In the Four Corners area, especially in Colorado and New Mexico, Cottonwoods were for many years known as Populus fremontii, Fremont's Cottonwood, but research in the 1970s and 1980s convinced most botanists that the Cottonwood of the Four Corners area of Colorado and New Mexico is Populus deltoides subspecies wislizeni. 

     Populus fremontii (aka P. deltoides subspecies fremontii), collected by John Fremont in California in 1846, is now generally accepted as the Cottonwood of California, Nevada, much of Arizona and Utah, and some of southwestern New Mexico. See the maps below.

     Populus deltoides subspecies wislizeni, the Rio Grande Cottonwood, collected by Friedrich Wislizenus in 1846-1847 on a trip from St. Louis through New Mexico and northern Mexico, is the Cottonwood of western Colorado, most of  New Mexico, far western Texas, northern Arizona, and eastern Utah.

     Populus fremontii is accepted as a species by many botanists, including the influential botanists of the Flora of North America. Some botanists, including John Kartesz of the Biota of North America Project maintain that it is a subspecies of Populus deltoides, the Eastern Cottonwood and that is the name given on this web site.

     In a paper published in 2014, Cushman et al. indicate that their research (and previous research) shows, as my friend Randy Bangert, Ph.D., summarizes:

"a genetic gradient between P. deltoides and P. fremontii with most trees between the Continental Divide and Eastern Utah as genetically mixed and trees farther west as P. fremontii and trees further east as P. deltoides. Cushman et. al. 2014 also state that the trees between P. deltoides and P. fremontii (the Four Corners region) have both species' genes and are largely P. deltoides types".

      The research by Cushman et al. focuses on the influence of habitat fragmentation on the genetic health of species, specifically Populus fremontii. The research concludes,

"Because most arid lands riparian systems are threatened by habitat loss, invasive species, water diversions, and altered stream flows..., increased habitat fragmentation and reduced gene flow threaten the plants that support much larger communities of organisms and their ecosystem process. Thus conservation efforts should focus not only on restoring riparian habitat, as in the case of large-scale efforts underway for targeted areas in the southwestern United States..., but also on re-establishing corridors that promote gene flow. Given that community structure in cottonwoods scales from local... to regional levels..., it is important to consider how gene flow across the landscape... , may influence the evolution of dependent community members..., and affect community diversity and ecosystem processes...."

     Populus deltoides and Populus fremontii are very similar and are distinguished primarily by floral characteristics. Here is how James Eckenwalder separates the two in his key in the Flora of North America:

P. deltoides (including P. deltoides ssp. wislizeni): Stamens 30-40(-55); pedicels 1-10(-17 in fruit) mm; ovaries ovoid, discs saucer-shaped, 1-3(-4) mm diam.; branchlets usually glabrous or thinly long-hairy.

P. fremontii: Stamens (30-)40-60(-70); pedicels 1-4(-5.5 in fruit) mm; ovaries spherical, discs cup-shaped, (2.5-)4-7(-9) mm diam.; branchlets glabrous, glabrate, or hairy.

      "Populus" is Latin for "people" and is the classical Latin name for the tree.  "Deltoides" refers to the leaf shape and is from the Greek alphabet letter "delta", thus "triangular" + "oides", which means "similar to".

     A word about the ending "i" or "ii" in "wislizeni"and "fremontii":  The International Code of Botanical Nomenclature several decades ago recommended dropping the double "i" at the end of botanical names.  But not everyone follows this recommendation and you will thus find varying spellings.  There also seems to be some disagreement about whether the International Code really did recommend dropping the "ii"! The Synthesis and The Flora of North America spell one subspecies, "fremontii", but the other "wislizeni".

     The pronunciation of "fremontii" is also controversial:  Although the Latin "i" should be pronounced "e", almost all American botanists pronounce "ii" as "e eye": "fremon tea eye". Yet every European botanist I have met pronounces "ii" as "e": "fremont e". A number of prominent American botanists (William Weber for one) and Latin scholars indicate that "e" is correct and that "e eye" is to be rigorously avoided . I agree. The "e eye" sounds like the secret code of a secret society, and its use also often coincides with the mispronunciation of a person's name. Folks who use the "e eye" ending then pronounce "fremontii" as "fremon tea eye". But one of the widely stated and accepted suggestions for botanical pronunciation is to stay with an accurate pronunciation of names. Thus, no matter whether we say "ee" or "e eye", we should say "fremont" and then add the ending: "fremont ee" or "fremont ee eye". I have never heard anyone use the latter pronunciation.

Semi-desert, foothills. Canyons, washes, streamsides. Spring.
Hunter Canyon, Utah, May 2, 2005.

These lovely specimens are about 50 feet tall and show all the Cottonwood features: broad, leafy canopy; dark, leaning, twisted trunks (notice the third trunk leaning out of the picture in the lower right); and canyon bottom habitat.

Friedrich Adolph Wislizenus was a St. Louis physician, friend of George Engelmann, and avid amateur botanists who made several trips to the West, including one through the Southwest into Mexico.  (More biographical information about Wislizenus.)

Semi-desert, foothills. Canyons, washes, streamsides. Spring.
Canyonlands National Park, October 28, 2009.

Semi-desert, foothills. Canyons, washes, streamsides. Spring.
Canyonlands National Park, March 24, 2001.

Cottonwoods sometimes seem to grow in very dry conditions, apparently hostile for them, but a closer observation will show that their roots are tapping into water at the base of a steep hill (as in this picture), or where water is relatively near the surface because of rock formations, or where washes periodically flood.  Trunks of older trees are typically gray and deeply furrowed, commonly leaning, sometimes even sprawling along the ground.  Orange tinted bark occurs occasionally.

Semi-desert, foothills. Canyons, washes, streamsides. Spring.
Dominguez Canyon, Gunnison River, May 13, 2003.

Where water is more consistent than in the dry rocky wash shown in the above photograph, Cottonwoods grow three foot diameter trunks (as shown here) to about 50 feet tall with broad canopies providing shade for grasses, shrubs, and hikers (and, unfortunately, cows).

Range map © John Kartesz,
Floristic Synthesis of North America

State Color Key

Species present in state and native
Species present in state and exotic
Species not present in state

County Color Key

Species present and not rare
Species present and rare
Species extirpated (historic)
Species extinct
Species noxious
Species exotic and present
Native species, but adventive in state
Eradicated
Questionable presence

Range map for Populus deltoides  

Range map for Populus deltoides subspecies fremontii

Range map for Populus deltoides subspecies wislizeni