16. Cyperus lupulinus (Spreng.) Marcks

Pl. 71 g, h; Map 260

C. filiculmis Vahl

Plants perennial with short, knotty rhizomes, lacking tubers, but the stem bases somewhat thickened and hard. Aerial stems 10–70 cm long, bluntly trigonous, smooth. Leaf blades 5–45 cm long, 1.0–3.5 mm wide, shorter than the stems, the margins sometimes inrolled. Inflorescences irregular umbels with 1–3 sessile spikes and less commonly with 1–4 rays, each ray smooth, ending in a short, headlike spike of spikelets. Inflorescence bracts 2–4, longer than the rays, spreading to more commonly reflexed. Spikes 10–25 mm long, with 10–70 spikelets, hemispherical to globose, the spikelets ascending to reflexed, radiating in several planes, attached immediately adjacent to one another on the axis, the spikelet bases densely overlapping such that basal portions are not visible when fresh (pressed specimens become distorted). Spikelets 3–22 mm long, linear to narrowly elliptic, pointed at the tip, somewhat flattened in cross-section, with 3–22 florets, jointed to the axis of the spike and usually shed as an intact unit, but the individual scales also sometimes easily shed. Spikelet axis not winged. Spikelet scales 2.0–3.5 mm long, strongly overlapping, ovate to oblong-elliptic, bluntly angled along the back, rounded to bluntly pointed, often minutely notched at the tip, the midrib extended past the body of the scale as a short point, straight to slightly incurved, with 7–13 nerves, straw-colored to light brown or nearly white, less commonly tinged or streaked with reddish purple, the midrib green. Stamens 3, the anthers 0.3–0.8 mm long. Stigmas 3. Fruits 1.4–2.2 mm long, elliptic in outline, sharply 3-angled in cross-section, the sides flat, the surface finely pebbled, brown to dark brown or nearly black, shiny. 2n=164, ca. 166. May–October.

Scattered to locally common throughout the state (eastern U.S. west to Colorado and Texas; Canada). Mesic to dry upland forests, upland prairies, sand prairies, and sandstone, chert, and igneous glades; also pastures, fallow fields, ditches, roadsides, railroads, and dry, open, disturbed ground, usually in acidic substrates with rocky or sandy soils.

This species was treated under the name C. filiculmis Vahl in most of the older botanical literature. Marcks (1972) showed that C. filiculmis is a related but distinct species, endemic to the southeastern United States and not occurring in Missouri.

Possibly because it is relatively widespread and locally common, C. lupulinus has been implicated as a parent in more interspecific hybrids than any other species of Missouri Cyperus. These hybrids all show some degree of reduced viability (reduced seed set and pollen stainability) (Marcks, 1972; Carter and Bryson, 1991), and those plants (from other states) studied by Marcks (1972) also show cytological irregularities. Hybrids usually occur at disturbed sites where both parents grow, but because vegetative propagation by rhizome fragments is fairly common in this group, hybrids are sometimes found in the absence of one or both progenitor taxa.

In the sand prairies of southeastern Missouri, nearly every population of C. grayoides occurs mixed with C. lupulinus, and hybrids can be relatively common. These plants generally have the larger fruits, ascending to spreading bracts, and more inflorescence branches typical of C. grayoides but vary greatly in size, shape, degree of overlap of spikelet scales, and anther length. The confusing array of morphologies present at a few sites indicates potential backcrosses to both parents.

The name C. ×mesochorus Geise has been applied to hybrids between C. lupulinus and C. schweinitzii, which are most frequently encountered in sand prairie and disturbed sandy areas in southeastern and northeastern Missouri, but also have been found locally in disturbed, sandy areas and along railroads in Jackson and Ray Counties, and the city of St. Louis. These hybrids tend to resemble C. schweinitzii in their ascending bracts and more highly branched inflorescences, but tend to have less roughness on the stems and more condensed inflorescences than are typical of that species.

A single specimen from a disturbed, sandy site in Scott County is apparently a hybrid between C. lupulinus and C. lancastriensis. It has spikes that resemble the latter species, but the spikelets are flattened, as in C. lupulinus.

Another uncommon hybrid is C. lupulinus × C. echinatus, which was reported by Marcks (1972) based on a specimen from Newton County, but which should be searched for elsewhere in the state. This hybrid generally resembles C. echinatus but tends to have longer spikelets with more florets.

Perhaps the most unexpected hybrid was reported by Carter and Bryson (1991); it apparently involves two relatively distantly related parents, C. lupulinus × C. strigosus. These species normally do not grow in proximity because of differences in their moisture requirements. However, plants at a site in Scott County occur on a highly disturbed slope between a moist ditch (with C. strigosus) and a dry ridge (with C. lupulinus). They have more open spikes and sharply pointed spikelet scales than are typical of C. lupulinus but have much broader spikelets than found in C. strigosus. This unusual hybrid does not key well to either parent.

Two intergrading subspecies of C. lupulinus are recognized as occurring in Missouri: