Cottonwoods for Cheyenne

By Sam Cox

It’s no accident that the most common deciduous tree in Cheyenne is the cottonwood, making up about 18% of the street tree canopy (Cheyenne TreePlotter Inventory 2020) and gracing the yards of seemingly every 3^rd house in the older parts of the city. One of only a handful of native deciduous trees tough enough to thrive in Wyoming, it is well-suited to the harsh growing conditions of the high plains with superior cold-tolerance and vigorous growth in the face of relentless, desiccating wind. Visitors to Cheyenne can’t help but notice the massive cottonwoods arching over Pershing Blvd near I-25, or shading the grass at Holiday Park, and providing a welcoming rustle of leaves during a summer afternoon stroll along just about every street in the old sections of the city. So ubiquitous is the cottonwood throughout the entire state of Wyoming, it was the designated state tree in 1947. Nebraska and Kansas also designated the cottonwood as their state tree. Yet, for all that, the cottonwood is regarded as a second-class tree by many arborists and residents, often earning labels like “undesirable”, “tree to avoid” or “garbage tree” (Chesapeake 2008, Donovan Arborists 2018, Bender 2015). How can a tree be simultaneously an icon of the plains states, the most common deciduous tree of Cheyenne’s urban canopy and a tree that many arborists will tell you to avoid planting? Learning a little about the cottonwood can help you determine the answer.

Confusion reigns over the general name ‘cottonwood’, for several cottonwood species lay claim to parts of Wyoming, and all are similar in appearance. All cottonwoods belong to the genus Populus, which also includes aspen and true poplars. The defining characteristics of species in this genus include fast growth, light-weight wood, extensive root systems, preference for moist locations, bright green triangular to lance-shaped leaves on long petioles that turn yellow in the fall and, most noticeably, cottony fibers attached to the millions of seeds released in early summer that give the cottonwoods their name. Members of the Populus genus hybridize freely, and since they have been around in North America for more than 50 million years, there have been quite a few opportunities for new species and subspecies to arise. The Populus genus contains between 25 and 35 species and at least that many subspecies, depending on who you ask. Perusing literature regarding cottonwoods specifically, you’ll likely come across at least a dozen species names, although there is no universal agreement on which ones are separate species or just subspecies. The National Audubon Society Field Guides recognize 5 cottonwood species (Little 1980, 1995), while the Natural Resource Conservation Service plants database lists 5 species, 5 subspecies and 7 hybrids (USDA 2020). This doesn’t even include the closely related hybrid poplars, of which there are more than a dozen (NDSU 2020). It’s a tangled, jangled web of names, genetics and phenology that is constantly in flux, and in the end it’s worth remembering that names are human constructs, and the trees are what they are regardless of their name of the moment.

All cottonwoods have a recommendation grade of A or B on the Colorado State University Extension Front Range Tree Recommendation List, with primary drawbacks being weak wood leading to broken branches in storms, short lifespan relative to many other hardwoods and fast growth and large size that can overwhelm some residential locations (CSU 2020). Cottonwoods are susceptible to a variety of bacterial and fungal pathogens that can further exacerbate their weak structure (Hilgert 2014). A soft breeze on a summer day can bring idyllic clouds of cottony seeds floating through the parks and streets and is a pleasant visual unless you have window screens or open car windows in the path of the seed blitz. Only female trees produce seeds and cotton, though with native seedlings, it’s impossible to know if the tree is male or female until it’s been in the ground 15 years. Most cottonwood cultivars marketed as “cottonless” are simply clones of male cultivars with outstanding growth habits and disease resistance. Many municipalities have outlawed planting female cottonwood trees specifically to avoid the cottony clouds they produce.

PLAINS COTTONWOOD

The most common cottonwood species encountered on the plains of Wyoming and along the streets of Cheyenne is a regional variety of the Eastern Cottonwood (Populus deltoides ssp. monilifera) called the Plains Cottonwood, often referred to as Populus sargentii, and sometimes referred to as Populus deltoides var. occidentalis (Cheyenne Botanic Garden 2020, Wyoming State Forestry 2020, Kennedy 1985). Its name is derived from the triangular shape of the glossy leaves (deltoides means triangular in Latin) that, like trembling aspen, and most other members of the Populus genus, are borne on long, flattened petioles that bend in the slightest breeze, such that the leaves will flap and slap against one another as the frequent southern breeze of summer flows through the canopy. Anyone who has spent time in the western plains in summer knows well the gentle rustling of cottonwood leaves on a warm breeze, sounding almost like the patter of raindrops on water. No better phonic backdrop to a summer picnic or hammock snooze could be asked for.

Plains Cottonwood provided the primary wood source for inhabitants of the Great Plains for centuries, used widely for canoes, huts, cabins, fenceposts and firewood and it is a tree tightly interwoven with western culture. Along the western migration routes across the hot, dusty plains of high summer, settlers must have been very happy to catch sight of verdant green cottonwoods on the horizon, for always the cottonwood is a sign of water. In the Texas Panhandle, a state historical marker near a centuries-old cottonwood describes “The Landmark Cottonwood” which was reportedly used as a navigation aid for stage drivers and buffalo hunters in the 1870’s as they came and went from Fort Elliott, near the Canadian River, and later was a routine campsite for mail carriers (Texas Forest Service 1970). Disdaining dry environs, cottonwoods are water-loving trees, and grow best where there is a constant source of water either on the surface, or shallow enough in the ground that they can sink their roots in to constantly-damp soil. Along the narrow rivers in eastern Wyoming, the plains cottonwood grows into massive riparian forests that stretch for hundreds of miles, providing wildlife habitat and cooling the water they shade for the benefit of fish and other aquatic life. Cottonwoods are not at all drought-tolerant and without water, they quickly succumb: One estimate put mortality of Plains Cottonwoods during the dustbowl years of the 1930’s at 55-59% for trees near intermittent streams or springs, but only 6% for those along continuously-flowing streams (USFS 2004). It’s no wonder that west of the Missouri River, there are too many “Cottonwood Creeks” to count.

Cottonwood lumber is soft and lightweight (28 lbs/ft³, compared to 44 lbs/ft³ for red oak; USFS 1931), and prone to rot and breakage, but can be worked easily into boxes, crates, pallets and low-end upholstered or painted furniture, and of course the soft wood is easily incorporated into pulp for paper products. Thus, cottonwood is a commercially important timber species because of its rapid growth and use in a wide array of products (Kennedy 1985).

Plains cottonwood commonly reaches heights of 60-80 ft and canopy spreads of in 50-60 ft in Wyoming, compared to the Eastern Cottonwood which commonly reaches heights of 100 ft in the upper Midwest (Cheyenne Botanic Garden 2020, Little 1980). Almost no tree you could plant in Wyoming surpasses the growth rate of a cottonwood, which can sprint upwards at 6 to 12 ft/year (USFS 2004), and virtually no tree will grow as tall or wide as a Plains Cottonwood. In fact, it is the fastest growing native North American tree (Kennedy 1985). The national champion Plains Cottonwood grows in Montana and rises 112 ft from ground (American Forests 2020). The largest Plains Cottonwood in Wyoming is an 80-foot specimen in Albany County that has an incredible 111-foot canopy spread (WY State Forestry 2020). Measured by height and spread, this is the largest tree in Wyoming. Most residential homeowners aren’t particularly interested in planting the largest tree species in their front yard, not only because it can dwarf a house, but because all too often we have seen newspaper photographs of cars and houses crushed under toppled giant cottonwoods. The lifespan of cottonwoods, like all members of the Populus genus, is disappointingly short, with most surviving no more than a century. The former US national champion Plains Cottonwood in Hygiene, CO, was believed to be just 150 years old at the time of its natural death. Under very favorable conditions, some cottonwoods have been known to survive 200-300 years. An unusually long-lived Plains Cottonwood along the banks of the Missouri River in North Dakota is believed to have sheltered a campsite of the Lewis and Clark expedition on their westward journey in 1804 (Fauske 2002). This 250-300 year-old cottonwood is part of the Smith Grove, a protected area near Mandan, ND. The giant cottonwoods in Cheyenne trace back to planting efforts of James Jenkins and other forward-looking citizens of Cheyenne around 1900, when thousands of cottonwoods were planted along the city’s streets (Citizens Tree Committee 1902). Aside from one or two which may have been planted earlier, these records indicate that most of the large cottonwoods in Cheyenne are no more than 125 years old. Despite their well-earned reputation for losing twigs and even larger branches in strong wind, heavy snow or hail, the cottonwood strategy of outgrowing the damage works to produce the largest trees in the city.

Cheyenne residents note well that while pines, spruces and other conifers grow vividly well in the city, deciduous trees of all sorts find it difficult to do so. The cold winters combined with desiccating winds defeat most eastern forest species despite the best-laid plans of careful gardeners. Cottonwoods, and other members of the Populus genus, seem curiously immune to cold. Plains Cottonwood grows reliably into zone 3, which stretches into Canada, and regularly withstands winter temperatures of -50°F. A cousin of the Plains Cottonwood, the Balsam Poplar, is the continent’s most-northerly growing broad leaf tree (USFS 2004), and of course everyone has seen aspen (Populus tremuloides) shrugging off snow at 11,000 ft in the Snowy Range. The Poplar genus is a cold-weather club. Whatever may befall the stately Plains Cottonwood in southeast Wyoming, it will rarely have anything to do with cold injury.

The Plains Cottonwood cultivar Populus Sargentii ‘Jeronimus’, also known as ‘Sargent Straight’, which is frequently offered through Rooted in Cheyenne, is one of the 17 recommended shade trees for Cheyenne (Cheyenne Urban Forestry 2020) and is a male (cottonless) cultivar. ‘Siouxland’ is a male clone (cottonless) cultivar of Eastern Cottonwood (Populus deltoides) that grows fast in western climates (USU 2020) and has a generally taller and more upright form than Plains Cottonwood.

NARROWLEAF COTTONWOOD

Another native Wyoming cottonwood is the Narrowleaf Cottonwood (Populus angustifolia). Shorter (30-50 ft tall) and narrower (20-30 ft canopy) than the Plains Cottonwood, it is more a mountain resident, growing happily in higher-elevation valleys and streambanks up to 8,000 ft in elevation (Cheyenne Botanic Garden 2020) and sometimes higher: A large grove of Narrowleaf Cottonwood grows at 8,500 ft along the Pole Creek Trail off Happy Jack Rd between Cheyenne and Laramie. Narrowleaf Cottonwood, as the name suggests, has narrow, slender leaves that look more like willow than cottonwood and are further distinguished by having round petioles that reduce the flapping sound of their leaves in the breeze. The bark is generally smoother than Plains Cottonwood, and the trees sucker profusely, much like aspen, so that one most often finds groves of this tree, rather than the picturesque solitary Plains Cottonwood often seen along irrigation canals and old farmyards.

Cold hardiness of the Narrowleaf Cottonwood is also very high. Braving 50 below zero temperatures in winter, it grows far north into Canada, and is generally considered a zone 3 tree, much like Plains Cottonwood. Like its cousin the aspen, just about every forest animal eats its twigs, bark and leaves, and its presence in mountain valleys makes it a frequent meal and building material for beaver (USDA NRCS 2020). The expedition journals of Lewis and Clark mention first encountering Narrowleaf Cottonwood somewhere near Great Falls, Montana on June 6, 1805, noting “a species of cottonwood with a leaf like that of the wild cherry” (Peattie 1953).

Its smaller size makes this species of cottonwood more desirable for residential planting. Its suckering habit can be considered either a drawback or a bonus, for it will yield additional trees without the homeowner planting them. There are no commercial cultivars for this species.

LANCELEAF COTTONWOOD

This naturally-occurring hybrid cottonwood is a cross between Plains and Narrowleaf Cottonwoods, and has a natural range throughout the western Great Plains and mountain states where the ranges of the two parent species overlap. It combines the generally more agreeable traits of both species in that it is smaller than Plains Cottonwood (40-60 ft tall, 30-40 ft canopy; Cheyenne Botanic Garden 2020), and therefore more readily planted in an urban landscape, and it suckers less than the Narrowleaf Cottonwood, a bonus for most homeowners. The leaves of the Lanceleaf Cottonwood give away its parentage, as they are almost a perfect middle ground between the long narrow leaves of the Narrowleaf Cottonwood, and the wide triangular leaves of the Plains Cottonwood. The Lanceleaf Cottonwood is generally more upright and tidier in appearance than Plains Cottonwood, and makes a very nice-looking landscape tree that retains the flattened petioles that cause the leaves to flap and shimmer in the slightest breeze. In good years where an early snow doesn’t kill the leaves in September, the leaves turn brilliant clear yellow before dropping in October or November. Male, cottonless clones are readily available for sale. Alternatively, clip a 12-inch shoot off an established tree in early spring before the buds leaf out, set it in a jar of water for 2 weeks and you’ll have a fully rooted tree ready for planting when the soil thaws.

OTHER COTTONWOODS

Silver Poplar (Populus alba) is not strictly considered a cottonwood but is very closely related and fills the same landscape niche as Plains Cottonwood. This huge tree can be seen here and there around Cheyenne if one knows what to look for. Native to Europe and western Asia, it was brought to North America during colonial times as a large landscape tree (Little 1995; More and White 2013). Its five-lobed leaves are dark green above with white tomentose hairs below, so that when the wind blows, the tree shimmers pleasantly as the white undersides of the leaves flutter upwards, undoubtedly providing the inspiration for this tree’s common name. The bark of the Silver Poplar remains light colored even into maturity, graying to dark furrows near the ground, resembling aspen. Also like aspen, Silver Poplar suckers freely, and can become a nuisance in landscape settings where new trees sprout up in flowerbeds, lawns and sidewalk cracks wherever the extensive root system happens to wander. Like cottonwood, the Silver Poplar grows very fast and attains heights of 80 feet or more in optimal conditions (Little 1995). Though it is hardy to zone 3, it is not particularly long-lived and has brittle wood that can lead to downed limbs in storms and high winds. The Morton Arboretum describes Silver Poplar in negative terms, and teeters on labeling it invasive (Morton Arboretum 2020). A University of Arkansas Extension leaflet calls the Silver Poplar, “…a train wreck when planted in the garden” (Klingaman 2012). So, perhaps residents of Cheyenne, when seeking a tree that looks like a cottonwood should just stick to the native Plains Cottonwood, which has a less shady reputation.

Fremont Cottonwood (Populus fremontii) is named after the explorer John Fremont, the same explorer after which Fremont County and Fremont Peak are named in Wyoming, who described it while traveling through Nevada, California and Arizona in the late 1800’s. It is found gracefully shading rivers and creeks at elevations below 6,000 ft throughout the Colorado River basin and south into Mexico. It is common in desert canyons throughout Utah and Arizona and the dry foothill valleys in California. Like all cottonwoods, Fremont Cottonwood grows always near permanent water sources, and differs from Plains Cottonwood only subtly: to a slightly shorter stature of 40-80 ft with a more sprawling form and leaves that are an inch or two shorter, and usually as wide or wider than they are long (Plains Cottonwood leaves are longer than wide). The largest Fremont Cottonwood grows in central Arizona and is 102 ft tall with an almost-unbelievable 160-foot canopy spread.

The Rio Grande Cottonwood (Populus wislizeni) is often regarded as a subspecies of Eastern Cottonwood (Populus deltoides spp. wislizeni) that grows abundantly throughout the southern Rocky Mountains. In form and range it is so similar to Fremont Cottonwood that even professional botanists have a hard time telling the difference between the two (Taylor 2000).

Black Cottonwood (Populus trichocarpa) is the dominant poplar in the pacific NW from California through Alaska, and is the largest of all North American poplars. This massive species is at its best in the Coast Range of Oregon where it commonly exceeds 100’ in height. The national champion Black Cottonwood growing near Salem, OR is 141 feet tall, though that pales in comparison to records of specimens 200’ tall and boles 7-8 feet thick growing along the bottomlands of the Columbia River before the sawmills took them (Peattie 1953). Its leaves are more spear-shaped than inland cousins, being nearly twice as long as wide, dark green above with more of the poplar white underneath with rounded petioles, rather than the flattened petioles of the interior cottonwoods. In fact, the Black Cottonwood is more a poplar than a cottonwood, and many authorities recognize it as Populus balsamifera ssp. trichocarpa, a subspecies of Balsam Poplar.

Swamp Cottonwood (Populus heterophylla) grows generally east of the Mississippi River in wet soils along rivers and marshes. A tall tree, it has leaves that are broadly ovate with a pointed tip and rounded petioles (unlike most other cottonwoods), and the typical fine curved teeth along the margin that most cottonwoods exhibit (Little 1995). The largest Swamp Cottonwood is 148 ft tall with a crown spread of 120 ft, growing along the Mississippi River in Missouri. Because of its preference for wet, warm locations, it is unlikely to be seen growing well in Cheyenne.

Lombardy Poplar (Populus nigra) is a hardy male clone of Black Poplar that has cottonwood-like triangular leaves and grows in a conspicuous columnar habit (Little 1980). Very popular for its rapid growth and formal columnar appearance, it was frequently used in the 20^th century throughout the United States in shelterbelts and fencerows where it grew very rapidly, though typically died after 15-20 years. It is increasingly hard to find in the landscape because of the negative effects of European canker disease, and the introduction of superior hybrids with columnar form, such as Populus x canescens ‘Tower’ or Swedish Columnar Aspen (Populus tremula ‘Erecta’).

Like most things in life, the cottonwood forces gardeners to accept the bad with the good. In exchange for a beautiful, vigorous shade tree that can provide a shady picnic spot or a handy tire-swing limb within a decade, one must accept the brittle branches and maintenance issues that go along with a tree that, more than any other, lives fast and dies young.

References

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