Stems are usually 5 to 10mm in diameter when young, increasing up to 20mm with age. Young stems are often angled, with several ridges running down the length of the stem, but they become more or less rounded as they mature. Stems normally scramble, branch and root at the nodes, forming an intertwining mat.
Larger stems are hollow between the nodes and mats are buoyant in water.
Erect stems are produced prior to flowering, growing to a height of 1.5m or more. In fertile situations its growth rate can exceed 15cm a week. It forms dense floating mats which can quickly cover waterbodies, excluding other life forms as well as impeding water flow, navigation and recreational activities. Problems with flooding due to impedance by this plant have already occurred in New Zealand. Stem fragments may be spread by water movement, deliberate plantings or by drainage machinery.