Silphium terebinthinaceum Jacq.

Prairie Dock

© SRTurner

Family - Asteraceae/Heliantheae

Habit - Perennial forb with woody taproot.

Stem - Ascending to erect, to 2.5 m, usually solitary, usually circular in cross section, smooth, glabrous, sometimes slightly glaucous.

	Stem and reduced leaf.
© SRTurner

Leaves - Mostly basal, with highly reduced stem leaves alternate. Leaves thick and leathery, usually roughened and pubescent with spreading, strongly pustular-based hairs. Basal leaves simple, long-petiolate, the blade 15-50 cm long, ovate-triangular to nearly circular, usually cordate at the base, pointed at the tip, the margins otherwise coarsely toothed and with minute, appressed hairs. Stem leaves mostly restricted to a few leaves similar to the basal leaves and positioned near the stem base, the median and upper portions of the stem naked or with widely spaced, much-reduced, sessile, bractlike leaves, these 1-3 cm long, sessile, ovate, the base wrapped around the stem, the margins usually entire.

	Basal leaves.
© SRTurner

	Basal leaf, abaxial.
© SRTurner

Inflorescence - Loose, open clusters or panicles of heads, these short- to long-stalked or sometimes nearly sessile.

	Inflorescence.
© SRTurner

Heads - Involucral bracts 21-35, 13-25 mm long, those of the shorter outer series mostly broadly elliptic, those of the longer inner series oblong-elliptic, usually ascending at the rounded or broadly angled tip, the outer surface glabrous, the margins sometimes with a minute fringe of dense, spreading hairs. Receptacle 1.5-2.5 cm in diameter.

	Head and buds.
© SRTurner

	Head, lateral view.
© SRTurner

	Involucre.
© SRTurner

Flowers - Ray florets 13-21, pistillate, the corolla 17-30 mm long, yellow, showy. Disc florets numerous, the corolla 6-7 mm long, slightly shorter than to about as long as the chaffy bracts, 5-lobed, yellow. Style branches with the sterile tip somewhat elongate and tapered. Pappus absent or that of the ray florets of 2 short, triangular, awnlike extensions of the winged angles of the fruit 1-5 mm long (the tip of the fruit then appearing deeply notched), persistent at fruiting.

	Disk and ray florets.
© SRTurner

© SRTurner

Fruits - Achenes 7-13 mm long, the surface glabrous or finely hairy, the angles with relatively broad, lighter wings, each wing incurved but abruptly truncate at the tip, the fruit with a U-shaped to semicircular apical notch.

Flowering - July - October.

Habitat - Glades, upland prairies, tops of bluffs, savannas, openings of dry upland forests, fields, railroads, roadsides.

Origin - Native to the U.S.

Lookalikes - Vegetatively, Parthenium integrifolium.

Other info. - This highly recognizable species occurs in Missouri mostly in the east-central area of the state. It is rare to absent in the northwestern half of the state and the Mississippi Lowlands division. Its North American distribution predominantly occupies a tight band extending diagonally from southwestern Missouri through eastern Michigan, though scattered populations also exist out to the Carolinas and into Canada.

The flowering heads of prairie dock resemble those of many other species; however, the tall, smooth, and mostly naked stems and the large, heart-shaped basal leaves uniquely identify this species. Vegetative populations of numerous basal rosettes and few flowering stems are common. These leaves can appear similar to those of Parthenium integrifolium, which is also found in similar habitats. The leaves of that species, however, are usually narrower and not cordate at the base.

Praire dock is an important attractor for bees, which not only use the flowers for nectar, but also nest beneath or within the plants. The species has a long taproot, extending to 14 feet or more and providing resilience to fires or during periods of drought. Unlike many other species, it will often survive in areas converted from prairie to railway. The leaves are palatable to livestock despite their coarse and leathery texture, and populations will survive moderate grazing. Native Americans used a tea made from the plant to relieve lung bleeding, to minimize menstrual bleeding, as an emetic, and for liver issues, fever, and enlarged spleen. The smoke from the plant was used as a treatment for nerve pain, congestion, and rheumatism.

The genus Silphium is named after a now-extinct plant which was endemic to the Mediterranean region. The identity of this plant is not known for certain, but is believed to be of the genus Ferula in the Apiaceae. Modern relatives include fennel and asafoetida. The plants were highly valued by ancient Greeks and Romans for their aromatic resins, which were used for seasoning, perfume, and medicine. The reason for the extinction of the ancient species is unknown.

Photographs taken at Shaw Nature Reserve, Franklin County, MO, 08-08-2007, Victoria Glade, Jefferson County, MO, 08-25-2015, Little Lost Creek Conservation Area, Warren County, MO, 08-24-2017, and Don Robinson State Park, Jefferson County, MO, 09-01-2018 (SRTurner).