The leaves of woodrush flatsedge are linear, measuring 12-24 inches (30-60 cm) in length and 0.5 inches (1.27 cm) in width. They are basal, flat or V-shaped, with a glossy green surface and a distinctive purple-black base. The texture is smooth and glossy, aiding in water repulsion. These leaves are a prominent feature, with their linear form and unique coloration at the base, crucial for the identification of woodrush flatsedge.

Woodrush flatsedge displays clusters of greenish-white flowers. Terminal flowers comprise 5-11 groups of densely packed spikelets. Each spikelet is small, about 0.2 inches (5 mm) long, tightly arranged in an umbrella-like inflorescence. There is no noticeable scent. These flowers typically bloom in late summer to early autumn, providing visual subtlety with their muted color. The arrangement of spikelets makes them identifiable by their unique texture and structure rather than vibrant hues or large petals.

The stem of woodrush flatsedge is distinctly triangular in cross-section, exhibiting a 3-sided profile. It is primarily green and has a smooth, firm texture. Typically, the stem can reach up to 3 feet (90 cm) in height and maintains a uniform thickness ranging from 0.1 to 0.3 inches (2.5 to 7.5 mm). The stem usually grows erect with occasional slight bending. A prominent feature is the absence of branches, which helps in distinguishing woodrush flatsedge from other species.

The fruit of woodrush flatsedge is composed of small, brown achenes that house numerous seeds. In an exceptionally prolific large plant, it can produce up to 1 million seeds. These achenes are typically dry and do not open upon maturity. The size of the achenes is not specified, but achenes generally have a subtle texture and a hardened outer surface. Their brown coloration can aid in camouflage among the soil and dead plant matter, which is a common habitat for woodrush flatsedge. Key identification features of the fruit include its brown hue and the remarkable seed output.