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Spring 2010

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

cottonwood releasing seeds

Insects have begun to eat holes in this tree's leaves. A favorite of livestock, cottonwood leaves are rich in protein, offering more amino acids than many common grains.

May 18, 2010

Cottonwood seeds:

A poplar lottery


Drifting on breezes and riffles, cottonwood seeds dot the sky and fleck local waterways. The odds that any one seed will germinate are remote.

Also known as Eastern poplar, a single cottonwood tree can release more than 25 million seeds, each suspended by a frizzy mass of cottony fibers that can transport the seed far from the tree.

Seeds set sail at about this time of year, when water levels are dropping, revealing freshly scoured gravel bars, sandbars and stream banks, ideal spots for young cottonwoods.

The fluff must settle onto the right surface soon. Seeds are viable for only one to two weeks, and they stand a chance of germinating only if they fall onto sunny, moist, exposed soil.

Lucky seeds sprout within a day, after which they grow as much as a quarter of an inch in their first 24 hours. To survive long enough to be saplings, seedlings need constantly moist soil with abundant sunlight. Most seedlings are either trampled and eaten by herbivores, overrun by exotic invasive plants, beaten down by rain, swept away by floods or -- if they survive long enough -- gouged by winter ice.

Young saplings can tolerate inundation, but a drought will kill them. They devote much of their energy to sinking roots down toward the water table, piercing the soil by as much as a meter in their first year.

Once saplings have tapped a guaranteed source of water, they can grow 30 to 50 feet in five to 10 years, at which point they reach maturity. Female trees can then begin to disperse a multitude of their own fluffy long shots.

cottonwood seed

A cottonwood seed (actual size and enlarged four times) discards its parachute soon after landing. A million of these would weigh less than three pounds.

SOURCES: "Biology of Populus and its implications for management and conservation," by Reinhard F. Stettler, et al.; U.S. Forest Service