Feature Tree – January 2007
Green ash
 Fraxinus pennsylvanica (Oleaceae family)

Philip Hoefer, Retired Community Forester, Colorado State Forest Service 

Green ash is native to Colorado in areas where moisture is more abundant in northeastern Colorado and in many of the eastern draws and valleys. Green ash is widely planted throughout Colorado. It became one of the five most common street and park trees in cities. It was also widely planted as the tall, dense deciduous tree in multi-row farmstead and field windbreaks on the eastern plains. Many varieties were developed over the years, mostly to select male plants to eliminate the problematic seeds. Green ash today has taken second place to the more popular white ash, Fraxinus americana.

Habit: Growth habit is pyramidal when young but develops into a broadly oval to irregular crown. Often grows to 60’ with a width of 25’. There are several major branches that form with many coarse, twiggy branchlets. The crown is dense with summer foliage and with winter branches. Considered a fast growing tree. Growth rates vary depending on the variety of ash planted but it is usually over 1.5’ to 2’ per year during its growth phase. Green ash is seen growing in many or our higher elevation communities.

Hardiness:  Zone 3 to 9.  Because of this wide range of temperature hardiness, green ash became widely planted.

Leaves:   Opposite, pinnately compound. Up to 12” long with 5 to 9 leaflets (mostly 5-7 in our environment). Leaflets are lanceolate to ovate and are 2” – 5”. Leaflet margins are finely serrate or nearly entire (without teeth or indentations). Shiny above. Dull green below. Fall color is yellow.

Twigs: Stout, rounded, densely velvety or glabrous. Leaf scars are not notched like white ash. It is half circular. Vascular bundles form a “closed” C shape in the leaf scar.   

Bark: Older trees are characterized by its interlacing furrows, forming elongated diamond shaped ridges. This feature makes it distinctive. Younger trees have a smooth, somewhat orange colored bark.

Fruit: Samara about 1” to 2 ½” long and ¼” wide. Paddle shaped and in dense clusters. There are many varieties of green ash which are seedless: The male side of this dioecious tree.

Landscape Value: Widely planted because of its hardiness in our harsher climate. In many areas green ash was over planted where it reached 30% - 50% of the tree population along streets or in parks. Before planting an ash, check you neighborhood for tree diversity. Seedless varieties are recommended and mostly sold at nurseries. It is still a popular tree to grow and nurture. Another positive is its adaptability to a wide range of soil pH. Insect problems are beginning to diminish this popularity.

Diseases/Insects: Several insects attack the ash but the most destructive are the ash borers. These insects have caused most loses of ash trees in Colorado. Leaf chewers and scale insects are also common. There have been unusual loses during extreme climatic conditions of drought and wide temperature fluctuations. Green ash remains a sturdy plant for our plains and mountain regions.

Information Sources: 

Kuhns, Michael, “Trees of Utah and the Intermountain West.” Utah State U. Press, 1998

Dirr, Michael, “Manual of Woody Landscape Plants.” University of Georgia, 1998

Cranshaw, Whitney, “Garden Insects of North America.” Princeton University Press, 2004.

National Audubon Society, “Field Guide to Trees,” Eastern Edition.