Native Denver Trees: Cottonwood

Two species of cottonwood trees, the narrowleaf cottonwood and plains cottonwood, are not only abundant throughout Denver, but also a sight to behold. These majestic trees can tower as high as 60 feet high, and the female trees are well-know for releasing their cottony “snow” in the springtime (which also happens to be a source of great annoyance for many allergy sufferers)! The following profile of the cottonwood tree from Landscaping with Native Trees – a fantastic resource for extensive tree descriptions – contains just about everything you ever wanted to know about one of America’s most beloved species! Read on and enjoy…

Description

“Cottonwood” is a name that means different things to different people. Cottonwoods of various species and varieties are found almost throughout North America, and all look and behave very much alike. They are rugged, water-seeking trees that grow faster and larger than nearly of their associates in any region of the continent. But they are also weak and very prone to damage and decay, and female trees release a summer snow of cottony seeds that become entangled in window screens from the Atlantic to the Pacific.

Cottonwoods are most appreciated in the Great Plains, where other trees are rare and more difficult to grow. Here, they make vast riparian forests that shade the rivers and furnish the structural bones upon which wildlife habitat is built. Kansas, Wyoming and Nebraska have each designated a cottonwood as their state tree, but there is some confusion whether the honored tree is the eastern cottonwood (Populus deltoides) or the closely related plains cottonwood (Populus sargentii). Some authorities consider them to be varieties of the same species; their ranges do overlap in Kansas and Nebraska.

We have known old, hollow cottonwoods that had enough room inside for a poker game. The current national champion, in a pasture near Minadoka Dam in Idaho, has the spreading form characteristic of plains cottonwood; it is only 85 feet (26 m) tall but has a trunk 11.5 feet (3.5 m) in diameter. A comparable specimen 96 feet ((30 m) tall with a trunk 11 feet (3.2 m) in diameter grows in Gosper County, Nebraska. For many years, the recognized national champion was an eastern cottonwood growing along the Illinois and Michigan Canal in Illinois. Before it fell in 1991, entire grade-school classes could convene within the massive tree’s hollow base (although the entrance was too small to admit most teachers).

Leaves

Toothed and triangular, the leaves average about 4 inches (10 cm) long and broad. Those on the shoots of western species and varieties are generally smaller and more leathery, but the leaves on vigorous shoots of any species grow much larger. We have found that trees from different geographic regions planted together at Starhill Forest in Illinois retain the foliage characteristics of their home habitat.

The leaves hang from flexible petioles and clack together even when the breeze it too subtle to be felt on the ground. Their music is especially pleasant in late summer, when the leaves begin to dry and their sounding boards resonate. Cottonwood foliage becomes a warm yellow if the tree has had an insect-free summer and a gradual transition to fall.

The autumnal display is made more dramatic by the typical, fungus-induced early abscission of the oldest leaves, which, in falling, highlight the bare structural form of the branches.

Flowers and Fruit

All cottonwoods are dioecious, so only female trees bear the cottony seeds for which they are notorious. The fruit capsules begin as strings of green pearls in the early spring, and the ripe capsules split open synchronously to fill the late-spring air with a beautiful but messy display of cottony snow.

Seasons

  1. Fall (there is something unforgettable about a grizzly old cottonwood on a lonesome ranch, its foliage reduced to a smattering of golden leaves rattling in the breeze, the scene lit by a ray of sunshine breaking through the dark clouds of a lowering sky).
  2. Late spring (the cotton is truly the best and the worst of this tree; it is festive in wild areas where it may be enjoyed without inconvenience).

Native and Adaptive Range

The combination of eastern cottonwood and plains cottonwood blankets low ground and riparian habitats across the eastern and midwestern United States. Plains cottonwood extends into Alberta and Saskatchewan, north at least to Saskatoon. Other species, which are similar in most details, range north throughout much of Canada, west to the Pacific Ocean and southwest into Mexico, where cottonwoods are known as los alamos. Our eastern native species is adapted from the Gulf Coast north into USDA zone 3, but local races exist. So if you are doing some planting, look for trees of local provenance.

Culture

Cottonwood is probably our fastest growing largest tree. It is fairly easy to transplant when small, but it grows so readily from unrooted cuttings that transplanting an established tree seems pointless. Seed is perishable and difficult to handle. Tiny seedlings volunteer everywhere, however, and they may be moved about with abandon as they germinate.

The trick with propagating cottonwood is to start in late winter with a hardwood cutting of known gender. Plant it in open, weed-free soil, give it excessive amounts of water and get out of the way. We have seen groves of cottonwoods that grew more than 100 feet (30 m) tall in less than 20 years. Soil type is not critical, but the trees must have water, full sun and little competition to flourish.

Problems

Whole books have been written about the insects and diseases of cottonwood. Two cankers, Cytospora chrysosperma and Dothichiza populea, are especially troublesome on trees that have been injured by pruning or extreme weather. The trees are notoriously prone to damage from lightning, beavers, ice, wind, insects, decay and nearly every other force known to nature. Yet they are so resilient that some live to take their place among the largest of our deciduous trees. A few of the vulnerable cottonwoods that shaded Lewis and Clark on their Journey of Discovery in 1804 are still growing along the Missouri River.

Cottonwoods’ worst problems are amplified by their sheer size. This translates into massive, brittle limbs and extensive, invasive roots. Then, if your trees are females, there is the cotton. Some communities have actually passed ordinances prohibiting the planting of female cottonwoods. The cottonwood is a picturesque, fast-growing giant at its best where its negative traits are of no consequence.

Cultivars

Because cottonwood has immense value for paper pulp, many superior production clones with elaborate pedigrees are grown in forestry plantations. Cottonwood has been used in a vast forestry hybridization program with Europe, Asian and western North American poplar species. Several nurseries offer “cottenless” ornamental cultivars, which are nothing more than staminate trees growing one need only look around during the blooming period for a male tree, then return in late winter to harvest a dormant cutting.

Related Species

Eastern cottonwood and plains cottonwood are the primary species in the eastern and midwestern United States. People in the northern part of our area will find balsam poplar (Populus balsamifera) an aromatic species that ranges north as far as the Arctic Circle. IT has narrower leaves than cottonwood, and it seldom grows as large.

Swampy areas in the eastern United States sometimes support swamp cottonwood (Populus heterophylla). It has beautiful emerging foliage in early spring and becomes a large, tall tree, like eastern cottonwood. The national champion (named “the Little Big Tree” from associations with Native American lore) grows along the Black River in Spencer, Ohio, and stands 140 feet (43 m) tall with a straight trunk nearly 9 feet (2.75 m) in diameter.

There are other cottonwood-like poplars in western North America. They include black cottonwood (Populus trichocarpa), a giant tree of the Pacific Northwest; narrowleaf cottonwood (Populus angustifolia) of the Rocky Mountains; and several varieties of Fremont cottonwood (Populus fremontii) in the Southwest. The aspens are poplars, too, although they have more in common with some European and Asian species than with other North American poplars. They are covered separately.

Comments

Enter a cottonwood grove on a hot summer day and you will receive a standing ovation from the clapping leaves and the comfort of the dappled shade. In the nearly treeless landscape of the Great Plains, this can be a memorable and welcome experience. If contemporary life were not so dependent on window screens, air conditions, swimming pool filters and all manner of sensitive gadgets that clog and choke, we might also appreciate the drifting summer snow of cottonwood seeds, just as it must have been admired by the early Native Americans who revered this great tree.

The Arapaho believed that great cottonwoods cast the stars into the sky, and many tribes found mythic and pragmatic value in virtually every part of the tree. The famous photographic portfolios of Edward Sheriff Curtis, compiled at the beginning of this century as the sun was setting on the ancient ways of Native American life, help document the importance of cottonwoods to his subjects. Two of his more dramatic images depict a Navaho weaver’s loom set beneath the exposed root of a huge cottonwood and a ceremonial hat made from cottonwood leaves for the Sun Dance of Cheyenne.

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