Key to Wisconsin Equisetum

Author: Robert W. Freckmann

• 1a.Teeth of sheaths reddish, thin, and papery, coherent in 3-4 groups of 2-5 each; sterile and mature fertile (cone-bearing) stems with branches that rebranch E. sylvaticum

• 1b.Teeth of sheaths brownish or black (at least along the midrib), firm, and separate or cohering in pairs, or teeth jointed at base and dropping readily; stems unbranched, or with whorls of branches which do not rebranch (except rarely in E. arvense) 2

• 2a.Fertile (cone-bearing) stems produced in spring, pale brown or whitish, soft and without branches, or with branches developing when cones shed spores; sterile stems absent in spring, later stems green with regular whorls of branches 3

• 2b.Fertile stems usually produced in summer, green, similar to the sterile stems (except for bearing cones), or fertile stems absent 4

• 3a.Fertile stems fleshy, tan, without stomates, the teeth uniformly brownish; fertile stems unbranched and drying out as sterile stems appear E. arvense

• 3b.Fertile stems whitish, becoming green and branched, persisting as sterile stems appear, the teeth with whitish margins E. pratense

• 4a.Stems hard, persisting over-winter, usually unbranched, or branched irregularily after damage to the main stem 5

• 4b.Stems relatively soft, often flattened when pressed, not persisting over winter; cones with a rounded apex; stems often with several whorls of branches (absent in E. laevigatum) 10

• 5a.Sheaths grayish-white with a black border above and below (E. x ferrissii lacks black border below); teeth 14-50, jointed at base and readily breaking-off; stems usually more than 40 cm tall 6

• 5b.Sheaths greenish, blackish only above (entirely green in E. x nelsonii ); teeth 3-12, not jointed, persisting (except E. variegatum); stems rarely more than 40 cm tall 7

• 6a.Spores green and spherical; sheaths about as long as wide, with a black border at base E. hyemale

• 6b.Spores white and deformed; sheaths about one and a half times as long as wide, without a black border at base E x ferrissi

• 7a.Stems sprawling, recurving, or zigzag, with 6 ridges and no central canal; sheaths with 3 teeth E. scirpoides

• 7b.Stems erect and straight, with 4-17 ridges and a central canal; sheaths usually with more than 3 teeth 8

• 8a.Stems flexible, dying back nearly to base in winter; sheaths entirely green; ridges of stem few, rounded with a single row of tubercles; teeth with a central groove only at tip, not deciduous E. x nelsonii

• 8b.Stems firm, perennial; sheaths green below, dark above; ridges of stem sharply angled with two rows of tubercles; teeth with a lengthwise central groove 9

• 9a.Ridges of stem 3--12 angled, forming deep grooves; teeth dropping quickly; spores green and spherical E. variegatum

• 9b.Ridges of stem 10--17 angled, forming shallow grooves; teeth persisting. spores white and deformed E. x mackaii

• 10a.Sterile stems with 8-many whorls of branches arising at most stem nodes; the first internode of the branches usually longer than the subtending stem sheath; branches with 3 or 4 angles and 3-4 teeth per sheath [or with 5-6 teeth and stem ridges rough] 11

• 10b.Sterile and fertile stems unbranched or with a few branches confined to mid-stem nodes; first branch internode shorter than the subtending stem sheath; branches with 5 or more angles, 5 or more teeth per sheath, and smooth stem ridges 13

• 11a.Central canal more than 1/3 the diameter of the stem; teeth brownish without a thin pale margin; sheaths in face view about as high as wide; stem ridges relatively smooth; teeth on sheaths of branches with elongated tips 12

• 11b.Central canal 1/6-1/3 the diameter of the stem; teeth with a thin white margin; sheaths in face view higher than wide; stem ridges rough; teeth on sheaths of branches triangular, without elongated tips E. pratense

• 12a.First internode of lowest whorl of branches much longer than the sheath; spores green and spherical E. arvense

• 12b.First internode of lowest whorl of branches slightly longer than the sheath; spores white and deformed E. x litorale

• 13a.Sheaths with 10-32 black dots at the top; teeth jointed at base, dropping early; stems without whorls of branches; plants not usually growing in shallow water or marshes E. laevigatum

• 13b.Sheaths without black dots at top; teeth firmly attached to sheath; midstem nodes often bearing a few whorls of branches; plants usually growing in shallow water or water-logged soil 14

• 14a.Sheaths square in face view, with 12 or more blackish teeth; central canal very large, about 9/10 the diameter of the stem E. fluviatile

• 14b.Sheaths higher than wide in face view, with less than 11 teeth, the teeth with whitish margins; central canal about 1/3 the diameter of the stem E. palustre