Aquatic Plants of Pennsylvania. Timothy A. Block

      Subulate arrowhead often occurs with long-lobed arrowhead, another species of freshwater tidal flats. See discussion above under Sagittaria calycina on how to distinguish these two species.

      Sagittaria subulata ×2/3

       Sagittaria subulata

       Sagittaria subulata

BEGGAR-TICKS/BUR-MARIGOLD

      BIDENS L.

      Aster Family—Asteraceae

      Bidens species are herbaceous annuals with opposite leaves and small flowers clustered in heads surrounded by an involucre which consists of large, leafy outer bracts and smaller membranous inner bracts. Additional (receptacular) bracts are associated with each individual floret.

      Of the species treated here, only Bidens laevis produces ray flowers; all, however, are insect-pollinated. The fruits are achenes with 2–4 awns. These species are plants of shallow water of stream or lake margins or freshwater tidal marshes. The seeds are eaten by waterfowl.

      Note: Bidens frondosa and Bidens cernua are included in the key due to their similarity to Bidens discoidea and Bidens laevis respectively. These species are not described below as they do not fit the criteria for aquatic plants treated in this work, despite the fact that they may occasionally occur in marginal habitat areas.

       Key to Aquatic Species of Beggar-ticks and Bur-marigold

      A. leaves compound

      B. outer involucral bracts 3–5, not ciliate ............................................. Bidens discoidea

      B. outer involucral bracts 5–10, ciliate ............................................. Bidens frondosa

      A. leaves simple

      C. leaves sessile; showy ray florets present

      D. rays 1.5–3 cm long, receptacular bracts red-tinged ...................... Bidens laevis

      D. rays less than 1.5 cm long, receptacular bracts yellowish .... Bidens cernua

      C. leaves with distinct petioles; ray florets not present ................. Bidens bidentoides

Swamp Beggar-ticks

Bidens bidentoides (Nutt.) Britton

       PNHP

       Native

       Rooted, emergent annual

      Swamp beggar-ticks is a plant of freshwater tidal marshes. It has opposite leaves that are simple and coarsely toothed; the heads contain only disk flowers. A characteristic that distinguishes it from all the other Bidens species in our range is the upward pointing barbs on the awns of the achenes (visible with a 10X lens).

      Swamp beggar-ticks typically grows in the upper zone of the tidal marsh where its roots are inundated at high tide but the upper stem remains above water. Its range includes freshwater tidal marshes of the estuaries of coastal rivers in New York, New Jersey, Pennsylvania, Delaware, and Maryland.

      Bidens bidentoides—achene ×2, note upward-pointing barbs on awns

      Bidens bidentoides ×1/3

       Bidens bidentoides

       Bidens bidentoides

Small beggar-ticks

Bidens discoidea (Torr. & A.Gray) Britton

       PNHP

       Native

       Rooted, emergent annual