10a. Equisetum hyemale subsp. affine (Engelmann) Calder & Roy L. Taylor, Canad. J. Bot. 43: 1387. 1965.
Equisetum robustum A.Braun var. affine Engelmann, Amer. J. Sci. Arts 46: 88. 1844; E. hyemale var. affine (Engelmann) A.A. Eaton; E. hyemale var. californicum J.Milde; E. hyemale var. pseudohyemale (Farwell) C.V. Morton; E. hyemale var. robustum (A.Braun) A.A. Eaton; E. prealtum Rafinesque
Aerial stems persisting more than a year, unbranched, 18--220 cm; lines of stomates single; ridges 14--50. Sheaths when mature dark-girdled, brown to gray above girdle, squarish in face view, 4.5--17 × 3.5--18 mm; teeth 14--50, articulate and promptly shed or persistent. Cone apex pointed; spores green, spheric. 2 n =216.
Cones maturing in summer, old stems sometimes developing branches with cones in spring. Moist roadsides, riverbanks, lakeshores, woodlands; 0--3000 m; Alta., B.C., Man., N.B., Nfld., N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Alaska including the Aleutian Islands, all other states; Mexico; Central America in Guatemala.
In southern and central to western regions plants tend to be taller and have more persistent teeth ( Equisetum robustum , E . prealtum ); in the Far West they often have bituberculate ridges ( E . hyemale var. californicum ). Equisetum hyemale subsp. hyemale is found in Europe and Asia to northwestern China in Xinjiang.