Spikes:

Separate staminate (male) and pistillate (female) spikes, with 1 to 4 staminate spikes up to 2 inches long at the top of the stem. Below the staminate spikes are 2 to 6 erect to ascending pistillate spikes arising singly from the nodes; often the pistillate spikes have a few staminate flowers at the tip (androgynous). Pistillate spikes are short-stalked to stalkless, slender and cylindrical, up to 4 inches long, each with a leaf-like bract at the base of the stalk, the bract of the lowest spike over-topping the terminal spike.

Leaves and stems:

Leaves are basal and alternate, up to 15 leaves per stem, 2.5 to 8 mm wide, the upper leaves longer than the flowering stem. Stem leaf sheaths snugly wrap the stem and are translucent whitish, firm and U-shaped at the top. The ligule (membrane where the leaf joins the sheath) is longer than wide, usually pointed at the tip, and with loose tissue around the edge. Leaves are V-shaped in cross-section when young, and hairless though slightly rough along the edges. Old leaves may persist to the next season before withering away.

Bases are wrapped in a brown to reddish-brown sheath that is not fibrous, the lowest sheaths having leaf blades similar to other stem leaves. Stems are 3-sided, smooth to slightly rough along the angles, erect to ascending to arching, elongating at maturity but mostly shorter than the leaves, and single or multiple from the base. Not all plants produce flowering stems. Plants are not clump-forming but form loose to dense colonies from long rhizomes.

Fruit:

Fruit develops starting in early summer, the pistillate spikes forming clusters of seeds (achenes), each wrapped in a casing (perigynium), subtended by a scale. Pistillate spikes are densely packed with 20 to 100 fruits, the perigynia mostly ascending and overlapping like shingles.

Pistillate scales are somewhat variable, lance-oblong to narrowly urn-shaped, blunt to pointed at the tip, the edges pale to dark reddish-brown to purplish-brown, the midvein narrow or broad and green or pale brown, lacking awns, and shorter or longer than the perigynia. Perigynia are light green turning light brown at maturity, 2 to 3.2(3.6) mm long, 1.3 to 2.3 mm wide, veinless, hairless, loosely wrapping the achene, usually widest above the middle, abruptly tapering to a stalk-like base, rounded on the tip end and abruptly tapering to a minute, toothless beak. Achenes are 1.3 to 1.5 mm long, lens-shaped, brown at maturity.

Notes:

Carex aquatilis is a common sedge of wet places in the northern half of Minnesota, less common farther south, found in marshes, wet ditches, floating mats, and even in water up to 12 inches deep. It is a circumboreal species, found in northern latitudes and alpine environments in North America and Europe.

Carex is a large genus, with over 600 species in North America and 150+ in Minnesota alone. They are grouped into sections, the species in each group having common traits. Carex aquatilis is in the Phacocystis section (a combination of former Acutae and Cryptocarpae sections); some of its common traits are: typically clump forming and rhizomatous, leaves V or M-shaped in cross-section when young and hairless except along the edges, spikes long and cylindric, spikes drooping to erect on slender stalks to nearly stalkless, terminal spike usually all staminate or with a few perigynia at the tip, perigynia densely packed, green to brownish, 2-ribbed, weakly veined and short-beaked, achenes lens-shaped with 2 stigmas, growing in wetlands and other wet places.

Carex aquatilis is among a group of 5 very similar species in Minnesota; the other 4 are Carex emoryi, C. haydenii, C. lenticularis and C. stricta. There are a few key characteristics that, in combination, help distinguish them from each other: clump-forming or not, the basal sheaths fibrous or not, the length of the lowest pistillate bract relative to the terminal spike, whether the perigynia is broadest above the middle or not. In some cases, the shape of the ligule, the length of the pistillate scales relative to the perigynia, or other characteristics are also relevant to a correct ID. Most references mention perigynia veins (or lack therof) as an important diagnostic, but we have found these to be pretty obscure in most cases, even under magnification, and sometimes the perigynia base is a bit pleated at the base which might be mistaken for veins.

Of these five species, only C. aquatilis and C. emoryi are colony-forming and not clump-forming. Only C. aquatilis and C. lenticularis have the lowest pistillate bract consistently overtopping the terminal spike. Only C. haydenii and C. stricta have fibrous basal sheaths and form dense hummocks. Only C. aquatilis and C. haydenii should have perigynia widest above the middle, though any of these species may be widest at the middle, at least occasionally. C. aquatilis and C. haydenii have perigynia veinless on both surfaces but veins can be quite faint on the others. Inspect multiple specimens in a population to get a consensus on these traits, and mature specimens are always best though not always available.

C. aquatilis is distinguished by the combination of: colony-forming, not clump-forming, basal sheaths not fibrous, the lowest pistillate bract consistently overtopping the terminal spike, perigynia usually widest above the middle and more or less rounded at the tip, and the lowest sheaths have well-developed leaf blades. Perigynia are veinless and pistillate scales may be shorter or longer than the perigynia, and it has the widest leaves of this group (up to 8mm). C. aquatilis most closely resembles C. haydenii and C. stricta, both of which are clump-forming, have fibrous basal sheaths and the lowest sheaths are bladeless or nearly so. C. emoryi, the other non-clump forming species, has a distinctive ligule wider than long.

There are 4 recognized varieties of C. aquatilis, 2 of which are found in Minnesota: var. aquatilis, only reported as present in Cook County, has 2 to 4 staminate spikes and pistillate spikes are up to 4 inches (10cm) long; var. substricta, found elsewhere in the state, has 1 to 3 staminate spikes and pistillate spikes are mostly less than 2½ inches (to 6cm) long. Pistillate scale coloration and width of the midvein is noted as another distinction, but we have not found this to be consistent in the field and the color of dried scales can be pretty much solid brown, which obfuscates this trait. In either case, the scales are pretty variable but should be uniform within a spike.