Flowers are borne in catkins, with male flowers smaller than female flowers. Narrowleaf cottonwood shows both asexual and sexual regeneration. Pollen is wind dispersed with fertilization occurring within 24 hours of pollination. Subsequent seed maturation is temperature dependent, generally requiring 3 to 6 weeks.
Seeds readily germinate when deposited in favorable environments.
Successful germination and establishment is dependent upon a suite of abiotic and biotic conditions, with insolation and moisture availability the most limiting factors. Cloning is important in narrowleaf cottonwood gallery maintenance. Male and female trees have similar clonal abilities. Shoot sprouting produced tightly clustered clones around buried root crowns. Root suckers produced wider spacing than shoot sprouting.