11. Equisetum ×ferrissii Clute, Fern Bull. 12: 22. 1904.
Equisetum hyemale Linnaeus var. elatum (Engelmann) C.V. Morton; E. hyemale var. intermedium A.A. Eaton
Aerial stems having basal part persisting over winter, unbranched, 20--180 cm; lines of stomates single; ridges 14--32. Sheaths elongate, 7--17 × 3--12 mm, becoming dark-girdled with age; teeth 14--32, articulate and promptly shed or persistent. Cone apex pointed; spores white, misshapen.
Cones maturing in late spring--early summer but spores not shed. Moist lakeshores, riverbanks, roadsides, prairies; 0--2500 m; Alta., B.C., Ont., Que., Sask.; Ariz., Ark., Calif., Colo., Conn., Del., D.C., Idaho, Ill., Ind., Iowa, Kans., Maine, Md., Mass., Mich., Minn., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.Dak., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; n Mexico including Baja California.
The hybrid between Equisetum hyemale and E . laevigatum , E . × ferrissii , was mistaken for E . laevigatum by Schaffner and some subsequent authors. Although sterile, it exists outside the range of E . laevigatum , and apparently it is dispersed vegetatively (R.L. Hauke 1963). Perhaps it has persisted in some areas from a time when the parents were both there. Equisetum × ferrissii has been reported from Maine, New Hampshire, Rhode Island, South Dakota, and Vermont, but I have not seen specimens from those states.