6a. Populus fremontii S. Watson subsp. fremontii
Populus arizonica Sargent; P. fremontii var. arizonica (Sargent) Jepson; P. fremontii var. macdougalii (Rose) Jepson; P. fremontii var. pubescens Sargent; P. fremontii var. thornberi Sargent; P. fremontii var. toumeyi Sargent; P. macdougalii Rose
Plants to 30 m. Branchlets, leaf blades, and petioles glabrous or glabrate to densely hairy, hairs whitish. Neoformed leaves: blade broadly triangular-ovate, usually about as wide as long, rarely wider, base truncate or cordate, apex short-acuminate. Capsules (6-)7.5-11 mm.
Flowering Feb-May; fruiting Mar-Jun. Floodplains, canyons, springs, other moist places; (-60-)0-1500 (-2200) m; Ariz., Calif., Idaho, Nev., N.Mex., Utah; Mexico (Baja California, Baja California Sur, Sonora).
Subspecies fremontii is the common cottonwood of southwestern North America west of the Continental Divide. It intergrades with subsp. mesetae along a line from southeastern Arizona south along the border between Chihuahua and Sonora.