Helianthus silphioides Nutt.

Rosinweed Sunflower


CC = 7
CW = 5
MOC = 13

© DETenaglia

Family - Asteraceae/Heliantheae

Habit - Perennial forb, sometimes with very short, thick rhizomes, often occurring as clumps.

Stems - Ascending to erect, to 2.5 m, single or multiple, sparsely to densely pubescent with short, ascending, slender hairs and/ or more or less spreading, short, stiff, pustular-based hairs toward the base, usually glabrous above the midpoint.

Helianthus_silphioides_stem.jpg Stem and node.

© DETenaglia

Helianthus_silphioides_stem2.jpg Stem and node.

© SRTurner

Leaves - Opposite, simple, petiolate, at 8-15 nodes along stem. Petioles 1-4 cm long, not winged. Blades 3-15 cm long, 1-15 cm wide, ovate to broadly ovate or nearly circular, mostly 1.0-1.7 times as long as wide, the uppermost leaves occasionally somewhat narrower, rounded or abruptly short-tapered at the base, rounded or broadly angled to a bluntly pointed tip, the margins flat and finely toothed or scalloped to nearly entire, the surfaces strongly roughened with moderate, minute, stout, pustular-based hairs, but lacking sessile glands, with 3 main veins, the lateral pair branching from the midnerve at or just above the base of the blade, arching upward.

Helianthus_silphioides_leaf1a.jpg Leaf adaxial surface.

© SRTurner

Helianthus_silphioides_leaf2a.jpg Leaf abaxial surface. The stiff hairs visible in these views bestow a very rough, sandpapery texture to the leaf surfaces.

© SRTurner

Helianthus_silphioides_leaves2.jpg Pressed leaf, adaxial.

© DETenaglia

Helianthus_silphioides_leaves1.jpg Pressed leaf, abaxial.

© DETenaglia

Inflorescence - Open clusters or panicles of flowering heads, sometimes reduced to a solitary terminal head.

Heads - Radiate. Involucre 8-15 mm long, 10-20 mm in diameter, slightly shorter than to about as long as the tips of the disc corollas, the bracts in 3 or 4 noticeably unequal, overlapping series, oblong to oblong-ovate, rounded or narrowed to a bluntly pointed, tightly appressed tip, the margins often with a fringe of minute, spreading hairs, the outer surface glabrous or sparsely hairy toward the base. Receptacle convex, the chaffy bracts 8-10 mm long, narrowly oblong to narrowly oblong-oblanceolate, angled or short-tapered to a sharply pointed, green, straw-colored, or purplish-tinged, glabrous tip, occasionally with a pair of additional short, broad lobes toward the tip, the outer surface also usually glabrous.

Helianthus_silphioides_head1.jpg Flowering head.

© SRTurner

Helianthus_silphioides_involucre.jpg Involucre.

© DETenaglia

Flowers - Ray florets 12-15, sterile, the corolla 1.5-2.0 cm long, glabrous or the outer surface with inconspicuous, short, slender hairs along the veins. Disc florets perfect, the corollas 6-7 mm long, reddish brown to dark purple at the tips, the lobes and tip of the tube often minutely hairy on the outer surface. Pappus of 2 scales 2.5-3.0 mm long, these lanceolate to narrowly triangular, sometimes with an irregular pair of small basal lobes, tapered to a sharply pointed, often minutely awnlike tip.

Helianthus_silphioides_flowers.jpg Flowering head.

© DETenaglia

Helianthus_silphioides_disk.jpg Florets.

© DETenaglia

Fruits - Achenes 3.0-4.5 mm long, narrowly wedge-shaped to narrowly obovate, flattened but sometimes more or less 4-angled in cross-section, the tip and margins moderately to densely pubescent with short, ascending hairs, the surface otherwise glabrous, uniformly dark brown or more commonly with fine, darker and lighter brown mottling.

Flowering - August - October.

Habitat - Streambanks, sand prairies, upland forest openings, fields, fencerows, roadsides. Typically on acid soils.

Origin - Native to the U.S.

Lookalikes - Other species of Helianthus and Silphium.

Other info. - This showy species can be found in the extreme south of Missouri, particularly toward the southeastern corner. It is a relatively uncommon species, with a total range restricted to a few states in the lower U.S. Midwest and Gulf Coast states. The plant is recognized by its showy yellow-rayed heads with brown centers, rounded involucral bracts, and broad leaves with very rough surfaces. The plant is attractive, and according to Steyermark, does well in cultivation.

The species epithet silphioides means "like Silphium." Plants in the Silphium (rosinweed) genus are sometimes confused with the sunflowers, as the flowering heads appear similar. Two points of differentiation are 1) the involucral bracts, which in Silphium are typically flat and rounded; and 2) fertility of the florets within the head. In Helianthus, the ray florets are sterile and the disk florets fertile, a situation which is reversed in Silphium. Fertility of a floret is indicated by the presence of a forked style, or later by the presence of a dark, hardened achene.

Photographs taken off Hwy C., Carter County, MO., 8-31-03 (DETenaglia); also in Stoddard County, MO, 9-7-2019 (SRTurner).