What’s an Ash Tree?

Ash trees (Fraxinus species) are common in Wisconsin, with estimates as high as 765 million trees in wooded areas and over 5 million in urban areas. Ash is a component of three forest types in Wisconsin including 1) Elm / Ash / Cottonwood, 2) Northern Hardwood and 3) Oak / Hickory.  Ash trees can reach heights of over 50 feet when mature and often turn yellow in fall or reddish-purple (white ash).

Green ash tree, showing fall coloration. Photo Credit: Paul Drobot; UW-Herbarium

There are four native species of ash in Wisconsin: green ash (Fraxinus pennsylvanica), white ash (F. americana)black ash (F. nigra), and blue ash (F. quadrangulata).  Horticultural cultivars of these species are commonly planted in urban areas.  Ashes are often very tolerant of urban environments and have been planted extensively as street trees. Until now, ash trees have had few serious insect and disease problems. Green ash and white ash are the most commonly found ash species in the Midwest with blue ash being rare. 

Some other woody plants, such as mountain ash and prickly ash, have “ash” in their names but are not from the “true” ash group (Fraxinus spp.) and are not attacked by EAB.

Ash Trees Can be Readily Identified by Two Key Features:

1. Opposite branching pattern—two branches come off the main stem directly across from each other.  Most of Wisconsin’s trees have an alternating branching pattern.  Maples have an opposite branching pattern, but have simple leaves.

Branching Pattern. Credit: WI-DNR

2. Compound leaves with 5-11 leaflets (depending on the species of ash). Leaflets are moderately toothed and may be stalked or un-stalked (sessile).  Most of Wisconsin’s trees have simple leaves.  Some potential ash look-alikes with compound leaves include hickories and walnuts, but these have alternate (rather than opposite) branching patterns.

Compounds leaf of an ash tree with seven leaflets. Photo Credit: Michigan Department of Agriculture, Bugwood.org

Other helpful features include rough bark with diamond-shaped ridges (mature trees) and clusters of “oar” shaped seeds (samaras).

Bark of white ash. Photo Credit: Keith Kanoti, Maine Forest Service, Bugwood.org

Ash Seeds. Photo Credit: Keith Kanoti, Maine Forest Service, Bugwood.org

A helpful video illustrating how to identify ash trees can be found here.

A printable guide to identifying ash trees is available from Michigan State University.

Additional information about Wisconsin’s native ash species can be found at the Wisconsin State Herbarium site.