Interesting Facts
Blue ash is also referred to as square-stemmed ash because its twigs and stems are square, distinguishing it from other ashes in the state. The name "blue ash" is a reference to the blue dye produced when the inner bark is placed in water and/or the sap, which turns bluish when exposed to air.

Identifying Features

Bark
Blue ash bark is bluish gray to brown gray. It is thickly furrowed and scaly, becoming shaggy with age. The shaggy, ridged bark of the mature tree is a distinctive characteristic. 
Twigs
The twigs are light green-gray to brown and stout. They are square in cross-section - another distinguishing feature of blue ash. They can be either sparsely hairy or smooth, with a sap that turns bluish on exposure to air. 
Buds
The buds are rounded, slightly hairy and up to 1/2 inch long. 
Leaves
The leaves are opposite, from 8-12 inches long, and pinnately compound with 5-11 leaflets. Leaflets are up to six inches long, lance-shaped with rounded or sometimes asymmetrical bases, and are coarsely toothed along the edges. They are light green and smooth on the upper surface and paler with sparse hair along the veins underneath.
Flowers
Individual flowers have both male and female components (stamens and pistils) - botanists refer to this as a "perfect" flower. Flowers appear before leaf out as small, branched clusters of purplish flowers without petals. 
Fruits
The fruits are oblong, winged, and enclose a single seed at the base. They are 1-2 inches in length, 1/2 inch broad, and dark green to gray. Songbirds and rodents eat the seeds.

Uses
Blue ash wood is heavy and hard. It is used for flooring. The inner bark produces a blue dye when crushed and placed in water.