Hats off to Patrick Belardo, who ID'd the mystery plant as Nutsedge.

The purveyor of this mystery, Edith Wallace, wrote:

"[The plant in the photo] is nutsedge, a noxious weed, but in Spain we purchased a special beverage, horchata, made of nutsedge tubers called chufa."

I think we all knew that already.

Not. (Thanks, Edith!)