Knowlton's hop-hornbeam, western hop-hornbeam, ironwood

Ostrya baileyi Rose

Trees , to 9 m; crowns open, narrowly rounded. Bark brownish gray, broken into narrow vertical scales or rather ragged strips. Twigs sparsely to moderately pubescent, often with stipitate glands. Leaves: petiole covered with stipitate glands. Leaf blade ovate or broadly ovate-elliptic to broadly elliptic or nearly orbiculate, 2.5--6.5 × 1.5--5 cm, base narrowly rounded to cordate or cuneate, margins sharply and unevenly doubly serrate, apex acute; surfaces abaxially pubescent, especially on veins. Inflorescences: staminate catkins 2--3 cm; pistillate catkins 0.6--1 cm. Infructescences 2.5--4 × 1.8--2.5 cm; bracts 1--1.8 × 0.5--1 cm.

Flowering late spring. Streamsides and rocky slopes in moist canyons; 1200--2400 m; Ariz., N.Mex., Tex., Utah.

Ostrya knowltonii occurs sporadically throughout the arid Southwest, including both rims of the Grand Canyon. On the basis of morphology and phytogeography, it appears to be more closely allied with Ostrya in mountainous western Mexico than with the eastern O . virginiana .