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Blue Ash

Fraxinus quadrangulata

Olive Family (Oleaceae)

Introduction: Blue ash is a beautiful, underutilized Kentucky native that may be difficult to find in the nursery trade. It enjoys alkaline soil and is very much at home on the limestone outcrops in central Kentucky. Blue ash bears tiny but attractive flowers, pretty clusters of dangling seeds and handsome foliage.

Culture: Blue ash makes a nice, large shade tree. It thrives in full sun on rich, moist, well-drained soils but it is particularly tolerant of limestone soils and drought. Its ability to survive in adverse conditions makes this a good tree for urban landscapes. The lilac borer is the main pest problem with blue ash. The lilac leaf miner, ash borer and fall webworm may be minor pests on this tree. Blue ash does not present the problem of weediness exhibited by green ash.

Botanical Information

Native habitat: Michigan to Arkansas and Tennessee.
Growth habit: Upright oval to rounded crown with spreading branches on a straight, slender trunk.
Tree size: Blue ash grows moderately fast to 50 to 70 feet in height.
Flower and fruit: Bisexual flowers bloom as leaves emerge. The tiny, purple, petal-less flowers hang in loose panicles. Fruit is a winged samara that is 1 to 2 inches long and hangs in clusters. Samaras fall from the tree over several weeks in autumn.
Leaf: Blue ash has a very large 7- to 14-inch leaf that is divided into two rows of 2- to 5-inch, coarsely toothed leaflets on either side of a central stem. Leaves are dark green in summer and fade to pale yellow in autumn. The stem has small ridges that make it appear square.
Hardiness: Winter hardy to USDA Zone 4.

Additional information: Blue ash's native range includes all of Kentucky, particularly the inner Bluegrass region. Blue ash is an upland species that thrives on limestone outcrops. In the inner Bluegrass blue ash is often seen along with bur oak in old pastures. Statesman Henry Clay named his Lexington estate "Ashland" in honor of this species.

Blue ash was introduced into cultivation in North America by 1823. The national champion blue ash (86 feet tall with a 66-foot spread) is in Danville, Ky.

The blue ash genus name, Fraxinus, is from the Latin name for the Old World ash species. Ash is a member of the olive family and consequently has some unlikely relatives, including forsythia, lilac, privet and olive.

Blue ash has a gelatinous substance on its inner bark that turns blue when exposed to air. It was used by pioneers as blue dye. This is the quality for which blue ash is named.

The wood of blue ash is used to make tool handles. The airiness of blue ash's dark foliage contrasted with the white undersides of its leaves creates a nice shade.This species' stems are stout and square with four corky ridges.