Cyperus alternifolius is also known as Umbrella Sedge, and Umbrella Plant. It is a grass-like plant in the large genus Cyperus of the sedge family Cyperaceae. They are considered aquatic plants, need lots of water and/or waterlogged soils. But as you can see from the photos they do all right on land too.
The leaf-like structures that form the "umbrella" are actually involucral bracts that occur just below the inflorescence. Cyperus alternifolius is one of the most commonly used Cyperus species for landscaping purposes, frequently cultivated as an ornamental plant in gardens in the ground, pots, in ponds, and as a houseplant. However, while some consider them ornamental, others consider them aggressive invasive reeds.