Helophilus pendulus on a Butomus umbellatus

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The flowering rush (Butomus umbellatus), also known as grass rush, is a perennial aquatic plant, constituting the family Butomaceae. Its name is derived from Greek bous, meaning "cow", "ox" etc and tome, a cut (the verb 'temnein' meaning "to cut"), which refers to the plant's swordlike leaves.

Other than suggested by its English common name, it is not a true rush. It is native to Eurasia and grows on the margins of still and slowly moving water down to a depth of about 3 m. It has pink flowers. Introduced into North America as an ornamental plant it has now become a serious invasive weed in the Great Lakes area.

Butomus is a rhizamatous perennial with linear leaves up to 1 metre long, or more. The leaves are triangular in cross-section and arise in two rows along the rhizome.

The inflorescence is umbel-like consisting of a single terminal flower surrounded by three cymes. The flowers are regular and bisexual. There are three petal-like sepals which are pink with darker veins. They persist in the fruit. The three petals are like the sepals but somewhat larger. 6 - 9 stamens. Carpels superior, 6 - 9 and slightly united at the base. When ripe they are obovoid and crowned with a persistent style. Ovules are numerous and found scattered over the inner surface of the carpel wall, except on the midrib and edges. Fruit is a follicle. The seeds have no endosperm and a straight embryo.

from wikipedia