Sharp-pointed Flatsedge, Short Flatsedge, Taper-tip Flatsedge: Cyperus acuminatus (Synonyms: Cyperus acuminatus var. cyrtolepis, Cyperus cyrtolepis) - Native.
Ballast Sedge: Cyperus aggregatus - Introduced.
Shining Flatsedge: Cyperus bipartitus (Synonyms: Cyperus niger var. rivularis, Cyperus rivularis) - Native.
Asian Flatsedge: Cyperus difformis - Introduced.
Tall Flatsedge: Cyperus eragrostis (Synonym: Cyperus vegetus) - Introduced.
Redroot Flatsedge, Red-rooted Flatsedge: Cyperus erythrorhizos (Synonyms: Cyperus cupreus, Cyperus erythrorhizos var. cupreus, Cyperus halei, Cyperus occidentalis, Cyperus washingtonensis) - Introduced.
Yellow Nutgrass, Yellow Nutsedge: Cyperus esculentus var. leptostachyus (Synonyms: Chlorocyperus phymatodes, Cyperus esculentus var. angustispicatus, Cyperus esculentus var. phymatodes, Cyperus fulvescens, Cyperus phymatodes, Cyperus repens, Cyperus tuberosus) - Native.
Brown Flatsedge, Brown Galingale: Cyperus fuscus - Introduced.
Black Flatsedge: Cyperus niger (Synonym: Cyperus melanostachys) - Introduced.
Scented Sedge: Cyperus odoratus (Synonym: Cyperus ferax) - Introduced.
Sand Flatsedge: Cyperus schweinitzii (Synonyms: Cyperus alterniflorus, Cyperus bushii, Cyperus lupulinus, Cyperus lupulinus ssp. lupulinus, Cyperus lupulinus ssp. macilentus) - Native east of the Cascades, introduced to Seattle.
Awned Flatsedge: Cyperus squarrosus (Synonyms: Chlorocyperus inflexus, Cyperus aristatus, Cyperus aristatus var. runyonii, Cyperus inflexus, Dichostylis aristata, Mariscus squarrosus) - Native.
False Flatsedge, False Nutsedge, Straw-colored Flatsedge: Cyperus strigosus (Synonyms: Cyperus hansenii, Cyperus stenolepis, Cyperus strigosus var. hansenii, Mariscus stenolepis, Mariscus strigosus) - Native.