Those in section Aegiros are large deciduous trees 20-45 m tall, distinguished by thick, deeply fissured bark, and triangular-based to diamond-shaped leaves, green on both sides (without the whitish wax on the undersides of balsam poplar leaves), and without any obvious balsam scent in spring. An important feature of the leaves is the petiole which is flattened sideways, so that the leaves have a particular type of movement in the wind. The aspens (Populus section Populus) share this characteristic, but not the balsam poplars.
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.
In the past up to five or six species were accepted, but recent trends have been to accept just three species, treating the others as subspecies of P. deltoides.
The Eastern Cottonwood Populus deltoides is one of the largest North American hardwood trees, although the wood is rather soft. It is a riparian zone tree. It occurs throughout the eastern United States and just into southern Canada. The leaves are alternate and simple, with coarsely-toothed (crenate/serrate) edges, and subcordate at the base. The leaf shape is roughly triangular, hence the species name, deltoides.
In the typical subspecies deltoides (Vermont south to northern Florida and west to about Michigan), the leaves are broad triangular, 7-15 cm across at the base. Further west (Minnesota south to eastern Texas), the subspecies molinifera (Plains Cottonwood; syn. P. sargentii) has somewhat narrower leaves 5-10 cm wide at the base. This is also the state tree of Nebraska, Wyoming and Kansas. In western Texas, New Mexico and Colorado the subspecies wislizeni (Rio Grande Cottonwood; syn. P. wislizeni) occurs.
The Fremont Cottonwood Populus fremontii occurs in California east to Utah and Arizona and south into northwest Mexico; it is similar to Eastern Cottonwood, differing mainly in the leaves having fewer, larger serrations on the edge, and small differences in the flower and seed pod structure.
The third species, Black Poplar Populus nigra, native of Europe and western Asia, is distinct in its much smaller leaves, 5-11 cm across, with a more rhombic (diamond) shape; see the link for further details.
Felling a cottonwood tree usually involves making an initial deep chainsaw cut to drain the water.
Cottonwood bark is often a favorite medium for artisans. The bark, which is usually harvested in the fall after a tree's death, is generally very soft and easy to carve.