Seventeen species of ash trees (Fraxinus spp.) grow throughout the United States, scattered throughout woodlands, canyons, uplands and lowlands rather than occurring in thick stands. Worldwide, about 65 species exist, nearly all with pinnately compound, feathery leaves that usually have six to nine leaflets. Ash trees are widely used as ornamentals, providing shade and fall color, and they're also used in public plantings and in wild-land restorations. They furnish food and cover for wildlife and stabilize soil. All kinds of ash trees, however, are under threat from the emerald ash borer, a 1/2-inch-long, metallic-green beetle believed to have been introduced to the United States from Asia in the 1990s. It has destroyed millions of ash trees in more than 20 U.S. states.
The two most widely distributed American ash trees grow throughout the eastern United States and into the Midwest. They are white ash (Fraxinus americana) and green ash (Fraxinus pennsylvanica), both of which are hardy in U.S. Department of Agriculture plant hardiness zones 3 through 9a. Growing 50 to 80 feet tall with a 40- to 60-foot-wide canopy, white ash is useful as a shade tree and a street tree. The white ash cultivar "Autumn Purple" (Fraxinus americana "Autumn Purple") furnishes maroon, purple or deep-red leaves in fall when grown in full sun. It is hardy in USDA zones 3b through 9a. The green ash reaches 60 to 70 feet tall and is useful as a shade and street tree, too, and it can grow in urban areas because it tolerates poor or compacted soil and air pollution.
Although they're not as cold-hardy as their eastern relatives, ash trees native to the South have other useful attributes. Texas ash (Fraxinus texensis) is native to Texas and Oklahoma and hardy in USDA zones 5 through 9a. Usually growing 25 to 30 feet tall and about as wide, Texas ash has a symmetrically shaped canopy and dark-green leaves. It has a high drought tolerance and is useful as a multistemmed, ornamental small tree, although it can be trained to have a single trunk. A species that grows in the Southeast and is hardy in USDA zones 7 through 10, Carolina ash (Fraxinus caroliniana) is useful as an addition to bogs or water gardens because its native habitat is in swamps and bottomlands with wet soils. It grows to 30 feet tall in full sun or partial shade.
If you want a small, well-contained tree or large shrub with fine foliage, consider littleleaf ash (Fraxinus greggii), native from Arizona to Texas and Mexico. Growing 18 feet tall and 15 feet wide where it is hardy, USDA zones 7b through 10, the semi-evergreen plant can be pruned to have a central trunk. Use it as a patio tree or in a garden with limited space. A larger tree that gives reliable gold fall color, Arizona ash or velvet ash (Fraxinus velutina) inhabits canyons and foothills from California to Texas. Hardy in USDA zones 7 through 11, it reaches 40 feet tall and wide. It is used in residential and commercial plantings as a shade, lawn and street tree.
Native to the Pacific Northwest, Oregon ash (Fraxinus latifolia) is useful in streamside settings and areas with moist soil that mimic part of its native habitat. Under these conditions, it reaches 60 to 80 feet tall. It can grow quickly when young, a useful quality for canopy development in an ornamental shade tree. It can tolerate drier conditions, which lead to slower growth and a shorter tree. Oregon ash is hardy in USDA zones 6 through 8. Useful because of its tolerance to heat, sun, drought and alkaline soils, single leaf ash (Fraxinus anomala) also contributes brilliant yellow fall color. Hardy in USDA zones 5 through 7 from California to Wyoming and south to New Mexico, this ash differs from most other ashes because it has simple, undivided leaves.