Cottonwoods are dioecious, meaning their flowers grow on separate male and female trees. Their tiny flowers hang from branches in long, thin clusters called catkins. In very early spring, usually March, the colorful red male catkins produce pollen, which floats on the wind and lands on the stigmas of the female catkins on the female tree. Cottonwood pollen is known to be highly allergenic, but like most pollen, it is invisible to the naked eye.

The fertilized female parts develop into light brown seeds enclosed within egg-shaped capsules. Each capsule may contain 30 to 60 seeds. When the capsules open, the seeds, with their numerous white, hair-like structures, are dispersed by the wind or water, eventually to land and germinate on a stream bank or beach, or, alas, to create “snowdrifts” in air conditioners, screens and other human contrivances.

The ubiquitous cotton may mislead hay-fever sufferers in June into thinking it must be the obvious cause of their discomfort. The true culprit, however, is the pollen from grasses and other plants that disperses at the same time. The female cottonwood is messy but innocent.

Eastern cottonwood, Populus deltoitis, is in the willow (Salicaceae) family and is native from Quebec west to North Dakota and southern Manitoba, south to central Texas, and east to northwestern Florida. In the plains, it intermingles with the plains cottonwood, Populus occidentalis, meaning “of the west.”

According to some sources, their genus name, Populus, derives from “arbor populi,” the Latin for “people’s tree,” because poplars were widely planted in Rome. “Deltoides” means triangular and describes the leaf, which resembles the Greek letter delta. There are numerous cottonwood species throughout the northern hemisphere.

Cottonwood leaves are toothed, with a broad, straight base and a distinctive pointed tip. They average 3 inches across and 4 inches long. Bright, glossy green all summer, they turn yellow in fall. Being poplar leaves, they grow on long stalks and are attached in a manner that allows them to spin in the slightest breeze.

A National Parks and Conservation journal article from spring 2007 states that some Western Native American tribes believed the gods whispered to them through the “rustle of the wind through the quaking leaves.” Donald Culross Peattie, poetic nature writer and author of “A Natural History of Trees,” wrote that “even on the hottest, driest day [they] reminded you, by the sound of their rustling leaves, of lake waters coolly lapping.”

The ubiquitous cotton may mislead hay-fever sufferers in June into thinking it must be the obvious cause of their discomfort. The true culprit, however, is the pollen from grasses and other plants that disperses at the same time. The female cottonwood is messy but innocent.

Cottonwood is a pioneer tree, being the first to move in on bare land such as new dunes and laying the groundwork for a succession of trees and shrubs. At the Indiana lakeshore, cottonwoods grow out of the dunes, their lower branches having been smothered in sand. To survive, the tree develops “adventitious” roots higher up on its trunk, at the same time helping to stabilize the dunes. To look at most dunes cottonwoods is to see, so to speak, the tip of the iceberg.

The average visible height of cottonwoods is 75-100 feet. They make their growth spurt in the first 40 years and typically live to be around 125. They are often double-trunked, and trunks average 12 feet in circumference. As they age, their bark becomes thick and furrowed. They begin producing seed around age five.

Cottonwoods love water. Settlers traveling west found that the necessities of timber, water and shade were scarce on the plains. Beacons to these resources were the nearest row of large trees, invariably cottonwoods, growing along a low-lying watercourse. Because of high water content and fast growth, cottonwood timber tends to be weak and brittle and to warp when dried. In the absence of alternatives, however, it became the tree of many uses, including fence posts, wind breaks, shade, houses, ox yokes, wagon wheels and coffins. Lewis and Clark and the Corps of Discovery used the plains cottonwood for their North Dakota winter shelters and turned six sizeable trunks into six canoes. Early French trappers shipped beaver pelts in cottonwood dugouts.

Remember the Golden Spike, the symbol that concluded the race to finish the Transcontinental Railroad? The spike was gold, but in the Great Plains, the Union Pacific’s crossties were primarily made from the only available lumber, cottonwood.

The wood was so wet it could only last two or three years, and a process that replaced the water with zinc oxide was expensive and did not work. The motto, according to Stephen Ambrose, was “Nail it down! Get the thing built! We can fix it up later.” Shortly after the railroad’s completion 300,000 cottonwood ties had to be replaced.

Male cottonwood clones have been planted to stabilize the banks along streams and ditches, to improve water quality and to reduce flooding. Because they are fast-growing, in the future they may be grown for biomass to produce energy.

They may also make suitable cattle feed because of their high protein and mineral content. As a precedent, Native Americans fed the bark to their horses when other food was covered by snow. Finally, research has shown that cottonwoods are vitally important feeding, resting and nesting corridors for neotropical (new world) migrating birds.

A “Big Tree” of exceptional girth, also known as “The Potawatomi Tree,” once grew at the intersection of Glenview Road and Edens Expressway. It has been gone now for over a century. While it was alive, local tales grew up, some almost as tall as the tree. In “A Natural History of the Chicago Region,” Joel Greenberg writes that it was said to be “130 feet tall with a circumference of 41 feet,” and was judged to be at least 500 years old. Its enormous hollow trunk might have sheltered 30 people simultaneously. Thieves hid their loot inside. It was once home to a black bear and two cubs. Probably only the age is exaggerated, for in photographs this tree is truly impressive.

A stately eastern cottonwood grows at the northeast corner of Lincoln Street and Sherman Avenue. In 2007 its circumference measured 15 feet. It is currently 222 years old.

Libby Hill

Libby Hill is the author of "The Chicago River: a Natural and Unnatural History. She has been writing about birds and trees and Evanston's natural history for the Roundtable since 2004.