Know Your Natives – Southern Prairie Aster

Southern prairie aster (Eurybia hemispherica) of the Aster or Sunflower (Asteraceae) family is a slender stemmed plant with spectacular, inch-wide, lavender composite flowerheads. The genus name is based on Greek words for “wide” and “few,” a reference to big-leaf aster (Eurybia macrophylla), most likely to the ligules of the ray flowers. The specific epithet derives from the Greek words for “half” and “sphere,” in reference to the involucre. Occurrence in the U.S. is primarily from eastern Texas and southeastern Kansas, east to southwestern Kentucky and the panhandle of Florida, excluding lowlands of the Mississippi River alluvial plain. In Arkansas, the species occurs nearly statewide, excluding some Mississippi Alluvial Plain and northern Ozark counties. Preferred habitat is sandy to rocky, mesic to dry soils of upland prairies and open woodlands.

Southern prairie aster is a long-lived herbaceous plant that forms loose clonal clumps. Plants have a short vertically oriented main root with many descending roots and shallow runners. Runners, originating from various levels of the main root, extend to 6 inches from the main root. Unlike roots, runners are modified stems, and are thus segmented at nodes, with each node subtended by a small fully clasping bract. One to several deciduous stems grow from the apexes of the main root and runners. Older portions of the main root and runners become reddish brown, while roots are light tan.

Photo 1: This plant has a vertical central root (¾ inch long) and several segmented runners. Runner extending to the left is 4½ inches long. In this mid-September photo, basal and cauline leaves (see below) have been lost.

The unbranched, mostly erect, wiry stems, to 20+ inches tall, are yellowish green to slightly reddish in sunlight. They are mostly glabrous, but may have short pubescence toward the inflorescence.

Photo 2: Plants have wiry unbranched stems topped with a single or several flowerheads. Photo – September 9.

Basal leaves, present in spring and fall, and cauline leaves have different shapes. Basal leaves may be spatulate with an acuminate apex to broadly lanceolate. They are flexible and glabrous except for the smooth margins which are minutely roughened.

Cauline leaves are alternate, stiff and ascending, lanceolate to linear, with an acuminate to obtuse apex and a narrowing base. They decrease in size from stem base (to 8 inches long and ¼ inch wide) toward the inflorescence (1½ inches long and ⅛ inch wide or smaller). The closely spaced leaves are clasping on the lower portion of the stem (base extending half-way around stem) to sessile along the upper portion. Leaves are a dull medium green above and a shiny light green below. While the lower leaf surface is glabrous, the upper surface and margins are roughened by minute hirsute pubescence. The leathery (coriaceous) leaves are up-folded along the midrib, which is slightly depressed above and prominently expressed beneath. Pinnate secondary venation is obscure. At the time of flowering, lower leaves tend to drop off. Tufts of small leaves (or small flowerheads) occur in leaf axils below the inflorescence, more apparent distally, but do not develop unless the stem above such a node is damaged.

Photo 3: Flexible basal leaves are spatulate to broadly lanceolate while the stiff cauline leaves are lanceolate to linear. Lower cauline leaves are clasping. Photo – September 2.

The inflorescence, with the bloom period extending from August into October, may be a single terminal flowerhead or a terminal flowerhead along with an additional 2-6 axillary flowerheads racemosely disposed immediately below. Flowerheads may also occur at the stem apex in a shortened corymb. Flowerheads are sessile or borne on peduncles to 3 inches long that may bear one to several scattered lanceolate leaf-like bracts. The hemispherical involucre is comprised of 50-60 phyllaries (bracts of the composite head) in 4 to 6 series. Involucres are to ½ inch long and 1 inch wide. The outer stout lanceolate phyllaries become elongate-triangular, smaller and thinner above. Phyllaries are spreading to ascending, with spiky mucronate tips, glabrous above and minutely hirsute beneath.

Photo 4: Four to six series of involucral bracts are mostly stout, but become shorter and weaker toward the center. Leaf like bracts may occur below the involucre.

Flowering sequence of racemes proceeds downward from the terminal flowerhead. The composite flowerheads consist of 20-35 pistillate (no stamens) ray florets and ± 50 perfect (with pistils and stamens) disk florets, flowering sequentially from the perimeter to the center of the disk. As heads approach anthesis, ligules of the ray florets are erect and disk florets are covered by the flattened pappus (long hairs attached to the ovaries). As heads develop, the light to dark lavender strap-like ligules expand to ½ inch long and become widely spread around the disk. The ¼-inch-long yellow disk florets have tubular corollas with five short-triangular erect lobes. Stamens have slender filaments and elongate anthers which join laterally to form a ring around the style. As the style elongates through the anther ring, it picks up the pollen and carries it to the surface of the disk where it is exposed to pollinating insects. The long stigma then divides (bifurcates), with the paired stigmatic surfaces slightly separating.

Photo 5: As flowerheads approach anthesis, disk florets are mostly obscured by the silky pappus. Peduncles often have leaf-like bracts below the 4 to 6 series of phyllaries. Photo – September 17.
Photo 6: Widely flared ligules are disposed in a single layer with occasional gaps and overlaps. Involucres have a hemispherical shape.
Photo 7: Pistillate ray florets and perfect disk florets are both fertile. Bifurcated stigmatic surfaces become slightly separated, touching at their tips.

With completion of flowering, the entire stem dries and the phyllaries loosen, with the eventual release of 1-seeded, strongly ribbed, indehiscent fruits (achenes). The ⅜-1 inch long, bullet-shaped, slightly flattened achenes are tipped with a ring of 20-30 rigid plumose hairs (pappus). The well-attached arched hairs provide lift for wind dispersal.

Photo 8: As stems die, seeds mature and pappus fluffs-up. Photo – October 29.
Photo 9: Achenes, equipped with a pappus of plumose hairs, are dispersed by wind. Squares = ¼ inch.

Southern prairie aster is a good selection for a native plant garden or natural area with mesic to dry soil. It is not aggressive but may form small colonies. With its narrow leaves and wiry stems, the plant will probably be unnoticed until it blooms. Flowerheads are large for an aster but occur in limited numbers.

An additional species of the genus which is known only historically from Arkansas (Benton County only) is big-leaf aster (Eurybia macrophylla), not observed in the state since 1926. Big-leaf aster has large heart-shaped leaves and flowerheads in open cymes. Southern prairie aster is easily distinguished from 22 other “asters” in the Symphyotrichum genus by the appearance of its leaves, stems and flowerheads.

Article and photographs by ANPS member Sid Vogelpohl

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