Answer to the January's Horticulture Challenge

This is Flowering Rush. Flowering rush is a regulated Class A noxious weed. This means eradication is required state-wide. It is also on the Washington statelist.Flowering rush is known as Butomus umbellatus and it is in the flowering rush family. Other names for this plant are grassy rush and water gladiolus.
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Source: King County Noxious Weed List