26 July 2021

Two trees in the Birch family (Betulaceae) are common in the Pittsburgh area but I’ve struggled with what to call them because they have the same names.

Both are called Ironwood because their wood is hard, close-grained, and very strong. Ironwood is a poor name choice, though. About 160 species around the world are called “ironwood”.

Their scientific names are different but their default common names are very similar: American hornbeam (Carpinus caroliniana) and American hophornbeam (Ostrya virginiana). Hornbeam refers to their hard, strong wood: horn (hard, bony structure) + beam (Old English for tree). Hop is the only difference.

Fortunately they are easy to tell apart in the field at any time of year. In the photo at top:

• The bark of American hornbeam looks like sinewy muscles (top left).
• American hophornbeam bark peels in narrow parallel strips (top right).

Both trees produce fruit enclosed in an involucre, a whorl or rosette of bracts surrounding the inflorescence. This is where “hop” comes in.

• The fruit of American hornbeam looks like a drooping whirligig (left below).
• American hophornbeam fruit looks like hops, hence its name (right below). Click here to see what hops look like.

Both species with fruit and bark paired.

They differ by a hop.

p.s. Because of their similar names I sometimes call “hornbeam” by another common name: blue beech. More confusion!

(photos by Kate St. John)