Plants of the Eloise Butler Wildflower Garden

Rosinweed is an erect perennial forb of the Upland Garden, growing up to 6 feet high, on sturdy resinous stems that are cylindrical to slightly angled, green with stiff small hair when young, becoming somewhat smoother when older and sometimes reddish when in full sun. Some plants may lack stem hair entirely. There may be branching at the top. It is native to the mid-continent parts of the U.S. south of Minnesota.

Varieties: There are two - see notes at bottom of page.

The leaves are opposite with very tiny teeth on the edge (or no teeth) and no leaf stalk, but leaves do not surround the stem. They are lanceolate (broadest in the lower half) to ovate with bases round to heart-shaped and pointed tips. Upper and lower leaves are similar in size and both surfaces have fine stiff hair giving a sandpaper texture to the touch. Basal leaves fall away before flowering.

The floral array is an open branched cluster of flower heads at the top of the stem and on side stems from the upper leaf axils.

Flowers: The composite flower heads are 2 - 3 inches wide when open, with 12 to 22 yellow ray florets that are female only and fertile. Each ray is 1/2 to 1 inch long with a branched style at the base. These surround a central disc, which is 1/2 to 1 inch wide and composed of 70 to 225 functionally staminate sterile tubular disc florets. The disc florets have a deep-yellow colored tubular 5-lobed corolla with spreading tips at the apex. The green floret tube is shorter than the corolla. Five stamens tightly surround a single unbranched style, which is exserted when the floret opens, but is non-functional. The phyllaries (floral bracts) around the outside of the flower head are in 2 or 3 series, all with pointed spreading tips. These are yellow-green and either hairy or smooth depending on the species variety. In the Silphiums, each ray floret is subtended by a phyllary. When in flower, the plant has a resinous aroma.

Seed: Fertilized ray flowers produce a dry oblong cypsela, 9–14 × 6–10 mm, without pappus, but slightly ridged for wind distribution. Silphium seeds require 60 days of cold stratification for germination.

Habitat: Rosinweed grows from a fibrous root system containing small rhizomes that allow vegetative growth. It needs full sun, poor to loamy soils, and mesic to dry conditions.

Names: "Rosinweed" is a common name applied to many Silphiums due to the resin in the plants. The genus name, Silphium, is from the Greek word silphion, which was a plant of North Africa, unidentified today, that was used as a seasoning, perfume, aphrodisiac, and medicine, and contraceptive. The sap was odiferous and delicious. It only grew in the Cyrenaica, and was thought to be extinct already by the time of Nero. Images of the the plant appeared on ancient Greek coins of the city of Cyrene. Silphium is used today for a group of plants with resinous juice. The species name integrifolium, is from the Latin for "entire or uncut leaves". The author name for the plant classification - ‘Michx.’ is for Andre Michaux (1746-1802), French botanist who made many exploring expeditions in the U.S. collecting and cataloging many species. Two important works were the Histoire des chênes de l'Amérique septentrionale (1801 - Oaks of North America), and the Flora Boreali-Americana (2 vols., 1803, published posthumously, which contained the description of this plant.). His son Francois, traveled with him and the father’s notes were later used for his 3-volume North American Sylva.

Comparisons: To separate this plant from other sunflower types, look for the rough opposite stalkless leaves, most with tiny teeth. Rosinweed is one of the four Silphiums in the Garden. Here is a comparison chart.