Equisetum xferrissii Clute is the naturally occurring sterile hybrid of E. laevigatum A. Br. and E. hyemale var. affine (Engelm.) A.A. Eaton (Hauke, 1958). Although it is morphologically distinct and relatively easy to identify, it does not consistently appear on floristic lists of Iowa's prairies, nor in broader discussions of Iowa or Midwest floras. Only recently has the commonness of E. xferrissii become recognized. Peck (1976) documented the presence of E. xferrissii in 74 of Iowa's 99 counties. He felt (pers. comm.) that a thorough search would turn up E. xferrissii in the remaining counties as well, making it as widespread in Iowa as its parent species. E. hyemale has been recorded in all counties, and E. laevigatum in 91 of 99 counties. The current study was designed to quantify the presence of E. xferrissii in one county considered representative of much of Iowa and to address ecological questions regarding the dispersal and maintenance of the sterile hybrid and its parental species. Ecological information on these three Equisetum taxa is sparse. Hartman (1958) recognized what are now known as E. xferrissii and E. laevigatum as plants of ditches and embankments, noting also that E. xferrissii grew in moister sites than E. laevigatum and that E. hyemale preferred a moister habitat than either E. Xferrissii or E. laevigatum. Because they are agricultural weeds and toxic to livestock, E. palustre and E. arvense have received considerable attention. Borg (1971) did an extensive study on E. palustre as a weed in Finland. He recorded rhizome growth of 30 cm to several meters per year, reaching depths of 180 cm below the soil surface. In regard to vegetative dispersal, he found that rhizome fragments of E. palustre were not good propagules and did not support the idea that E. palustre was spread by road building activity. As field weeds, E. palustre and E. arvense have been tested for herbicide responses. E. arvense topgrowth was found to be sensitive to most chemicals, and rhizome growth could be retarded to various degrees (Hoyt & Corder, 1962). Equisetum palustre, on the other hand, was found to be quite resistant to the chemicals tested. Topgrowth could be damaged, but rhizomes remained virtually unaffected (Holly, 1953). The ease of vegetative propagation in Equisetum has been noted several times (Hauke, 1963; Praeger, 1934; Schaffner, 1931; Wagner and Hammitt, 1970). The reports concentrate on the proliferation of aerial culm fragments floating on water or placed in or on saturated soil. Hauke (1963) suggested that E. xferrissii culms were transported outside the range of its parent species by human activity. He noted, however, that preventing desiccation during translocation was critical for success. Dosdall (1919) also found that culms rapidly lost viability upon drying and, in