Tall tree with rough gray bark. Leaves are triangular, with very large teeth, and are not opposite each other on the twigs. Flowers are catkins which appear in the spring before the leaves.  Fruits are small capsules that split open to release seeds embedded in a clump of cotton, as in the middle picture below.  Some members of the genus Populus are called cottonwoods because of their cotton-containing fruit.  The cotton blows away on the wind, carrying the seeds, and eventually falls to the ground, as in the picture on the right below.

An alien tree, often planted, and persisting in the Park from earlier cultivation. It spreads easily by sending up suckers from the roots. A large beautiful specimen grows near the picnic table north of the Outdoor Classroom; the suckers it sends up are mostly chopped down in mowing the meadow there.

The triangular toothed leaves are distinctive and the best way to identify it. In the spring, the flowers are are a good clue.  Later in the spring, the cotton is also a good clue.