Know Your Natives – White Oldfield Aster

White oldfield aster (Symphyotrichum pilosum), formerly Aster pilosus, a herbaceous perennial of the Aster (Asteraceae) family, occurs from Texas to South Dakota and Minnesota thence to the Atlantic and Gulf Coasts.  In Arkansas, this aster occurs statewide and is one of 22 species of aster (Symphyotrichum) konwn to occur in the state.   Habitats for this aster include old fields, open woodlands and roadways with a wide variety of dry to mesic soils.  The genus name is from Greek for “junction” and “hair”, apparently based on a European cultivar.  The specific epithet is from Latin for “hairy”.  Other common names include white heath aster, hairy aster, frost (based on dense hairs) aster, and awl (based on shape of phyllaries) aster.

In late fall, after flowering, basal leaf rosettes appear and that persist over winter.  Basal leaves are oval to elliptic with a narrowed base that may be glabrous or have scattered hairs on upper leaf surface as well as short marginal (ciliate) hairs.  These leaves, which may be 3 inches long and an inch wide, have entire margins or a few widely spaced shallow serrations.  The adaxial (upper) leaf surface is medium green, while the abaxial (lower) surface is a slightly lighter green.  Each basal rosette produces a stem.

White oldfield aster - Symphyotrichum pilosumPhoto 1:  In November, stems are frozen and overwintering basal rosettes appear.  Old stems retain pubescence.  Ciliate (marginal) hairs can be seen on new leaves.

White oldfield aster grows in clumps (cespitose) from a caudex and has both fibrous roots and rhizomes.  Young stems are a medium green, but as the terete stems mature the color changes to brown or reddish brown and stems become hardened.  Mature plants have tall lanky stems that may reach 5 feet.  The largest stems may have splits in their epidermal layer due to lateral growth pressure.  Pubescence of short hairs typically covers the entire stem.  The stout, straight stems tend to lean in various directions from the caudex.  Dead stems persist well into the next year, still exhibiting pubescence.

Stem leaves are alternate, simple and spaced 1½ to 4 inches apart.  Clusters of leaves (fascicles) arises from the leaf axils.  Leaves in these fascicles become alternate leaves along growing branches.  Lower, down-stem, branches may be an inch or two long, while higher, up-stem, branches may reach 12 or even 16 inches.  The longer branches also produce fascicles that become secondary branches.

White oldfield aster - Symphyotrichum pilosumPhoto 2:  In mid-May, new stems may be 3 feet tall.  Note pubescent stems, ciliate leaf margins, winged bases of stem leaves, and fascicles of leaves from leaf axils that will grow into branches.

Leaves along stems (cauline leaves) and branches differ in size and shape from the basal leaves.  The largest leaves, cauline leaves and those lower on branches, may be 4 inches long and ½ inch wide at mid-leaf and have a very gentle taper to leaf tips (acuminate).  Leaf width from mid-leaf to leaf base reduces down to narrow wings that continue to the sessile and clasping leaf base.  Leaves have entire margins or small widely spaced serrations.  Slight up-folding occurs along midrib which is especially prominent along the winged base.  Along with short ciliate hairs at leaf margins, similar scattered hairs occur on adaxial leaf surface while abaxial surface is more hairy with concentrations along primary veins and the leaf base.  Leaves are thin and feel slightly rough.  The color on both sides of leaves is a similar medium green.  As plants mature in fall, especially with drying soils, lower leaves wither and drop off.

White oldfield aster - Symphyotrichum pilosumPhoto 3:  In mid-October, pubescent stems are hard and brown.  Lower along stem, axillary branches that bear flowers are shorter, as shown.

Leaves on branches that bear the inflorescence become smaller upwards, first becoming broadly acicular (needle-shaped), to acicular, then to bract-like.  These leaves and bracts, also sessile, have the same color and pubescence as the larger leaves, though without winged bases.  The acicular leaves, which may be ½ inch long and 1/16 inch wide, further decrease in size to tiny acicular bracts.  Bracts are especially numerous at the ends of the longest branches and secondary branches.

The inflorescence, a series of composite flower heads along branches, occurs along and toward the ends of widely spreading and ascending, short to long branches.  The longest flowering branches tend to occur about mid-upper-stem with shorter braches below and above.  Branches have alternate leaves or bracts that are separated from each other by about ½ to 1/8 inch or less.  Leaf/bract size and separation decreases toward the ends of branches.  Leaves or bracts within the inflorescence subtend a single flower head or a fascicle that may produce a secondary flowering branch.  Flower heads at the ends of branches or secondary branches bloom first, with development continuing down-branch over a month or two if conditions remain favorable.  When stems lean to a great degree, flower heads bend to the sunny side, becoming secund (oriented to one side).  Bracts just below flower heads have acute tips that grade into the similarly sized and shaped phyllaries.

White oldfield aster - Symphyotrichum pilosumPhoto 4:  In mid-October, plants may be 5 feet tall with many branches and secondary branches.  Fragrant or aromatic aster (S. oblongifolium) is shown at lower right.

Flower heads, to about ¾ inch wide, have 20+ pistillate ray flowers and 25+ perfect yellow disk flowers.  Ray flowers have widely spreading strap-like ligules, normally white, about ¼ inch long and 1/16 inch wide.  Tubular disk flowers, with yellow 5-lobed corollas and yellow stamens, become reddish with age.  Flower heads are set in an elongated urn-shaped involucre composed of closely spaced, sharply pointed, 1/8 inch, awl-shaped, overlapping, stiff bracts (phyllaries) in 4 to 6 irregular layers (series).  Phyllaries are appressed on their lower portion and upward-flared on upper portion.  Involucres are the same medium green as leaves, but with lighter green bases due to bract growth.  Pedicels, subtended by acicular bracts, are not more than ¼ inch long.  The elongate inferior ovaries of the ray and disk flowers have bristly structures (pappus) at their upper ends.   The white styles of ray flowers and yellow styles of disk flowers have bifurcated stigmas.

White oldfield aster - Symphyotrichum pilosumPhoto 5:  Ends of branches and secondary branches bear acicular leaves and bracts that transition directly to the acicular phyllaries.  Note the pubescent branches and ¾-inch daisy-like flower heads.

White oldfield aster - Symphyotrichum pilosumPhoto 6:  The disk flowers changes from yellow to reddish with age.  Note the many acicular leaves and bracts below flower heads

With fertilization, ray and disk flowers produce 1-seeded achenes topped by tufts of white hairs (pappus).  Achenes, about 1/16-inch long and dispersed by wind, are slightly flattened and lightly ribbed.

White oldfield aster - Symphyotrichum pilosumPhoto 7:   Composite flower heads have pistillate ray flowers and perfect disk flowers.  Ovaries indicated by “x’s”.  Stigmas of ray flowers indicated by red arrows while those of disk flowers indicated by yellow arrows.  Stamens indicated by blue arrows.  Pappus indicated by white arrows.

Asters are important nectar and pollen sources for many insects in the fall.  White oldfield aster may not be suitable for small gardens due to large size and tendency to readily spread by seed.  Removal and disposal of seeds before maturity lessens degree of spreading.

Several other native Arkansas asters have white flower heads and may get confused with white oldfield aster.  Wreath aster (S. ericoides), typically found in tallgrass and blackland prairies of western Arkansas, tends to be about 2 feet tall and bushy with smaller more closely spaced leaves; in addition, flower heads have fewer ray flowers.  White woodland aster (S. lateriflorum), known from throughout the state, has narrowly lanceolate leaves, prefers shadier and usually moister woodlands, has smaller flower heads with fewer ray flowers, and the pubescence tends to be limited to stems (although the mid-vein on the underside of leaves may be hairy, as well).  Bottomland aster (S. ontarionis) has abaxial side of leaves uniformly soft pubescent, upper leaves and bracts that are broader and more leaf-like, and is strongly rhizomatous; it occurs in lowlands of eastern Arkansas.  Tall white aster (S. lanceolatum), found scattered throughout much of the state in moist woodlands and disturbed areas, has mature leaves that are glabrous with wedge-shaped bases and stem hairs in lines, and it lacks the numerous small leaves and bracts found on upper branches of white oldfield aster.

White oldfield aster can be distinguished by a combination of characteristics: It 1) prefers drier and often more disturbed habitats, 2) has new basal growth of oval to elliptic leaves, 3) has evenly pubescent stems, and 4) has leafy fascicles during mid-summer growth.

Article and photographs by ANPS member Sid Vogelpohl

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