Silphium laciniatum L.

Compass Plant

© SRTurner

Family - Asteraceae/Heliantheae

Habit - Perennial forb with woody taproots.

Stem - Erect, to 2.5 m, usually solitary, circular in cross section, stout, moderately roughened and pubescent with long, slender, pustular-based hairs, often also with minute, spreading hairs, some of these usually gland-tipped.

	Stem and node.
© SRTurner

	Upper stem and leaves.
© SRTurner

Leaves - Alternate and basal, thick, leathery, roughened with slender, spreading, pustular-based hairs, often also dotted with scattered, sessile to impressed glands. Basal leaves present at flowering, long-petiolate, the blade 30-60 cm long, ovate in outline, 1 time or more commonly 2 times pinnately deeply lobed or divided, the lobes or divisions mostly sharply pointed at the tip, broadly attached at the base, the margins otherwise entire or few-toothed and with minute, appressed hairs. Stem leaves progressively reduced from the stem base, the lowermost leaves similar to the basal leaves, the uppermost leaves 4-15 cm long, mostly short-petiolate to nearly sessile, the blade 1 time pinnately lobed.

	Leaves.
© SRTurner

	Leaf.
© SRTurner

	Leaf abaxial.
© SRTurner

	Leaf venation.
© SRTurner

Inflorescences - Racemes or narrow, racemelike, short-branched panicles, the heads short-stalked to nearly sessile.

Heads - Involucral bracts 25-45, 20-40 mm long, ovate, often spreading or recurved at the sharply pointed tip, the outer surface sparsely to densely hairy and usually also glandular, the margins with a conspicuous fringe of dense, spreading hairs. Receptacle 2-3 cm in diameter.

	Heads.
© SRTurner

	Involucre.
© SRTurner

Flowers - Ray florets 20-35, pistillate, the corolla 20-50 mm long, yellow, showy. Disc florets numerous, staminate, the corolla 6-7 mm long, yellow, usually extending beyond the tips of the chaffy bracts. 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. Ray florets are pistillate, with exserted, forked styles at their bases. Disk florets are relatively large and 5-lobed, with a nonfunctional style protruding through the dark tube of fused anthers. The composite head structure shown in this view is typical for plants in the family.
© SRTurner

Fruits - Achenes, 8-14 mm long, obovate in outline, strongly flattened, the surface smooth, glabrous, brown to black, gland-dotted, the angles with relatively broad, slightly lighter wings, each wing incurved and abruptly angled at the tip, the fruit with a relatively deep, sometimes narrowly U-shaped to nearly square apical notch.

Flowering - July - September.

Habitat - Glades, upland prairies, savannas, dry forest openings, railroads, roadsides.

Origin - Native to the U.S.

Lookalikes - Broadly, other species of Silphium, also Helianthus spp.

Other info. - This tall and unmistakable species is common across much of Missouri, except for parts of the southeastern quadrant of the state where it is uncommon or absent. Its North American range largely comprises a well-defined patch in the American Midwest.

Although the flowering heads of compass plant resemble those of many other species, a quick look at the overall plant will dispel any doubts as to its identity. The large, coarse, deeply divided leaves are unique among Missouri's flora. The species can cross with at least one other member of the genus, S. terebinthinaceum (prairie dock), with the resulting plants typically having leaves which are only slightly lobed. The flowering heads are large, with large florets which provide an excellent study of the composite flower head. The common term "compass plant" refers to the tendency of the plant's leaves to adopt an orientation with the edges pointing north and south and the faces east and west, for maximal utilization of sunlight. In practice, this orientation is quite variable and unreliable as a navigational aid.

Compass plant is beneficial to wildlife, with its seeds providing food for birds and small mammals, and its stems serving as sturdy hunting and observation perches for birds such as the eastern kingbird. The plant was used by Native Americans for a variety of medicinal purposes, including as a general tonic for both man and horse. The resinous sap was chewed to clean the teeth and freshen the breath. Doctors of the 1800s used compass plant preparations in antipyretic, diuretic, emetic, expectorant, tonic, styptic, antispasmodic, stimulant, and diaphoretic applications. Livestock will graze on the young plants but the species will not persist in overgrazed areas.

Photographs taken at Shaw Nature Reserve, Franklin County, MO, 06-18-2006, 07-15-2006, 07-18-2018, 8-3-2018, and 7-22-2020, and in Weldon Spring Conservation Area, St. Charles County, MO, 7-6-2007 and 7-27-2009 (SRTurner).