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Wisconsin Plant of the Week

Helianthus strumosus L.

Asteraceae (Sunflower Family)

Common Name:

rough-leaved sunflower

09/05/2000

Rough-leaved sunflower is a native perennial that grows in a wide variety of habitats -- dry sandy prairies, moist soils and riverbanks, edges of oak woods or where woodland openings haven’t been shaded by fire suppression. Plants spread by creeping rhizomes and so may form large patches.

Mature plants are from 1 to 2 m tall with a strong stem that’s green and fairly smooth, at least well below the flower heads.   Leaves are thick with a very rough upper surface and arranged opposite each other on the stem. The leaves are pointed at the tips, widest below the middle, rounded at the base and gradually taper to a stalk or petiole at least 5 mm long.  Leaves have smooth or at most finely toothed edges.

Flowers bloom in August and September. The central disc portion of the flower head may be 2 cm across and the ”petal-like” ray flowers up to 4 cm long. Phyllaries -- the green leaf-like bracts behind the flower head -- equal or slightly exceed the size of the disc.

In all Helianthus, the disc flowers are female and form seeds, but the ray flowers are male and do not. These male flowers fall off once the disc flowers set seed. This distinguishes the genus Helianthus from two similar ”sunflowers”: Heliopsis, where both disc and ray flowers set seed; and Silphium, where only ray flowers set seed.

While the genus is easy to recognize, most species of Helianthus are highly variable in leaf arrangement, shape and other characters depending on growing conditions and possible hybridization. Positive identification to any species usually requires examining several specimens from a site, looking at the complete plant including the roots and not relying on any one character.

Some other Helianthus have at least some, if not all, alternate leaves. Wisconsin’s only other Helianthus with opposite leaves and a petiole longer than 5 mm is Woodland sunflower (H. decapetalous). Woodland sunflower is distinguished from Rough-leaved sunflower by thinner leaves with coarse teeth and much longer phyllaries.

  • Origin of the name: Helianthus, helio, Gr., sun; antho-, Gr., flower; strumosus, L., cushion-like or swollen, possibly refers to a medicinal use for treating swelling
  • Range:  Eastern US, ME to ND, s. to TX, FL
  • WI Range:  Statewide, except extreme NW
  • Common associates: Baneberry, shooting star, Penn sedge
  • Wetland Indicator Status: UPL
  • Coefficient of Conservatism: C = 5 (S&W), C = 4 (MI)
     
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