Plants perennial; cespitose but shortly rhizomatous, with tough, fibrous
roots. Culms 25-60 cm. Sheaths dull and fibrous basally, with scattered,
contorted hairs to 5 mm, margins glabrous; ligules 0.2-0.7 mm; blades
(5)8-20 cm long, 1-2.5 mm wide, flat to folded, glabrous or scattered-pilose on
both surfaces, margins glabrous. Panicles 5-20 cm long, (0.6)1-8 cm wide,
longer than wide, narrowly pyramidal, open to somewhat contracted, not diffuse,
well-exserted from the upper leaf sheath; lower nodes with 1-2(3) branches;
primary branches 0.6-7 cm, appressed or spreading to 70° from the rachis,
not capillary, without spikelets on the lower 1/3; pedicels 0.8-5.5 mm,
appressed to spreading. Spikelets 4.5-6.6 mm, plumbeous. Glumes
unequal, lanceolate, membranous; lower glumes (2)2.5-4.2 mm; upper glumes
3.8-6.5 mm, at least 2/3 as long as the florets; lemmas 5-6.5 mm, ovate,
membranous, glabrous, acute; paleas 4.8-6.5 mm, ovate, membranous; anthers
3-4.2 mm, yellow to purplish. Fruits about 3 mm long, 1.5-1.7 mm thick,
pyriform-globose; embryo dark brown to blackish; endosperm reddish-brown.
2n = 30.
Sporobolus interruptus grows on rocky slopes and in dry meadows of open
yellow pine and oak-pine forests and pinyon-juniper woodlands, at elevations from
1500-2300 m. It is an Arizonan endemic that is morphologically similar to S.
heterolepis, but the two species are separated geographically, the range
of the latter lying to the north and east of Arizona. The only reliable morphological
difference between them is anther length (3-4.2 mm long in S. interruptus,
1.7-3 mm long in S. heterolepis). Cytologically, S. interruptus
appears to be triploid, while S. heterolepis appears to be an octoploid
(2n = 72).