Spikes:

6 to 15 spikes each 6 to 12 mm (to ~½ inch) long, all at the tip of the stem, overlapping and tightly crowded to slightly separated, the inflorescence (group of spikes) up to about 2 inches long, usually erect, occasionally slightly nodding. All spikes are stalkless, ascending to erect, rounded at the tip, tapered at the base, club to cone-shaped in outline, with staminate (male) flowers at the base and pistillate (female) flowers at the tip (gynecandrous). At the base of the lowest spike is a scale-like bract with a bristle-like tip that may be longer than the spike and sometimes overtops the terminal spike.

Leaves and stems:

Leaves are alternate, up to 16 inches long, 2.5 to 7 mm wide, flat, hairless, rough along the edges especially near the tip. Leaves of flowering stems are on the lower half of the stem, vegetative shoots are leafier with leaves mostly on the upper half and may rise above the flowering stems. Stem leaf sheaths are mostly green nearly to the tip, the whitish translucent tip extends above the leaf base and is U- to V-shaped across the top edge. Sheaths loosely wrap the stem especially at the tip, the leaf midvein and edges becoming continuous wings or ridges down the sides and back of the sheath. The ligule (membrane where the leaf joins the sheath) is longer than wide.

Bases are wrapped in a brown sheath that may be somewhat fibrous, with old leaves often persisting to the next season. Stems are hairless, erect to prostrate, 3-sided in cross-section, mostly smooth except just below the spikes. Stems may elongate to 40 inches at maturity. Plants are clump forming from a mix of flowering and vegetative shoots. Late in the season, old prostrate stems frequently form buds at the nodes that take root, forming new plants the following year.

Fruit:

Fruit develops in early to mid summer, the spikes forming clusters of seeds (achenes), each wrapped in a casing (perigynium), subtended by a scale. Perigynia are ascending to appressed . Each spike contains 30 or more fruits.

Pistillate scales are lance to egg-shaped, translucent whitish to brown-tinged with a green or pale midrib that does not extend all the way to the tip, mostly pointed at the tip, 2 to 3 mm long, about half as long as the perigynia. Perigynia are 3 to 5.4 mm long, 1.1 to 1.7 mm wide, light green to straw-colored to light reddish-brown at maturity, hairless, 3 to 6-veined on the front, 2 to 4-veined on the back (both sides may be faint), flattened, not inflated, the body lance-elliptic, tapering the base, tapered to a beak 1 to 1.2 mm long, and has a translucent, whitish, papery wing .1 to .2 mm wide around the edges that is widest above the achene and abruptly narrows near the middle of the perigynia body. Achenes are lens-shaped, brown at maturity, 1 to 1.8 mm long, .6 to .9 mm wide, oblong-elliptic, distinctly longer than wide; the distance from the tip of the achene to the tip of the beak is 1.4 to 2 mm.

Notes:

Carex tribuloides is an occasional to frequent sedge of floodplain forest, wet meadows, ditches, shores and swamps and reaches the western edge of its range in Minnesota.

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 tribuloides is a member of the Ovales section, a notoriously difficult group. Some common traits are: usually clump forming, basal sheaths brown and somewhat fibrous, leaves V-shaped when young; 2 to 20 stalkless spikes all at the stem tip and crowded or not, spikes usually all pistillate at the tip and staminate at the base (gynecandrous), lowest bracts scale-like usually with a bristle tip, pistillate scales blunt to pointed at the tip and sometimes awned; perigynia erect to spreading, hairless, veinless to conspicuously veined on one or both surfaces, flat, beaked, usually with a translucent, papery wing; achenes lens-shaped.

Some traits to look at in Ovales are whether spikes are all crowded at the tip or more loosely arranged, whether the inflorescence is nodding or mostly erect, the shape of the spike (round vs. elliptic vs. club-shaped), the size and shape of the perigynia particularly the body (e.g. round vs. elliptic), the width of the wing and whether it extends all the way to the base, whether there are distinct veins on one or both sides of the perigynia, the length of the pistillate scale relative to the perigynia, the shape of the achene, leaf width, and whether sheaths are papillose, but strong magnification (30x or more) is required to see this. Habitat can also be a factor, and a metric scale is essential since fractions of millimeters make a difference.

Carex tribuloides is distinguished from other Minnesota Ovales species by its leafy stems, widest leaves 4 to 7 mm wide, loose sheaths that are winged or ridged down the sides and back (continuous with the leaf midvein and edges); 6 to 15 spikes usually crowded together at the stem tip, spikes rounded at the tip and mostly tapering at the base, the pistillate portion round to oval, the inflorescence erect and up to 2 inches long, lowest perigynia on a spike ascending to appressed; pistillate scales shorter than the perigynia and the midvein not reaching the tip; perigynia 3 to 5 mm long, the body lance-elliptic, veined on both sides (sometimes faintly) and narrowly winged above the base; pistillate scales shorter than the perigynia. The distance from the tip of the achene to the tip of the beak is 1.4 to 2 mm. It is usually found in moist to wet soil, often in shade or dappled sun.

The loose, winged sheaths and prostrate stems rooting at the nodes are traits shared with Carex projecta and Carex cristatella and when fruiting stems are not present they can be very difficult to distinguish. C. projecta spikes are more loosely arranged usually in a nodding or bent head, and C. cristatella spikes are more globular, consistently rounded at the base with the lowest perigynia in a spike widely spreading to reflexed. C. tribuloides may resemble other Ovales sedges as well, in particular C. scoparia, which has tight sheaths, spikes tapered at both ends, larger perigynia, 4 to 7 mm long, and prefers open, sunny areas. There are 2 recognized varieties of C. tribuloides: var. sangamonensis, present to our south, has widest leaves to 4.5 mm wide and perigynia 2 to 3 times as long as wide; var. tribuloides, present in most of the eastern half of North America including Minnesota, has widest leaves to 7 mm wide and perigynia 3 to 5 times as long as wide.