1 Stems and sheaths pale pinkish brown, fleshy, unbranched, bearing a strobilus at its terminus; [subgenus Equisetum].

  2 Sheaths at midstem 8-15 mm long (not including the teeth); teeth of sheaths with a black center and narrow brown margins

  2 Sheaths at midstem 6-10 mm long (not including the teeth); teeth of sheaths with a narrow brown center and broad whitish margins

1 Stems and sheaths largely green, wiry or fibrous, unbranched or branched, bearing a strobilus or not.

    3 Stems annual, deciduous, the sterile (vegetative) stems flexible; sterile and fertile stems either alike in appearance (monomorphic) or distinctly different in appearance (dimorphic), usually branched (often copiously so) but sometimes unbranched or sparsely and irregularly branched (as in E. fluviatile and E. palustre); [subgenus Equisetum].

      4 Branches compound (the primary branches producing secondary branches at their nodes); teeth of the sheaths on the main stem 3-15 mm long, orange, partially fused along their margins into 3-5 groups; stem ridges with needlelike spicules

      4 Branches simple; teeth of the sheaths on the main stem 0.5-6 mm long, primarily brown or blackish, not fused and separate, 7-25; stem ridges lacking spicules or the spicules blunt-tipped.

        5 Midstem sheaths 4-8 mm long (not including the teeth); sheath teeth 7-25 per node; stem 2-9 mm in diameter.

          6 Sheaths of the midstem with 15-25 teeth, the teeth 1.5-3 mm long; central cavity of stem ca. 0.9× the diameter of the stem (the stem very easily flattened); culms to 1 m or taller

          6 Sheaths of the midstem with 7-12 teeth, the teeth 3-6 mm long; central cavity of stem 0.15-0.3× the diameter of the stem (the stem not easily flattened); culms 0.2-0.8 m tall

        5 Midtstem sheaths 1.5-4 mm long (not including the teeth); sheath teeth 4-15 per node; stem 1-3 mm in diameter.

             7 First internodes of the branches of the lowest whorl of branches equaling or surpassing the adjacent sheath and teeth of the main stem; teeth of the sheaths of the side branches acuminate-attenuate; ridges of the main stem with small bumps, feeling nearly smooth to the touch; central stem cavity approx. 25% of stem diameter; [widespread in our area but absent or sporadic in the southern most states]

             7 First internodes of the branches of the lowest whorl of branches shorter than the adjacent sheath and teeth of the main stem; teeth of the sheaths of the side branches with elongated, peglike, blunt-tipped spicules, feeling rough to the touch; central stem cavity approx. 35-50% of stem diameter; [n. NJ and Long Island northward]

    3 Stems perennial (or annual in E. laevigatum), evergreen, stiff; sterile and fertile stems monomorphic and either unbranched or with 2-3 short and unequal branches per node; [subgenus Hippochaete].

               8 Stems 0.3-5 dm tall, 0.5-2 mm in diameter; stem ridges 3-12; [PA, OH, IN, IL northwards].

                 9 Stem ridges 6; teeth 3 per sheath; stems tortuous; stem lacking a central cavity

                 9 Stem ridges 3-12; teeth 3-12 per sheath; stems straight; stem with a central cavity

               8 Stems 2-20 dm tall, 1.5-18 mm in diameter; stem ridges 10-50; [collectively widespread].

                   10 Main erect stems usually with 2-3 branches at the nodes; stems 1.5-7 mm in diameter; stomatal lines 1-2 on each slope of the stem ridges

                   10 Main erect stems unbranched (rarely branched as a result of injury); stems 3-18 mm in diameter; stomatal lines 1 on each slope of the stem ridges.

                     11 Sheaths of the mid or upper stem after their first August tan or grayish, with a black circumferential band at the sheath summit (just below the teeth) and a black band at the base of the sheath ; cone apiculate.

                       12 Spores not produced, or white and misshapen; sheath teeth 14-22 per node

                       12 Spores green, spherical; sheath teeth 15-35 per node