Ashes are popular, rapidly growing landscape shade trees, according to Colorado State University Extension's Plantalk. Both green and white ash tree varieties are commercially available. The more cold-tolerant of the two, green ashes provide yellow fall color. White ash foliage is purple in autumn. Because ash trees are vulnerable to storm damage, they benefit from pruning on a 3- to 4-year schedule to maintain a tidy form. They may suffer potentially fatal ash borer attacks.
Blue ash (Fraxinus quadrangulata) is native to the central United States from Arkansas north to Michigan and east to Tennessee. Its inner bark once provided American settlers with blue dye. Standing from 50 to 75 feet high and up to 60 feet wide, it's a narrow-crowned tree with lance-shaped, feathery 4- to 5-inch green leaves. They become pale yellow in fall. Its purple clusters of self-pollinating April and May flowers give way to winged seedpods (samaras). Up to 2 inches long, they may remain on the trees all winter.
Blue ash, says the Missouri Botanical Garden, tolerates winter temperatures to minus 30 degrees Fahrenheit. It's susceptible to webworms, ash borers, and leaf miners. Plant it as a street or lawn tree in full sun. For best performance, give it moist, well-drained humus-rich loam. (References 2 and 5)
Native to southern Europe and Asia Minor, flowering ash (Fraxinus ornus) tolerates winter temperatures no lower than minus 10 degrees F. Between 40 and 50 feet high and wide, it has a short, smooth, gray-barked trunk. In April and May, flowering ash produces abundant 5-inch clusters of creamy white, fragrant flowers. They contrast well with its deep-green, compound 8-inch leaves. Like blue ash, flowering ash produces samaras that may remain through the winter.
This tree's major threats are ash borers and oyster shell scale. It also suffers from powdery mildew, rust, canker, and other diseases. Plant it as an ornamental shade tree in full sun and rich, moist well-drained loam. It's happiest where summers are cool.
A moisture-loving tree native to the eastern United States, pumpkin ash (Fraxinus profunda) thrives in swamps, bottomlands, and coastal and flood plains. Hardy to minus 20 degrees F, it has up to 18-inch compound leaves with greenish yellow surfaces and downy undersides. The foliage becomes reddish-purple or bronze in autumn. In April and May, the trees have inconspicuous green flowers. Clusters of samaras follow in late summer to mid autumn.
Like flowering ash, this tree is highly susceptible to ash borers and oyster shell scale. It also suffers from fungal leaf spot, rust, powdery mildew, and several other diseases. Because of its drought intolerance and size--up to 80 feet high and 50 wide--pumpkin ash is seldom used in home landscapes. It requires consistently moist or wet, sandy or clay loam and full sun to partial shade.