Thalia dealbata, powdery alligator-flag

powdery alligator-flag
Thalia dealbata

Hardiness: Zones 6 to 10
Plant type: Herbaceous perennial
Height: 6- 10in
Spread: 3- 6ft
Sun: Full sun
Water: Wet, requires standing water
Bloom time: July to August
Bloom color: Violet
Soil: Neutral, loamy soil 

More information on this plant here.