Aerial stems persisting less than a year or only the proximal part overwintering, unbranched, 20--60 cm; lines of stomates single; ridges 6--14. Sheaths green, elongate, 3.5--7.5 × 2--4 mm; teeth 6--14, centers brown and margins white, prominent, tip usually brown, long, filiform. Cone apex pointed; spores white, misshapen.

Equisetum × nelsonii , the hybrid between E . laevigatum and E . variegatum , is often mistaken for small forms of E . × ferrissii .

Perennial herbaceous fern ally 20 - 60 cm tall

Spores: white, oddly misshapen, not spheric or rounded, and never released from spore sacs (sporangia).

Stems: one to several, erect, straight, unbranched, green (often dying back in winter), round with hollow center and six to fourteen lengthwise rows of ridges with alternating valleys on outside. The stem joints have distinct nodes, and the stomates are sunken and arranged in single lines on each side of the stem valleys.

Sheaths: green, 3.5 - 7.5 mm long, 2 - 4 mm wide, elongate, longer than wide in face view. The sheaths are actually small, fused, whorls of leaves.

Sheath teeth: six to fourteen, erect, persistent, with brown centers, prominent white edges, and usually brown, long, slender tips.

Similar species: Equisetum x nelsonii is most similar to its two parent species, E. laevigatum and E. variegatum, but both those species produce viable, round, spheric, green spores. Equisetum laevigatum further differs by having entirely green stem sheaths since the sheath teeth are shed early, and its cone is rounded or at least blunt narrowed at the tip. Another hybrid, E. x ferrissii, is often confused with this one, but it has dark-girdled stem sheaths lacking the slender, brown and white teeth since they are shed, also there are typically more than fourteen teeth before they are shed. Also somewhat similar is E. x mackaii, but it differs because its stem sheaths are often entirely black or have a black girdle in addition to having the dual-shaded teeth. Members of the other subgenus have clearly rounded cones on the unbranched fertile stems, the vegetative stems have branches, and the stomates are neither sunken nor arranged in distinct rows or lines.

Habitat and ecology: Somewhat rare, localized to marly fens, pannes, interdunal swales, springy areas, or flats.

Occurence in the Chicago region: native

Notes: This plant may have lower parts of the aboveground stems persist through the winter, but often stems last less than a year. Since the plant does not produce fertile spores, it can only reproduce vegetatively. The Type specimens, those used as reference when the hybrid was first officially named, were collected from the Chicago Region in East Chicago, Indiana, and South Chicago, Illinois. The specimens are in the collections of The Field Museum and can be viewed by choosing to see the specimens in vPlants at the right or by performing an advanced search.

Author: The Field Museum