Asters are icons of the autumn garden, feeding pollinators with their late flowers while ushering in fall foliage season. Most of the 100-plus North American "aster" species have been reclassified into the genus Symphyotrichum. Here's advice on growing them, and a handful of my favorites.
Choosing and Tending Asters
Many species and selections are more prone to powdery mildew than others. Seek those that have been found to be resistant. Cut and discard all stems in autumn to curtail the presence of overwintering spores.
Cut the taller species down by half in early July. This will delay flowering by approximately 10 days, however the plants will be much sturdier and more self-supporting when they bloom.
Avoid the highly stoloniferous species and selections. If the nursery pot is crowded with stems when you purchase it, you can be sure it will sprint once released into your beds.
Attempt to expand your seasons of interest by selecting early-, mid- and late-blossoming species and selections.
Favorite Native Asters
I would not be without the following aster cultivars in my garden:
We employed 'Prince' calico aster (Symphyotrichum lateriflorum ‘Prince’) as an indispensible, low and herbaceous hedge at Heronswood for decades. Alan Leslie, of Monksilver Nursery in England, selected this superb, compact version of a species found naturally from Manitoba to northeast Florida for its black-plum foliage densely arranged on a sturdy framework to 18 inches. In late summer, the entire plant is cloaked with tiny white-ringed pink flowers, these concealing every bit of foliage. Unlike numerous species that possess a propensity to travel by stolons—creating soon too much of a good thing—S. lateriflorum is a solid clumper and a long-term garden addition. Another cultivar, ‘Lady in Black’, has equally dark foliage to 'Prince' and an arresting number of flowers, but along a taller, more open framework to three feet. Some may argue that the latter possesses more grace.
The heath aster, S. ericoides, is represented in the nursery trade by innumerable and superb cultivars, yet many (most likely) represent complex hybrids born in Europe. The species itself is native to much of North America east of the Rocky Mountains, in prairies, glades and dunes. It is in this last habitat, indeed on the dunes and shores of the Great Lakes, that one of the best of the lot occurs naturally, with nary a hand of the European court improving its looks. We consider S. e. var. prostratum one of the best groundcovers we grow, forming flat, finely textured mats of grayish-green foliage in poor, droughty soils and full sun. In late September through mid-October, tiny white flowers that are highly attractive to pollinating insects smother the colony. ‘Snow Flurry’, the name for the form generally available in commerce, aptly describes the effect, though I cannot say how much this cultivar differs from its purely wild counterparts.
New England aster
The countless cultivars of S. novae-angliae—the New England aster—that made their way home across the Atlantic may very well be hybrids, but S. n. ‘Purple Dome’ is the real McCoy. This sensational dwarf, richly hued variant was spotted along Pennsylvania Route 100 near Allentown by Mr. Robert Seip. It was subsequently given to Mt. Cuba Center in Delaware, which introduced it in 1989. Rich purple, semi-double flowers cap its tidy, (not floppy) 18-inch mounds of mildew-resistant foliage in September. It truly has become a standard bearer as a low-maintenance, long-lived and handsome garden stalwart that always attracts attention for the right reasons.
Many of the named forms of S. novae-angliae are prone to powdery mildew and to becoming too tall to support themselves. The cultivar ‘Andenken de Alma Pötschke’ (pronounced purrsk) may be mildew resistant—a good trait indeed—however it does become too tall in rich soils, requiring either staking or midsummer pruning. The bit of extra effort is entirely worth it. By mid- to late September and well on into the later days of October it produces a potch and a punch of near-radioactive red-pink flowers centered with yellow atop three-foot stems. It was selected at Pötschke Nursery near Stuttgart, Germany, and named in memory of the proprietor’s mother.
Even taller but equally worthy of growing is S. laeve ‘Calliope’. Masses of small, lilac-blue, yellow-centered flowers appear in very late summer atop nearly black four- to five-foot stems, which contrast beautifully with the floral display. The species itself has an enormous natural distribution, from the Yukon Territory to Florida, where it grows in dry, open areas. It should be cultivated in well-drained soils for best effect.
It is not a particularly gratifying task to nominate so few of the countless asters I have grown over the years when they really were Aster. Don’t hesitate to more fully explore this genus of plants exceptional in both flower and foliage. Few will ever disappoint.
Adapted from "By Any Other Name," Horticulture November/December 2018.
Calico aster by Fritz Flohr Reynolds/CC BY-SA 2.0
'Snow Flurry' aster by Ali Eminov/CC BY-NC 2.0
'Purple Dome' by Drew Avery/CC BY 2.0
Smooth aster by Joshua Mayer/CC BY-SA 2.0