Know Your Natives – White Leafcup

White leafcup (Polymnia canadensis) of the Aster (Asteraceae) Family is a coarse,  short-lived perennial. The genus name is in reference to the Greek Muse Polymnia (also spelled Polyhymnia), goddess of music, song and dance. The specific epithet refers to the plant’s occurrence in Canada. In the U.S., its greatest concentration occurs in eastern Oklahoma across Arkansas and Missouri and, from there, scattered north into Minnesota to Vermont as well as south into northern Alabama. In Arkansas, it is primarily limited to the Interior Highlands (Ozark Mountains, Arkansas River Valley and Ouachita Mountains). Other common names include small-flower leafcup, white-flower leafcup, white bear’s-foot and Canadian leafcup*. White leafcup’s habitat preference is light to medium shade in consistently moist, well-drained, rocky deciduous woods, ravines and talus slopes.

White leafcup is an herbaceous plant with several stems from 2 to 5 feet long that grow from short fibrous roots. Plants are mostly erect and may have a few lower branches along with shorter branching in the inflorescence. Lower branches grow straight from the main stems at a wide angles. Terete stems and branches are fistulose (hollow) and larger stems have noticeably longitudinal, rounded ridges. Stems and branches vary from light green to purplish with light green dominating on younger portions of branches and along shady sides of smaller stems and branches. Stems and branches are puberulent (covered with fine short down) throughout the plant, with pubescence within the inflorescence becoming longer and glandular (sap at hair tips). The glandular pubescence causes upper portions of flowering branches to feel viscid (sticky). Leaves are opposite or the uppermost alternate. Leaf margins are narrowly turned-under (revolute).

Lower leaves, to 12 inches long and 8 inches wide, have two to four somewhat opposite deeply cut lobe pairs with irregularly large-scale serrated margins. The terminal lobe, lateral lobes and major secondary lobes have acuminate (gradually tapering) tips. The largest lobes may be 2½ inches long and 1 inch wide. The soft leaves, dull light to medium green on upper and lower surfaces, are puberulent on the upper surfaces and less so on lower surfaces. The upper half of the petioles have wings with the same characteristics as the leaf lobes, including irregular margins. Wings lower on the petioles/midribs are narrower. Petioles, to 4 inches long and rounded with flat tops, have short pubescence with longer scattered hairs along their edges. Pinnate venation is slightly suppressed on upper surface and strongly expressed on lower surface, with lower midrib of larger leaves standing in sharp, rounded relief. Crushed leaves have a strong lemony scent.

White Leaf Cup - Polymnia canadensisPhoto 1: A young plant bearing the characteristic large ragged leaves of the species. Photo in mid-June

The bases of opposite leaves may be expanded and fused so that they wrap around the stem to form a leafy cup (the basis of the common name “leafcup”).

Leaves up-stem into the inflorescence become smaller, with shapes varying from hastate to broadly triangular. These small leaves may have entire (undivided) margins or shallowly crenulated margins with tiny tips that mark ends of tertiary veins. Smaller leaves do not have expanded bases and petioles are not winged.

White Leaf Cup - Polymnia canadensisPhoto 2: Display showing large and smaller leaves and upper and lower surfaces. Small leaves at top have crenulated margins with tips. Note ribbed stem, purplish coloration and barely expanded leaf bases.

Flowering, consisting of composite flower heads, may occur from late spring into mid-fall over a two month period. Upper portion of branches bear several clusters of flower heads. Clusters are subtended, going from those lower to upper in the inflorescence, by small hastate leaves, to smaller triangular leaves, to even smaller lanceolate leaves and to tiny bract-like leaves. Each flower head in a cluster (except for terminal flower heads) is subtended by a short linear bract about ⅜ inch long and 1/16 inch wide. Flower heads are on weak pedicels to about ¼ inch long. Terminal flower heads of a cluster bloom first.

White Leaf Cup - Polymnia canadensisPhoto 3: Stem and branches topped by clusters of flowerheads. Stems tend to be purplish.

White Leaf Cup - Polymnia canadensisPhoto 4: In a shady open site, branches are especially long and low-angled. Some leaves and branches are opposite and some are alternate.

Buds of the flower heads are covered by involucral bracts (phyllaries) in two series, the inner slightly longer than the outer. Margins of phyllaries bear long, twisty and sticky hairs (viscid pilose pubescence). As florets mature, apices of phyllaries curve backward.

When the disk of the flower head is first exposed, its flat surface is covered by light green, pointed, elongate-triangular and concave bracts, which individually wrap around each disk and ray floret. Ray florets develop ligules that extend outward. Each disk floret opens directly upward. The first florets to reach anthesis are ray florets along with the outermost ring of disk florets followed incrementally by smaller rings toward the center of flower head. The width of flower head when ray florets are at anthesis is up to 1¼ inches. The round involucres are about ⅓ inch deep and wide.

White Leaf Cup - Polymnia canadensisPhoto 5: Pubescence of branch, phyllaries and floral bracts can be seen. Note ligules just beginning to show on bud at upper left and leaf shapes of cluster on right.

Flower heads bear up to a dozen pistillate (no stamens) ray flowers and 25 or more light yellow staminate (pistils not functional) disk flowers. Ray florets have obovate, white ligules with pleated surfaces and three equal-size shallow, apical lobes and pubescent pinched bases. Ligules, about ⅓ inch long and broad, are sharply bent at their bases so that ligules flare outward. Styles of the ray florets are white with white bifurcated stigmas. Disk florets have pale yellowish-green flared corollas with five triangular lobes that join to form a skinny stalked tube. Corollas of disk florets enclose an exserted column of five light yellow stamens with yellow pollen. The column of stamens encircles a style with an infertile stigma based on a rudimentary ovary with ovules that do not develop.

White Leaf Cup - Polymnia canadensisPhoto 6: Staminate disk florets produce pollen while pistillate ray florets bear bifurcated styles.

White Leaf Cup - Polymnia canadensisPhoto 7: Display showing parts of a flower head that held eight pistillate ray florets, receptacular bracts that subtended the ray florets, two involucral bracts and a few staminate disk florets.

With fertilization, a flower head produces hard brown ovoid achenes. Achenes are about ⅛ inch long with rounded bases and three-angled apexes.

A second species of the Polymnia genus occurs in Arkansas, namely Cossatot leafcup (Polymnia cossatotensis). This species, endemic to Arkansas (five known sites in Polk and Montgomery Counties), is found on novaculite talus slopes. This annual species has heart-shaped leaves and flower heads with relatively small yellow centers of disk florets and usually two to three white ligules.

Peruvian daisy or quick weed (Galinsoga quadriradiata), recorded from seven scattered Arkansas counties, has flower heads similar in appearance to those of white leafcup. This non-native invasive species is a much smaller plant with coarsely toothed but unlobed leaves and occurs in open, disturbed areas.

  • The terms “small-flower” and “white bear’s-foot” are in contrast to structures in the closely related bear’s-foot or yellow leafcup (Smallanthus uvedalius), previously classified in the Polymnia genus. Bear’s-foot, a larger plant than white leafcup, has larger yellow flowers and broader leaves and is most often encountered on stream and river terraces.

Article and photographs by ANPS member Sid Vogelpohl

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