Ozark Sunflower (Helianthus silphioides) is a tall herbaceous perennial with large simple leaves and composite flowers with a purplish brown central disk and bright yellow ligules. The genus name is based on Greek words for sun (helios) and flower (anthos). The specific epithet notes that appearance of flowers and involucre are similar to those of rosinweeds in the Silphium genus*. Endemic to the U.S., most-widespread occurrence is reported from Arkansas, southern Missouri, Tennessee and northern Mississippi with less widespread occurrence in Oklahoma, Indiana, Kentucky, Alabama and Louisiana. In Arkansas, Ozark Sunflower occurs primarily in the Ozark Plateaus, Arkansas Valley, Ouachita Mountains and on northern portions of Crowley’s Ridge along with higher elevations of the West Gulf Coastal Plain. It is adaptable to various soil types, but prefers sunny to partially sunny, well-drained soils that range from moist to dry, such as in prairies and glades, along streambanks and woodland borders and openings. It is also known as Rosinweed Sunflower.

Plants, with many long ropy roots, have a knobby central rootstock that gradually widens near-surface as new stems sprout from bases of previous year’s stems. Erect stems, multiple for older plants, may grow to 8+ feet with a ½-inch-wide stem-base. Leaves along the stem’s lowest portion are opposite and become alternate a short distance below ascending, axillary branches along a stem’s upper third. Bases on the lower opposite leaves are clasping and encircle the stem while bases of alternate leaves that do not subtend branches are not clasping. The spreading branches, positioned from 30⁰ to 45⁰ off the stem, typically terminate with an open cluster (panicle) of flower heads. Branches range from 2+ feet long (lower ones) to those that are less than an inch in the congested apical portion of the inflorescence. From stem base into the branched portion of stems, leaf spacing along stems ranges from 3 to 5 inches (may be tighter at mid-stem where leaf size changes; see below) before decreasing to less than an inch near stem/branch apices. Most surfaces of stems and branches are uniformly covered with short hirsute pubescence. Pubescence decreases distally, becoming sparse-hirsute to absent (glabrous) along upper portions of stems and branches. Thereafter, shorter hirsute pubescence is found along the ultimate branches and stalks (peduncles) of individual flower heads. The solid-core stems and branches, terete without ribbing, are light green to mostly reddish, varying with plant growth.

Ovate cauline leaves are large below the branches [e.g., to 9 inches long (including a 2½-inch petiole) and 5 inches wide] while orbicular leaves subtending branches are much smaller [e.g., 4 inches long (including a 1-inch petiole) and 2½ inches wide]. Longest branches may have 1-2 opposite pairs of small leaves at about mid-branch which may subtend very short secondary branches with a few additional flower heads or a peduncle. Leaves have medium to dark green upper surfaces and light green lower surfaces. Leaves typically have a broadly cuneate apex and a rounded to truncate base abruptly contracted onto a narrowly winged petiole. The very firm, robust leaves have a blade-surface that undulates between principal veins. Flat to crinkly leaf margins are variously serrulate, to dentate to serrate – marginal cuts being small in comparison to the size of larger leaves. Margins of the smallest leaves may be entire. The short-hirsute pubescence on leaves makes them feel sandpapery (scabrous) across the upper and lower surfaces, although pubescence of the upper surface is less dense. Lower surface pubescence is more prominent along veins and continues along lower side of petioles. Stout ascending petioles have ridged, pubescent margins (ciliate pubescence) that are winged. Upper sides of petioles are grooved and glabrous.

Venation is arcuate-pinnate. Upper veins are impressed, with major veins having a raised center line. Lower veins are expressed, with the major veins elevated in round-relief. The most prominent pair of secondary veins is immediately above the petiole. Arcuate tertiary veins extend from secondary veins toward leaf-margin-side only. Main veins of upper surface are a lighter color of green or reddish while veins of lower surface are a paler green. Venation of petiole wings, extending from leaf blade, parallels the petiole.

Flower heads bloom from mid-September into late October. Stems have to 12+ branches on their upper ½ to ⅓. Branches terminate with panicles of 2-5 flower heads; with longer branches also having a few flower heads at mid-branch. The ascending, sturdy, pubescent peduncles, ½ to 1½ inches long, may have 1-2 elongate bracts positioned well below the flower head (see Photo 24). With the long branches and relatively small leaves, the overall floral array per stem is elongate and open. Flower heads, 2+ inches across, face skyward but do not track the sun.

Domed flower head buds are covered by stubby, broadly elongate bracts (phyllaries) in several imbricated series. Bracts have short hirsute pubescence on their exteriors, more notable along margins. With anthesis, phyllaries recurve to form a bowl-shaped involucre containing a flattened to convex receptacle. The firm phyllaries are about ¼ inch long and ⅛ inch wide.

The purplish brown central disk consists of tightly packed, fertile disk florets. The numerous tubular florets reach anthesis successionally from outer edge of the disk to the center. The pale green to translucent floral tubes (fused petals) have 5 triangular reddish-purple lobes that spread wide at anthesis. Florets, ¼+ inch long and 1/32+ inch wide, have 5 stamens (filament + anther) and a pistil (ovary + style + stigma). Stamens have elongated dark-purple anthers with connate side-margins which exsert as an elongated anther ring well above the corolla. Yellow pollen, released inside the anther ring, is pushed to the outside by the elongating yellow style/stigma. Thereafter, anthers shrink, and the stigma bifurcates to expose an opposite pair of elongated stigmatic surfaces, sharply recurved just above the corolla. Each disk floret is subtended, on their exterior side, by a thin, lanceolate pale-green bract.

The central disk is encircled by the bright-yellow, strap-like ligules (aka laminae or rays) of infertile ray florets. Ray florets have an ovary but lack style/stigma and stamens. A flower may have 10 – 25 overlapping ligules in a single series. Ligules, about ¾ inch long and 3/16 inch wide, are oblanceolate with a pinched base and an acute apex, with several pleats extending their length. Lower side of ligules are slightly duller than upper side. As the final flower heads pass anthesis, heads, peduncles and stems/branches dry while the lower portion of the stems remain viable into mid-fall. If the weather remains favorable, limited secondary flowering may occur lower on the stem.

Fertilized disk florets produce ⅛-inch-long and 1/16-inch-wide mottled, dark brown oblong achenes (aka cypselae in Aster family) with pinched side-edges. They have rounded bases and truncated tops each with two, narrow-lanceolate, weakly attached scales that are almost as long as the achenes. Outer lateral edges of the somewhat flattened achenes and the rim of the truncated tops are covered with minute translucent scales. Scales are on a thin covering which, as achenes mature, splits-off to reveal glabrous, plump achenes. In the dense dry head, bracts subtending individual disk florets becomes chaff that remains as achenes drop from the head.

In considering Ozark Sunflower for a garden, note: 1) Number of stems increases from year to year and the rootstock gradually expands, 2) May self-seed freely, 3) Foliage and plant structure attractive and bold throughout the growth-year, 4) Stems may exceed 8 feet but remain erect, 5) Numerous yellow composite flower heads late in the growing season and 6) beneficial for wildlife, insects and arachnids. Suitable as a specimen plant in a sunny native plant garden with sufficient space or in prairie or natural settings. To restrict self-seeding, stems and branches can be topped soon after flowering.

Fifteen additional species within the genus Helianthus occur in Arkansas. Ozark Sunflower can be distinguished by: 1) its large ovate lower leaves and small orbicular upper leaves, 2) winged petioles, 3) purplish-brown disk, 4) rounded involucral bracts with pointed apices and 5) mottled achenes with weakly attached scales.

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*A previous article of this series addressed 2 Silphium species.

Article and photographs by ANPS member Sid Vogelpohl

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