Margins are usually finely toothed. Foliage is dark green above and pale and smooth below. Foliage is tomentose when young, becoming glabrous or somewhat hairy as it ages. The flowers bloom in early spring and are in catkins. Female catkins have few flowers, while the male catkins are larger and many-flowered.
Male trees produce no fruit, and the flowers are shed after pollen is shed.
The fruit ripens before leaves are full-grown. The male and female flowers occur on different trees. The wood is light and soft. The wood requires special attention while drying in order to prevent warping. Swamp poplar makes good paper pulp.