recumbent than the species, especially during the bloom period.

The white wood aster is particularly fetching planted in drifts in high-shade landscapes. The habit of white heath aster, Symphyotrichum ericoides ‘Snow Flurry,’ set it apart from any other asters in the trial. The low, arching stems create an undulating mounded habit only 8 inches tall but nearly 4 feet wide. The mat-forming habit makes a great groundcover and is an excellent choice to cascade over walls or containers. ‘Snow Flurry’ is aptly named for the snowstorm of white flowers that blanket plants in September and October. ‘Snow Flurry’ is also disease free and tolerant of dry conditions.

Calico aster, Symphyotrichum lateriflorum, is another aster with a singular habit and floral display. Wiry stems terminate in horizontally arched branches that give the plant its graceful habit. Sprays of small white blossoms cover the upper sides of the horizontal branches in late August to mid-September. New leaves emerge bronze but fade quickly to dark green. On the other hand, the leaves of ‘Lady in Black’ remain strongly purple all summer—full sun is essential to get the best color. While its bushy, vaselike habit is similar to the species, ‘Lady in Black’ was slightly more compact in our trial. As the flowers age, the disks change from yellow to purplish pink, which is particularly pleasing against the purple leaves.

Calico aster was untroubled by powdery mildew despite the fact that many neighboring species were infected. Unfortunately, all calico asters were browsed periodically by rabbits.

‘Raydon’s Favorite’ is one of the popular cultivars of the aromatic aster, S. oblongifolium. The slightly sticky, aromatic leaves superficially resemble S. novae-angliae but in this case the basal lobes do not clasp the stems. An abundance of blue-purple flowers are produced over a long period beginning in early August; in fact, precocious flowers often pop up a few weeks before blooming begins in earnest. The billowy habit of ‘Raydon’s Favorite’ has an informal look that suits it well to mass plantings and naturalizing. Aromatic aster is a good alternative to New England aster because it is more resistant to powdery mildew.

A range of habits
Plant habits range from less than a foot to more than 6 feet tall, and from clumping to rhizomatous. In fact, some species can be aggressive spreaders. A common problem of many garden asters is that their lower leaves desiccate during the summer, becoming unsightly and resulting in bare lower stems. Careful placement in the garden so that other plants mask the dying leaves mitigates this problem.

With such a large and diverse group of plants it is not easy to summarize their cultural needs in a few words. Generally speaking, asters grow best in moist, well-drained soils with plenty of sunlight. A few simple guidelines are helpful in choosing the best locations for specific asters. Asters can be lumped into three broad groups based on their native environments and garden requirements. The first group includes species that are native to meadows, prairies, marshes, and roadsides, and thrive in average to rich, evenly moist soils in full sun.

Symphyotrichum novae-angliae (formerly Aster novae-angliae), S. novi-belgii (formerly A. novi-belgii), S. lanceolatum (formerly A. lanceolatus) and Doellingeria umbellate (formerly A. umbellatus) do best with steady soil moisture. S. ericoides (formerly A. ericoides), S. laeve (formerly A. laevis), S. lateriflorum (formerly A. lateriflorus), S. oblongifolium (formerly A. oblongifolius), S. oolentangiense (formerly A. azureus), S. sericeum (formerly A. sericeus) and S. turbinellum (formerly A. turbinellus) prefer moist soils but are more tolerant of dry sites than others. Tatarian aster, A. tataricus, seems to have the greatest adaptability to a wide range of soil conditions.

The second group contains other sun-loving species such as Aster amellus, A. ×frikartii, and A. tongolensis that prefer cool night temperatures and need average to rich soils with excellent drainage for optimum growth. These asters are native to seashores and mountainous areas where good soil drainage is common. This group may be short lived in gardens, especially if good winter drainage is not provided.

The third broad group includes woodland asters that tolerate deep shade but bloom best in light to partial shade. Symphyotrichum cordifolium (formerly A. cordifolius), S. drummondii (formerly A. drummondii), Eurybia divaricata (formerly A. divaricatus) and E. macrophylla (formerly A. macrophyllus) grow best in moist, humus-rich soil but tolerate dry conditions too. Despite their adaptability to shade, these species will produce the best flower displays when given morning sun.

Disease pressure
Several foliar diseases such as powdery mildew and rusts are problematic for some asters, notably New England aster (S. novae-angliae) and New York aster (S. novi-belgii). The usual 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.

Classic companions
Asters are classic autumnal flowers of gardens, roadsides and native landscapes.

They are wonderful companions to a variety of other late-season perennials and grasses, whether peppered through a landscape or planted in large sweeps.

Copious flowers adorn asters in a stellar show of late-season color, blending beautifully with other perennials and ornamental grasses. The variety of habits and colors makes it easy to find an aster for any situation. Symphyotrichum ericoides ‘Snow Flurry’ is good for rock gardens or grown on a low wall where the stems cascade downward. Bushy asters such as S. oblongifolium ‘Raydon’s Favorite,’ S. lateriflorum ‘Lady in Black’ and A. tataricus ‘Jindai’ are excellent choices for perennial gardens or naturalistic landscapes.

With so many possibilities, selecting the best asters can be overwhelming, but any of these proven asters from the trials are a good place to begin.

See the entire evaluation report at www.chicagobotanic.org/downloads/planteval_notes/no36_asters.pdf.