Ficinia Nodosa / Knobby Club-Rush
Thanks to Meg for identifying this one for me: Previously known as Isolepis nodosa, but more closely related to the Ficinia spp. of southern Africa, which mostly have a more conspicuous disc below the nut.
Description: Perennial, tufted or rhizomatous sedges. Leaf blades often poorly developed. Inflorescences terminal, spikelets clustered in dense heads or panicle-like. Small hypogynous disc (gynophore) often falling attached to the base of the achene.