Silphium terebinthinaceum (Prairie Dock)
|Also known as:|
|Habitat:||sun; average to moist, sandy or loamy soil; prairies, fens, railroads, roadsides|
|Bloom season:||August - October|
|Plant height:||3 to 12 feet|
|Wetland Indicator Status:||GP: none MW: FAC NCNE: FAC|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Clusters of stalked flowers at the tip of branching stems and arising from upper leaf axils. Flowers are 2 to 3 inches across with 15 to 30 yellow petals (ray flowers) that are fertile and have a split style protruding from the short tube at the base. The center disk is green until the disk flowers bloom; disk flowers are sterile, yellow, tubular with 5 triangular lobes and a column of brown stamens with long, stringy style-like tips. Bracts are in 2 or 3 layers, egg to teardrop-shaped with blunt or rounded tips. Flower stalks and bracts are smooth.
Leaves and stems:
Leaves are mostly basal and persist through flowering. Basal leaves are erect, up to 18 inches long and 12 inches wide, long-stalked, coarsely toothed, wavy around the edges, heart-shaped at the base, and with prominent veins perpendicular to the midrib. Stem leaves are few and alternate, or absent altogether. Surfaces are smooth or rough. Stems are round and hairless.
Fruit is a dry, flat seed that forms from the ray flowers on the outer ring of the disk.
Prairie Dock's natural range extends to Wisconsin and Iowa and does not quite reach Minnesota, but it is available in the nursery trade and may be included in prairie seed mixes and in restored plantings. There are 2 recognized varieties, var. pinnatifidum, the less common, has lobed leaves and var. terebinthinaceum, described above, has unlobed leaves. Both are otherwise easily distinguished from the other Silphium species in Minnesota by the very large, persistent basal leaves and the round, hairless and usually leafless stems. Growing up to 12 feet tall, it has a tendency to topple over in a strong wind.
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Where to buy native seed and plants ↓
- garden-grown Prairie Dock
- Prairie Dock in a restored prairie
- flowering stems toppled over
- a sense of scale: Prairie Dock leaves with Swamp Milkweed
Photos by K. Chayka taken at a restored prairie in Washington County. Photos courtesy Peter M. Dziuk taken in a private garden in Anoka County.
Have you seen this plant in Minnesota, or have any other comments about it?