Helianthus strumosus, known by the common name pale-leaf woodland sunflower or just woodland sunflower (a general name also used for several other native species of sunflowers), of the Aster (Asteraceae) family, is a deciduous rhizomatous perennial which occurs throughout the eastern U.S. from eastern Texas and North Dakota to the Atlantic and Gulf Coasts. In Arkansas, it is reported from across much of the state and possibly occurs statewide. The genus name is based on Greek words for “sun” and “flower”. The specific epithet relates to having strumae (cushion-like swellings) at the base of hairs on the upper stems. This plant is usually found in full to partial sun in dry to mesic soils of open woods, glades, prairies, roadsides and river banks.
The plant, a colonizer with rigid, straight, rounded (terete) stems, grows to 5 feet or more tall and tends to lean. Stems and secondary floral branches vary from glabrous to scabrous and may be partially covered with a bluish waxy coating (glaucous). Winter-killed stems remain upright into the following year.
Photo 1: New stems of Helianthus strumosus grow from rhizomes in mid-April.
Opposite, widely-spaced, rough leaves are medium green on the upper surfaces and lighter green on the lower surfaces. Pairs of leaves are off-set 90 degrees from the pair immediately below or above. Leaf margins are mostly entire, but also may have shallow, widely-spread teeth (serrations). Lanceolate stem leaves with broadened bases range from 5½ inches long and 1½ inches wide to 8 inches long and 3¼ inches wide. From their broadest dimension near the base, leaves have a gentle taper to a long-pointed (acuminate) tip, while below that point the width sharply tapers toward the midrib before continuing at a more gentle taper onto the petiole. Petioles, including the winged portion, range from ¼ inch to 1 inch long. Hairs on the upper and lower leaf surfaces are stiff and hooked, and thus the leaves feel scabrous. Longer hairs occur on leaf margins and along abaxial (underside) veins. Leaves, when gently pulled between fingers, will not slide. Primary veins consist of three prominent veins; namely, the central vein (midrib) and a pair of lateral veins that originate near the base of leaf but well within the leaf blade. The two primary lateral veins gently arch toward the tip while secondary veins throughout the leaf are pinnate. Smaller leaves at the top of the plant are lanceolate and without broadened bases.
Photo 2: Display showing upper and lower leaf surfaces along with a lower and upper portion of a main stem. Primary lateral veins originate off midrib at a point well within leaf blade (see arrow).
The inflorescence consists of six or so straight floral branches from an inch to about 8 inches long that grow from leaf axils from near the top of the stem. These floral branches, which bear one or two pairs of opposite, ovate-lanceolate leaves ½ to 1 inch long (including petioles), produce about three to eight composite flower heads on ¼- to ¾-inch peduncles. Flower heads are borne in loose clusters with about 20 to 25 total flower heads on a stem. The central flower head of a cluster blooms prior to others in that cluster. The period of bloom at mid-summer lasts for about a month.
Photo 3: Roughness of leaves is evident in upper leaf pair. Small leaves near inflorescence remain opposite and retain petioles. Involucres are composed of imbricated bracts (phyllaries) in several series.
Flower heads are composed of ray florets without stamens or pistils (sterile) and fertile disk florets that have stamens and pistils (each a perfect flower). The surface of the central disk, before opening of florets, is covered by yellowish-green bracts (chaff) that subtend disk florets. Flower heads, from about 1 to 3 inches across, have about 10 to 15 ray flowers with bright yellow corollas (ligules) up to 1 inch long that are strap-like and slightly infolded along the long axis. Ray flowers encircle up to about 50 1/8-inch-long tubular disk florets with five up-pointed lobes and a truncated, constricted base. Five closely packed stamens with purplish anthers covered with yellow pollen become exserted well above the corolla. Thereafter, the stamens withdraw before the pistil, with a recurved bifurcated stigma, becomes exserted. Disk florets, pollen and stigma are a pale orange. Flower heads are subtended by an involucre composed of up to about 30 light green bracts (phyllaries) that have an elongated triangular shape with up-pointing tips and occur in several series. The length of phyllaries is about two-times their width.
Photo 4: Floral bracts (chaff) of the receptacle are pushed aside as disk florets open. Pollen bearing stamens are purplish.
Photo 5: Display showing phyllaries (#1), sterile ray florets (#2), fertile disk florets with dark anthers exserted (#3), fertile disk florets with exserted bifurcated stigma (#4) and bracts (chaff) that subtend florets (#5). The needle-like appendages (arrow) will become awns on achenes.
White inferior ovaries are attached to the constricted bases of disk florets. With fertilization, ovaries develop into dry, flattened, glabrous, dark brown, 1-seeded fruits (achenes). Achenes have an elongated rounded base and a truncated top with two relatively long, weakly attached awns. Seeds are spread by dropping from long, leaning stems and by foraging wildlife.
Several characteristics help distinguish Helianthus strumosus from two other widespread and very similar native woodland sunflowers, namely, Helianthus divaricatus and Helianthus hirsutus. Helianthus strumosus has: 1) stems that are typically rough, but not especially hairy, 2) leaf petioles that are ¼ inch or more long with leaf blade extending onto petioles, 3) two primary lateral veins near the leaf base that originate at a point well within leaf blade, and 4) phyllaries that are about twice as long as their widths. Nevertheless, it should be admitted that even the experts often have difficulty distinguishing among these 3 species of woodland sunflower. And without the roots, even Jerusalem artichoke, Helianthus tuberosus, becomes tough to tell apart from them.
FOOTNOTE: Woodland sunflowers are prone to naturally hybridize and species’ characteristics in hybrid plants may become muddled.
Article and photographs by ANPS member Sid Vogelpohl,
Pingback: Know Your Natives – Woodland Sunflower – Helianthus divaricatus | Arkansas Native Plant Society
Pingback: Know Your Natives – Jerusalem Artichoke | Arkansas Native Plant Society
Pingback: Know Your Natives – Hairy Woodland Sunflower | Arkansas Native Plant Society