The cottonwoods are poplars in the section Aegiros of the genus Populus.

It also commonly refers to any species of poplar in the United States, including some poplars not in the section Aegiros, notably the Swamp cottonwood Populus heterophylla (in the section Leucoides) and the Black cottonwood Populus trichocarpa (a balsam poplar in the section Tacamahaca).

The Eastern cottonwood, Populus deltoides, is one of the larger North American hardwood trees, although the wood is rather soft. It is a riparian-zone tree.

The leaves are alternate and simple, with coarsely-toothed (crenate/serrate) edges, and subcordate at the base. Leaf shape is roughly triangular, hence the species name, deltoides. An important feature of the leaves is the stem which is flattened sideways, so that the leaves have a particular type of movement in the wind. Some other Populus species such as the aspens (Populus section Populus) share this characteristic.

Male and female flowers are in separate catkins, appearing before the leaves in spring. The seeds are borne on cottony structures which allow them to be blown long distances in the air before settling to ground.

The cottonwoods are exceptionally tolerant of flooding, erosion and filling around the trunk. They are also able to grow in moister areas on the Great Plains where most other trees cannot grow. A traveling salesman once planted a cottonwood seedling in the middle of one of the upper plains states, many miles from the next nearest tree, and watered it on his route every week. The tree survived and, once its roots were able to penetrate the water table, needed no further watering and lived for many years.

Classification