New York Natural Heritage Program
Cypress-knee Sedge
Carex decomposita Muhl.
Monocots

General Description [-]
Cypress knee sedge is a densely tufted grass-like perennial. Its leaves are strap-like and 2.5-7.0 mm wide. Stems are 5-120 cm tall and towards its apex numerous secondary stems branch off of the main stem. Along these secondary stems as well as at the apex of the main stem are dense flower/fruit clusters (spikes). These spikes are densely arranged and are composed of separate female and male flowers. Female flowers develop into deep olive green to brown fruits (perigynia) which are 1.8-2.6 mm long (Cochrane 2002).

Best Life Stage for Proper Identification [-]
This species is relatively easy to identify when it has immature to mature perigynia. Perhaps the biggest challenge with finding this species is its rarity. When in fruit the inflorescences are large and plants should be visible from a distance.

Similar Species [-]
Carex decomposita superficially resembles the closely related C. diandra and C. prairea. Carex decomposita can readily be distinguished in the field by the larger inflorescences. Carex decomposita has inflorescences mostly 7-15 cm long, lowest inflorescence branch with 9-33 or more spikes, and adaxial leaf sheath apices concave and not prolonged beyond junction with the blade. In comparison, C. diandra and C. prairea have inflorescences mostly less than 8 cm long, lowest inflorescence brach with up to 12 spikes, and adaxial leaf sheath apices truncate to convex and prolonged beyond the junction with the blade.

Carex sparganioides is perhaps a little similar but it is an upland species and the lower branches of the inflorescences, if present, only have a few spikes and are short.
Cypress-knee Sedge Images
click to enlarge
The Best Time to See
Carex decomposita starts to produce perigynia in mid-June and these persist till about the end of July or perhaps longer. Towards the end of this season the fruits are starting to shed. Therefore, the best time to survey for this species is from mid June till late July.
J F M A M J J A S O N D
Fruiting
The time of year you would expect to find Cypress-knee Sedge fruiting (green shading) in New York.