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| 1 Stems annual, deciduous, the sterile (vegetative) stems flexible; sterile and fertile stems dimorphic or monomorphic, usually branched (often copiously so) but sometimes unbranched or sparsely and irregularly branched (as in E. fluviatile and E. palustre); [subgenus Equisetum]. |
| ..2 Sterile and fertile stems monomorphic (alike in appearance), strobili appearing in summer borne on green stems that persist throughout the season; sterile and fertile stems simple or sparsely and irregularly branched; grooves of branches rounded (U-shaped in cross-section). |
| ....3 Stems shallowly 9-25 ridged; central cavity of stem > 1/2 the diameter of the stem; culms to 1 m or taller |
| ....3 Stems deeply 5-10 sulcate; central cavity of stem < 1/3 the diameter of the stem; culms 0.2-0.8 m tall |
| ..2 Sterile and fertile stems dimorphic, strobili appearing in spring borne on soft pink or brown stems with green stems appearing later in the season (or strobili produced early on fertile stems that later become green and branched as in E. pratense); fertile stems unbranched or branched; sterile (vegetative) stem whorls copiously and regularly branched; grooves of branches channeled (C-shaped in cross-section). |
| ......4 Sheath teeth reddish-brown or brown, papery, 5-10 mm long, without a conspicious hyaline margin; branches of the stem compound (branching more than once); first internode on lowest branch longer than associated stem sheath |
| ......4 Sheath teeth dark and firm, < 5 mm long, with a conspicuous hyaline margin (these margins thin or widened); branches of the stem simple or compound; first internode on lowest branch shorter or longer than associated stem sheath. |
| ........5 Fertile stems whitish to brownish, unbranched, soon withering; first internode on lowest branch longer than the stem sheath; branches of the (sterile) stem compound; central stem cavity approx. 25% of stem diameter; [widespread in our area but absent or sporadic in the southern most states] |
| ........5 Fertile stems becoming green and branched; first internode on lowest branch shorter than sheath; branches of the (sterile) stem simple; central stem cavity approx. 35-50% of stem diameter; [n. NJ and Long Island northward] |
| 1 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]. |
| ..........6 Stems 0.3-5 dm tall; stem ridges 3-12; [PA, OH, IN, IL northwards]. |
| ............ 7 Stem ridges 6; teeth 3 per sheath; stems tortuous; stem lacking a central cavity |
| ............ 7 Stem ridges 3-12; teeth 3-12 per sheath; stems straight; stem with a central cavity |
| ..........6 Stems 2-20 dm tall; stem ridges 10-50; [collectively widespread]. |
| ............ ..8 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 |
| ............ ..8 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. |
| ............ ....9 Cone apex rounded; aerial stems annual |
| ............ ....9 Cone apex pointed; aerial stems perennial. |
| ............ ......10 Spores not produced, or white and misshapen; most stem sheaths lacking a blackish band well below the teeth |
| ............ ......10 Spores green, spherical; most stem sheaths with a narrow to broad blackish band well below the teeth |