Helianthus divaricatus, another species known by the common name woodland sunflower, of the Aster (Asteraceae) family, is a deciduous, rhizomatous perennial that occurs in the U.S. from Louisiana to Oklahoma to Wisconsin thence east and south to the borders. In Arkansas, this species is recorded from throughout much of the state except for portions of the Mississippi Alluvial and West Gulf Coastal Plains. Preferred habitat is dry uplands in full to partial sun, such as found in woodlands, glades and prairie borders. It may occur in more moist habitats as long as soils are well drained. The genus name is based on Greek words for “sun” and “flower”. The specific epithet is perhaps in reference to the plant’s colonizing or “spreading” nature. It could also be in reference to the divergent nature of inflorescence branches, when present.
The plant, a colonizer with rigid, straight, rounded (terete) stems, ranges from 2 to 5 feet tall and tends to lean when in bloom. Stems, often reddish and well-covered with a bluish waxy coating (glaucous), are mostly glabrous, but may have scattered small, rough outgrowths that can be felt more easily than seen. Winter-killed stems remain upright into the following year.
Leaves are a medium green above and a lighter shade of the same green below. Leaves typically occur in widely spaced, opposite pairs that are offset 90 degrees from the pair immediately above or below. Alternate leaves may occur on the upper portion of main stems, but floral branches from those leaf axils have opposite leaves. A few single leaves may occur just below flower heads. Leaves are sessile or have petioles up to 1/16 inch long and which do not have wings. Smaller leaves of floral branches (for example, 3 inches long and 3/4 inch wide) are narrowly lanceolate toward the tip, with wider rounded bases. Larger leaves of the stem (for example, 6 inches long and 2½ inches wide) are lanceolate toward the tip, with bases that are significantly wider in proportion to their length. Bases of these larger stem leaves are broadly truncated. Upper leaf surfaces are covered by very short, coarse hairs while hairs on lower surface are denser and slightly longer; especially on veins. Although leaves are scabrous, leaves can be pulled between fingers with slight resistance. Leaf margins are mostly entire, but may have widely-spaced, shallow, serrated teeth or may be somewhat crenulate. Primary venation comprises three prominent veins; namely, the central vein (midrib) and a pair of lateral veins originating at the junction of leaf margin and petiole. These veins arch gently toward the tip while secondary veins throughout a leaf are pinnate.
Photo 2: Display showing upper and lower leaf surfaces of upper-stem leaves along with a lower and upper portion of a stem. Primary lateral veins originate off midrib at leaf blade and petiole junction (see arrow). Note the prominent bluish waxy coating on the stem.
The period of bloom at mid-summer lasts for about two months, with floral display varying considerably between small and large plants. Smaller plants, with no branching or short branches, may only produce a single or a few composite flower heads at the top of slender stems. Larger plants may have a dozen straight floral branches up to 2 feet long that arise from lower as well as upper portions of the stem. Floral branches have the same appearance as main stems; that is, the longer ones have widely spaced opposite leaves that may produce axial sub-branches. Final pairs of leaves at the top of floral branches often produce three flower heads or a central head along with two axial branches. Floral branches of larger plants terminate with loosely arranged composite flower heads with all branches on a plant in bloom simultaneously. Peduncles, at the extremities of floral branches, are about 1 to 2 inches long and may bear a tiny leaf (bract) or two.
Flower heads, up to about 2½ inches wide, consist of 10 to 20 sterile ray florets surrounding numerous fertile disk florets. The surface of the central disk, before opening of florets, is covered by light yellow floral bracts (chaff), each subtending a disk floret. Ray florets have a bright yellow, strap-shaped to oblong corolla (ligule) about an inch long. Ligules, with rounded to pointed tips, are slightly in-folded along the long axis. Tubular disk florets are about 1/8 inch long with five lobes and five closely packed brownish-black stamens topped with yellow pollen. Stamens shrink in size before the pistil appears with its recurved bifurcated stigma.
Involucres consist of up to about 50 ascending, imbricated phyllaries in several layers or series. Phyllaries have narrow bases in relation to their lengths. Length of phyllaries is about six times their width. The phyllaries have long, narrow, free-standing, twisting and pointed tips.
Photo 5: Display showing phyllaries (#1), sterile ray florets (#2), fertile disk florets with dark stamens exserted (#3), fertile disk florets with exserted bifurcated stigmas (#4), chaff bracts that subtend florets (#5), and a style with 2-lobed stigma (#6). Needle-like appendages (arrow) will become awns on achenes.
White inferior ovaries are attached to constricted bases of disk florets. With fertilization, ovaries develop into dry, flattened, glabrous, dark brown 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 divaricatus from two other very similar widespread woodland sunflowers; namely, Helianthus strumosus and Helianthus hirsutus. Helianthus divaricatus has: 1) stems that are typically smooth and well covered with a bluish waxy coating, 2) petioles that are absent or do not exceed ¼ inch in length, and, if present, leaf blades that do not extend onto petioles, 3) two lateral primary veins near leaf base that originate off midrib at leaf blade and petiole junction and 4) phyllaries with narrow bases and long twisting tips.
Fifteen species of Helianthus occur in Arkansas, of which three widespread species are often referred to by the general common name “woodland sunflower”. These are: Helianthus divaricatus (addressed herein), Helianthus strumosus (addressed in this previous article), and Helianthus hirsutus. Helianthus hirsutus has nearly sessile leaves with primary lateral veins much like Helianthus divaricatus,except Helianthus hirsutus has leaves and stems that are generally densely covered by long hairs. The dense hairiness of Helianthus hirsutus also helps distinguish it from Helianthus strumosus.
Photo 6: Helianthus hirsutus, as shown, has stems and leaves that are covered with dense longer hairs. Note venation and serrated leaf margins in comparison to the other two woodland sunflower species.
FOOTNOTE: Woodland sunflowers are prone to naturally hybridize and species’ characteristics in hybrid plants may become muddled–which of course adds to the difficulty of telling the three species apart.
Article and photographs by ANPS member Sid Vogelpohl