Ash Trees

In the past, ash trees were one of the more common street trees in many urban areas of the United States.   However, with the introduction of the Emerald ash borer, Agrilus planipennis, a member of the beetle family native to Asia, our native ash populations have suffered severe losses. 

Ash trees have been a popular “street tree” species for many urban areas.  It is estimated that by the late 20th century, 20% of all parkway trees in Chicago were ash species.  Green ash, Fraxinus pennsylvanica, likes moist soils, but grows well in most conditions.  White ash, Fraxinus americana, is very similar to green ash, but prefers well-drained sites.  Both have furrowed bark appearing as tight rectangular blocks on young trees, but deepening into longer furrows and ridges resembling a diamond-shaped pattern in mature trees.  Black ash, Fraxinus nigra, often found on wet sites, is covered with irregular, knobby bumps.  All the barks are grayish-brown.

In spring, ash trees grow small green or purple flowers with no petals.  Most are wind pollinated, although several bee species collect the pollen and contribute to some seed development.  Flowers appear on both male and female trees, but only female trees develop seeds.  Seeds are called samaras, or “helicopter seeds,” due to the distinctive whirling pattern they exhibit as they fall off the tree in a lazy spiral to the ground.  The seed is housed in a fully rounded shell at one end of the single wing of each samara.

Its pinnately compound leaves are another distinctive feature of the ash family. A compound leaf has a leaf blade divided into leaflets that are attached individually to the middle vein, each by its own stalk.  The term pinnately refers to the arrangement of leaves, opposite one another along the middle vein.  A terminal leaf, grown at the tip of the vein means there are always an odd number of leaflets, from 5 to 9 leaflets in ash species.  Buds and branches also grow in this pattern, which can be observed on rose plants, as well as hickory and walnut trees.

White ash is the main source of wood used in commercial applications.  It is of medium weight and springy, able to absorb a shock and bounce back.  This makes it valuable for bats, tennis rackets, bows and arrow shafts, along with a wide variety of other uses including tool handles, boats, and furniture.  The seeds provide a valuable food source for several bird species including quail, duck, and turkey.

Although many, many ash trees have been lost to attacks from Emerald ash borers over the past couple decades, a small number of green ash have proven more resistant.  They provide great shade for parks, yards, and large areas.  In fall, watch as white ash trees turn beautiful shades of gold, deep purple, and maroon.  Be sure to make note of them on your walks and check back in a few months to view their spectacular fall displays.