Smartweed

Polygonum hydropiperoides

Also known as: water pepper. 

Also known as: water pepper. 

Smartweed is a perennial plant that forms dense colonies in shallow water or moist soils and can grow to 3 feet tall. Smartweed leaves are oblong, and stems are jointed. Leaves are arranged alternately upon the stem. Flowers are white or pink and are small. Smartweed may be emersed or submersed with only the flowers being displayed.