Flower:

Male and female flowers are on separate trees (dioecious) in hanging clusters (catkins) from the leaf axils of 1 year old branches. Male catkins are 2 to 4½ inches long with tiers of red stamens. Female catkins are 2½ to 4¼ inches long with yellowish stigmas on top of a naked, round green ovary. 

Leaves and bark:

Leaves are alternate and simple with a mostly flat leaf stalk, blade is broadly triangular (deltoid), 2 to 4 inches long and up to 4¼ inches wide, the base mostly flat across, the corner angles rounded with coarse, rounded teeth, and tapered to an abrupt pointed tip. Both leaf surfaces are smooth, the upper bright green, the lower only slightly less so. Usually at the base of the leaf near the stalk are 2 small glands.

Twigs are greenish and smooth  turning a creamy yellow brown the second year. Buds are large, round and conical with a sharp point, the terminal bud up to ¾ inch, reddish or yellowish brown and shiny from a sticky, non-aromatic resinous coating. Bark is smooth and thin, creamy or greenish gray on younger branches becoming brownish to dark gray, thick and deeply ridged and furrowed on the trunk. Minnesota's champion tree cottonwood measures a whopping 10-ft diameter at breast height. Other massive 5 to 6-ft driver bottom specimens are not uncommon.

Fruit:

Fruits is a green, egg-shaped capsule on the long pendulous catkins, splitting into three or four parts when mature releasing copious amounts of cottony seeds to the winds.

Notes:

A massive tree of floodplains, stream valleys and lakeshores, it can also establish on drier upland soils where there is adequate subsurface moisture to see it through droughts. In urban plantings it is a fast growing species that can quickly dwarf the landscape though its weak, brittle wood predisposes it to wind breakage and the copious amounts of cottony wind dispersed seeds can quickly clog up window screens, air conditioner coils and lawnmower filters. Male hybrid selections such as Siouxland, as well as others, avoid the seeds and are frequently planted as field windbreaks and farm shelterbelts. Unlike most other poplar species, Cottonwood does not reproduce by root suckers and does not occur naturally in dense, pure clonal colonies. Attempts to utilize these hybrids in intensive biomass plantations, both here a abroad, have been hindered with issues like Cottonwood leaf beetle outbreaks and bacterial canker diseases. There are 3 recognized subspecies in North America: subsp. wislizeni limited to the southwestern US; subsp. deltoides (commonly called Eastern Cottonwood) in southern and eastern North America which has leaves with a short taper to the tip, 3 to 6 glands on the leaf, and usually hairless winter buds; subsp. monifilera in the central US and Canada which is found in Minnesota and has leaves with a longer taper to the tip, usually 2 glands on a leaf, and usually hairy winter buds.