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Ostrya knowltonii
Ostrya knowltonii
Ostrya knowltonii

Ostrya knowltonii (Western Hop Hornbeam)
Betulaceae (Birch Family)

Semi-desert. Wet, shady, canyon bottoms. Spring.
Behind the Rocks Wilderness Study Area, Utah, October 26, 2012.

Ostrya knowltonii late fall yellow leaves stand out in the canyon bottoms that it favors. Male catkins form at branch tips in the summer and persist through the winter; female catkins will grow in the early spring about the time leaves begin growing.

Ostrya knowltonii
 

Ostrya knowltonii (Western Hop Hornbeam)
Betulaceae (Birch Family)

Semi-desert. Wet, shady, canyon bottoms of Southeast Utah and Northern Arizona. Spring.
Behind the Rocks Wilderness Study Area, Utah, April 23, 2006.

Western Hop Hornbeam is a rare tree in southeast Utah and is otherwise found only in northern Arizona and a few scattered areas of southern New Mexico and Texas. 

The tree is quite prominent on the Chesler Park Trail of Canyonlands National Park where hikers walk through the narrow Breezeway before descending into Elephant Wash. The tree is also even quite abundant in moist, rocky canyon bottoms near Moab, Utah. 

Ostrya knowltonii typically grows to a maximum of 10-30 feet tall and 6-15 inches in diameter often with several main trunks and often in close proximity to a number of other Ostrya knowltonii.  The tree is prominent because of its cracked and shredded bark, hop-like seed pods, and male pollen-bearing catkins.  (See the photographs below and click for more plant details.)

"Ostrya" is Greek for "hardwood tree" and refers to the dense wood of the Hop Hornbeams (Western and Eastern) which gives them another common name, "Ironwood".   Botanist Frank Knowlton (1860-1926) discovered the tree in 1889 below the rim of the Grand Canyon.  (More biographical information about Knowlton.)

Ostrya knowltonii

Ostrya knowltonii (Western Hop Hornbeam)
Betulaceae (Birch Family)

Semi-desert. Wet, shady, canyon bottoms of Southeast Utah and Northern Arizona. Spring.
Behind the Rocks Wilderness Study Area, Utah, April 23, 2006.

In the picture at left, the maturing green seed pods on the female catkins (far left) resemble those of hops (various vine Humulus species).  The male pollen-bearing catkin is in the center of the photo and is also shown in the next photographs.

Since male and female flowers are separate but on each tree, the tree is termed "monoecious" (Greek for "one house"), in contrast to "dioecious" species which have male and female flowers on separate plants.

Ostrya knowltonii (Western Hop Hornbeam)
Betulaceae (Birch Family)

Semi-desert. Wet, shady, canyon bottoms of Southeast Utah and Northern Arizona. Spring.
Behind the Rocks Wilderness Study Area, Utah, April 16, 2008.

These long, fresh, male pollen catkins have just opened and tiny stamens protrude. 

Ostrya knowltonii

Ostrya knowltonii (Western Hop Hornbeam)
Betulaceae (Birch Family)

Semi-desert. Wet, shady, canyon bottoms of Southeast Utah and Northern Arizona. Spring.
Behind the Rocks Wilderness Study Area, Utah, April 23, 2006.

Bark on younger trees is gray, with darker, slightly raised, longitudinal ridges. Mature bark is shredded, as shown in the photograph below of a five inch diameter trunk.

 

Ostrya knowltonii

Ostrya knowltonii (Western Hop Hornbeam)
Betulaceae (Birch Family)

Semi-desert. Wet, shady, canyon bottoms of Southeast Utah and Northern Arizona. Spring.
Behind the Rocks Wilderness Study Area, Utah, April 23, 2006.

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 Ostrya knowltonii