Ovate spike-rush is an annual spike-rush that is found in North America in two regions: 1) the Northeastern US and Canada and 2) the northwestern US and Canada (Oregon, Washington, British Columbia and Alberta) (USDA 2011). Habitat includes "fresh, often drying shores, lake and stream beds, bogs, tidal estuaries, [and] disturbed places" (Flora North America 2011). In British Columbia, it has been reported from various areas in the southern part of the province. However, Lomer (2011) provides updated distribution information.

"A small annual spike-rush that was known for certain in BC only from Ellison Lake in the Okanagan. During the 2008 CDC survey [in the Fraser Valley], a population of over 1000 plants was found on the east shore of Hatzic Lake (UBC: Lomer 6867) and found again on the muddy receded shore of Latimer Lake, Surrey (UBC: Lomer 6883). At Hatzic it was growing on emergent mudflats with Limosella aquatica L., Eleocharis acicularis (L.) Roem. & Schult., and Crassula aquatica (L.) Schoenl. It looks much like the common annual spike-rush in the Fraser Valley, Eleocharis obtusa (Willd.) Schult., but the tubercles on the achene tops are narrower and more triangular in outline. The stems are mostly down-curved and are of variable lengths giving the plants a low starburst-like appearance. The Latimer Lake population numbers about 500 plants in the muddiest sites and though these plants were observed in previous years they were assumed to be E. obtusa, so it is quite likely that more sites will be found in the Fraser Valley and indeed in southern British Columbia once this species becomes better known". (Reprinted from Botanical Electronic News Issue # 435, with permission)

Flora North America (2011) provides a detailed descripition of this species, and says: "Although Eleocharis ovata has often been confused with E. obtusa, B. M. H. Larson and P. M. Catling (1996) showed that these species may be distinguished by non-overlapping widths of the tubercles, at least in Canada."