Though most often found on heavy clays, swamp cottonwood also grows on the edges of, but not in, the muck swamps of the Southeast. Optimum growth and development is in the deep, moist soils of shallow swamps and low-lying areas near tidewater.

Sites that are too wet for eastern cottonwood (Populus deltoides) will support swamp cottonwood. Examples are shallow swamps, sloughs, and very wet river bottoms where the water table remains near the soil surface for all but 2 or 3 months in the summer and early fall. In the southern part of its range, low, wet flats provide the driest sites occupied by swamp cottonwood. However, in southern Illinois it is a dominant or codominant tree on soils with available moisture varying from 3 to 21 percent for the 61- to 76-cm (24- to 30-in) layer.Swamp cottonwood grows naturally on at least eight major soil types common to the Midsouth: Alligator, Amagon, Arkabutla, Forestdale, Perry, Rosebloom, Sharkey, and Tensas. The soils represent several families and the orders Alfisols, Inceptisols, and Entisols. They range from 4.6 to 5.9 in pH and from 24 to 65 percent clay in the surface 0.3 m (1 ft).

Flowering and Fruiting- Swamp cottonwood is dioecious. Flowers are proteranthous, appearing from March to May. Staminate catkins are rather stiffly pendant, oblong, cylindric, and 5 to 10 cm 2 to 4 in) long; pistillate catkins are 5 to 15 cm (2 to 6 in) long, pendulous, slender, and raceme-like. Pollination is by wind. Fruits ripen and the seeds fall from April through July.

Seed Production and Dissemination- Trees start seed production at about 10 years. Reddish-brown obovoid seeds number about 330,700/kg (150,000/lb) (9). Seeds are very small, light in weight, and tufted with hairs, features that allow them to be blown over 100 in (330 ft) by wind and to float for a considerable distance in water. Water is an important transporting agent since the bottom lands normally flood during the seedfall period. Numerous seeds are produced annually, but under natural. field conditions they remain viable for no more than a week or two.

Seedling Development- Germination is epigeal. Best seedling establishment is from seeds that quickly settle on unshaded, moist mineral soil in shallow swamps, deep sloughs, and along often-flooded creeks or rivers. Seedlings require nearly full sunlight to survive and grow. They also need an abundance of moisture, especially during the early part of the growing season. Seedlings usually occur in groups but seldom cover a large area (4). Early growth is relatively rapid but will not match that of eastern cottonwood. On a well-drained soil in southern Illinois, swamp cottonwood seedlings grew at about the same rate as silver maple seedlings.