Plants medium-sized or rather large, green, brown, or when emergent above water surface with red secondary coloration; branch apices (and often shoot apex) ± pencil-like due to longly excurrent leaf costae; cells of stem epidermis not widened, or occasionally in small parts (< 10% of circumference) slightly widened; pseudoparaphyllia ± quadrate to much broader than long; axillary hairs with 2--7-celled distal portion, early brown. Stem leaves straight or more rarely falcate, gradually narrowed to a longly or usually very longly acuminate apex from an ovate to triangular base, concave; margins denticulate; costa strong, shortly or longly excurrent (excurrent portion up to 1/3 leaf length); alar cells in distinctly delimited, transversely triangular group that reaches costa, not decurrent, supra-alar cells weakly differentiated. Sexual condition dioicous.

Warnstorfia trichophylla is occasionally reminiscent of W. exannulata, but the distinctly pencil-like branch and shoot apices, excurrent leaf costa and long, early brown axillary hairs (shorter and normally hyaline in W. exannulata) separate the first species from the latter. For the differences between W. trichophylla and Drepanocladus longifolius, see the note after the latter.

Normally submerged or floating in water that is intermediately mineral-rich, but with a relatively high pH (5.1--6.7 in Europe), mostly in small lakes, water-filled kettle holes, ox-bow lakes and other small aquatic habitats; 0--250 m; Man., Nun.,; w Alaska; South America (Colombia), n Eurasia.

Warnstorfia trichophylla has been reported from many areas of Canada (R. R. Ireland et al. 1987). However, the species seems to be widespread only in western Alaska, and few specimens from outside this area have proved to belong to this species. It was reported also from two localities in Greenland by G. Mogensen (1995), but specimens that this report is based on are W. exannulata. In northern Europe, W. trichophylla has been found at higher altitudes (750 m) and there is no reason to assume that the species is restricted to lower altitudes in North America.