The leaves of narrowleaf cattail are tall, narrow, and stiff, growing up to 10 feet (3 meters) in height. They emerge from the base of the plant in a vegetative shoot and display a green color. The leaves have parallel venation and are sheathed at the base. Their outer surface is convex, while the inner surface ranges from flat to slightly concave. These distinguishing characteristics make the leaves identifiable and indicative of their species.

Narrowleaf cattail flowers appear as compact, cylindrical brown cattails, approximately 6-8 inches (15-20 cm) long. These structures are composed of hundreds of minute flowers, with staminate (male) flowers positioned above and pistillate (female) flowers situated below, separated by a distinctive 1/2 inch (1.3 cm) dividing line. The dense brown spikes are a key identification feature, though they lack a noticeable scent. Blooming occurs during the summer months, typically from June to August.

The stem of narrowleaf cattail is characterized by its stout, green appearance which is smooth and round in texture. Growing up to a height of approximately 6 to 12 feet (1.8 to 3.6 meters), the stem is robust and typically unbranched. Its smooth surface and cylindrical shape contribute to its distinctive look, making it a key identifying feature for plant enthusiasts. The thickness of the stem is generally uniform, providing structural support for the tall growth characteristic of narrowleaf cattail.

The fruit of narrowleaf cattail is characterized by compact, narrow, cylindrical brown clusters, typically measuring about 4-6 inches (10-15 cm) in length. Each cluster consists of numerous tiny achenes, which are dry, single-seed fruits that do not open at maturity. The achenes are enveloped by light, fluffy fibers that facilitate wind dispersal. These fibers give the fruit a distinctive soft and cotton-like texture, which turns the clusters into a woolly mass once mature. The fruiting clusters are prominent from late summer through fall and persist into winter.