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Common Name Prairie Dock
Latin Name: Silphium terebinthinaceum
[Pronounced: "SILPH-ee-um terra-bin-thin-ACE-ee-um"]
Type of Plant: Prairie Forb ("wildflower")
Identification Helps: Very large, "elephant ear" leaves at the base of the plant. Later in the season, tall, naked (no leaves attached) stems grow skyward, to 10 ft or more, topped with numerous 2-inch yellow sunflower-like blooms.
Similar Species: No other Ohio plant looks like prairie dock.
Preferred Growing Conditions in the Wild: Prairie dock grows in a number of prairie environments, from slumping prairie stream banks, hillsides, and flat open prairie sites. Except in some isolated river bottoms, the plant always grows in former prairies. If you find a naturally-occurring prairie dock growing in Ohio (except along rivers), you have discovered a former prairie site.
Seasons of Growth and Bloom: The species blooms from August through September.
Natural Distribution in Ohio: Prairie dock is found in all Ohio prairie regions (except for the pinnatifidum form which is not found in the Lake Plains Prairie Region nor in the Glaciated or the Unglaciated Appalachian Plateau Prairie Regions [Ohio Prairie Regions map]).
Description and General Information: This is Ohio's tallest wildflower. Once seen, prairie dock will be easy to identify, as no other native Ohio plant is similar. (Some would quibble and claim that compass plant, S. laciniatum, a similar plant commonly found on Wisconsin and Illinois prairies, was once native to Ohio. If so, it was rare and never a prominent Ohio prairie plant. There is, however, modern genetic evidence that compass plant once existed in Ohio prairies. See below.)
Prairie dock has deep roots and over time forms a football-sized basal body just beneath the soil. The plant is delightfully and preferentially grazed by cattle. It has large, dime-sized seeds that are consumed by goldfinches and other seed-eating birds in the fall. When germinated, the seeds usually produce only a single leaf in the first growing season. The plant grows in size each season and commonly flowers for the first time in the fourth through the seventh growing seasons. As a garden plant, prairie dock gains attention with its giant leaves, long naked stems, and multiple overhead flowers. But it can take several years to mature.
Except in the Lake Plain Ohio Prairie Region (see Ohio Prairie Regions map ), some populations of prairie dock have specimens with deeply incised leaves, of the taxonomic form Silphium terebinthinaceum pinnatifidum. With their deeply cut leaves, S. t. pinnatifidum plants can appear similar to compass plant, and genetic studies of Dr. Richard Fisher indicated that this leaf form originated from ancestral crossing of the two species.
Another curious form of prairie dock occurs in the Adams County prairies, Silphium terebinthinaceum var luciae-brauniae, named for the renowned botanist Dr. Lucy Braun. The Lucy Braun prairie dock is markedly smaller than the common form of the plant. It is confined to older, southern sites that were not glaciated by the last, Wisconsinin Glacier. This smaller prairie dock apparently predates the last stages of the Ice Age, when open prairie habitats to the south may have been maintained by the tree- and shrub-eating habits of large Ice Age mammals. Even then, prairies required open skies, and Pleistocene mammals may have devoured shading woody plants before prairie-maintaining landscape fires were set by humans in North America.
Like its compass plant relative to the west, prairie dock tends to orient its large leaf blade surfaces facing east and west. A few errant leaves often orient east and west, but the majority maintain a north/south edge orientation, minimizing direct, midday sun-induced evaporation. On a hot summer day, clasp a prairie dock leaf between your hands. You will notice that the leaf is several degrees cooler than the surrounding air. The cooling is caused by evapotranspiration, the loss of moisture that originated several feet in soils beneath the plant. Prairie dock tap roots can be as deep as 14 feet.
For those of us who have always known this plant as "prairie dock," new botanical authorities are suggesting that the common name be changed to "basal-leaved rosinweed," as most of the related species in the genus Silphium are commonly called "rosinweeds." Prairie dock or basal-leaved rosinweed, this is one of Ohio's greatest and most interesting prairie plants.