Are They Sunflowers?
Rosinweeds (Silphium species) and sunflowers (Helianthus species) are in the Asteraceae (Aster) Family. Rosinweeds and sunflowers are herbaceous perennials with yellow composite flowers (ray florets with strap-shaped petals, disk florets with petals fused into small tubes) on top of tall rough stalks with simple leaves. Rosinweeds and sunflowers produce an odd assortment of flowers or florets, some fertile and some sterile, some of the fertile perfect (with functional male and female parts) and some of the fertile unisexual, that is, pistillate or staminate. For rosinweeds, ray florets are pistillate and fertile while disk florets are also fertile but functionally staminate, producing pollen but no seeds; however, for sunflowers, the ray florets are entirely sterile, producing neither seeds nor functional pollen, while disk florets are fertile and perfect, taking on all of the reproductive chores, and leaving the rays to look pretty and attract pollinators. For rosinweeds, styles of the pistillate ray florets are divided (bifurcate) as are the styles of the perfect disk florets of the sunflowers. The staminate disk florets of rosinweeds, in addition to the functional stamens, also have non-functional undivided styles-an uncommon character for the composites.
Starry Rosinweed Starry rosinweed (Silphium asteriscus), the “type species” (see footnote) for the Silphium genus, is found from Texas to Illinois to Virginia and southward to the Gulf. It grows throughout most of western and central Arkansas but infrequently in the Mississippi Alluvial Plain. Plants grow naturally in prairies, meadows, open forests and woodlands, and roadsides. Plants three or more feet tall may have one or more stems. Stems feel rough to touch (scabrous) due to stiff hairs. Upright in sunny areas, stems in more shaded areas tend to recline. Starry rosinweed is drought tolerant. It is also a forage plant for deer. Leaf arrangement and characteristics change up-stem. Lowest leaves are opposite and lanceolate with the leaf base tapering into wings on long petioles. Higher up-stem, leaves are lanceolate, and become sessile and alternate with rounded bases. Lower leaves may be 6 inches or more long and 1.5+ inches wide. Upper and lower leaf surfaces are scabrous, the lower surface less so. Leaf edges, often mostly entire, may occasionally have widely spaced small teeth. Flower heads, singly or in one or several compact panicles, typically consist of 10+ fertile ray florets and a central disk of 30+ staminate disk florets. Bracts, forming an involucre of two or three layers, are acutely triangular, spreading, recurved, and overlapping (imbricate). Peduncles and involucre are covered with numerous stiff, spreading hairs (hirsute). Yellow ray petals, long and narrowly elliptical, are notched at the end. Starry rosinweed flowers in late spring into summer.
Photo 1: Starry rosinweed in an open woods setting. Alternate leaves with branching for flowering.
Photo 2: Involucre of starry rosinweed. Note hirsute involucres.
Photo 3: Starry rosinweed. Note appearance of ray florets and disk florets.
Wholeleaf Rosinweed Wholeleaf rosinweed (Silphium integrifolium), aka entireleaf rosinweed or prairie rosinweed, occurs primarily from Alabama to Texas, north to South Dakota and the Great Lakes. In Arkansas, it occurs more or less throughout the state. Wholeleaf rosinweed is found in dry tall-grass prairies, mesic prairies and rocky or dry open woods and glades. It is drought tolerant. It is also a forage plant for deer. Wholeleaf rosinweed reaches three to six feet in height and, with age, has many stout stems. Younger stems appear fuzzy and are scabrous, but roughness may disappear with age. Stems of plants in shade are green while those in sun are often purplish. Opposite, sessile leaves, up to 5 inches long and 2.5 inches wide, are broadly lance-shaped to ovate and are entire or slightly toothed. Leaves, which are medium green, are scabrous on top, but less scabrous on lower surfaces. Paired leaves are decussate (leaves rotated 90° from one pair to next). Flowering, occurring in mid-summer, produces panicles with flower heads 2 to 3 inches across. Twelve to 25 yellow ray florets surround numerous yellow disk florets. Yellow rays, long and narrowly elliptical, are notched at the end. Bracts, forming the imbricate involucre, are broad with very short hairs.
Photo 4: Wholeleaf rosinweed with opposite, decussate leaves and purplish stems.
Photo 5: Wholeleaf rosinweed with flowers in bud, in bloom, and past bloom. In flower past bloom, note winged achenes partially visible around infertile disk.
Photo 6: Wholeleaf rosinweed. Note divided styles of ray florets and stamens and
non-functional undivided styles extending from disk florets.
Footnote: “Type species” is a species that exhibits characteristics that define a genus and serves as a reminder of what is meant by a particular genus. Should a genus be divided, the type-species retains the original generic name.
Article and photographs by ANPS member Sid Vogelpohl