Fraxinus

Ash
Oleaceae
Deciduous, Trees

Fairly fast-growing trees, most of which tolerate hot summers, cold winters, and many kinds of soil, including alkaline sorts. Chiefly used as street, shade, lawn, and patio shelter trees. In most cases, leaves are divided into leaflets. Male and female flowers (generally inconspicuous, in clusters) grow on separate trees in some species, on the same tree in ­others. In the latter case, flowers are often followed by clusters of single-seeded, winged fruit, often in such abundance that they can be a litter problem. When flowers are on separate trees, you’ll get fruit on female tree only if a male tree grows nearby.

Ash trees are prone to borers. In some parts of California, ash whitefly is a problem; these chalky white, 1/8-in.-long insects colonize in patches on leaf undersides. Outbreaks are usually controlled by natural enemies; avoid spraying with broad-spectrum insecticides, which are likely to wipe out these beneficial predators.

Fraxinus americana

Native to eastern U.S. Grows to 80 ft. or taller; straight trunk and oval-shaped crown to 50 ft. wide. Leaves up to 15 in. long have five to nine dark green, ovalleaflets, paler beneath; turn purplish in fall. Leaf edges burn in hot, windy areas. Regular water.

Male and female flowers are on separate trees, but plants sold are generally seedlings, so you don’t know what you’re getting. If you end up with both male and female trees, you will get a heavy crop of seed; both litter and seedlings can be a problem. Seedless selections include ‘Autumn Applause’, ‘Autumn Purple’, and ‘Royal Purple’, all with exceptionally good, long-lasting purple fall color; ‘Rosehill’, with bronzy red fall color; and ‘Skyline’, an upright, somewhat narrow oval with brown and purple fall color.

Fraxinus greggii

Native from Arizona to Texas. Shrubby tree to 18–20 ft. tall and 10–15 ft. wide, with bright green leaves divided into three to seven leathery leaflets. Useful in desert; good patio tree. Survives on little water but grows faster with moderate irrigation.

Fraxinus latifolia

Native to Sierra Nevada and west of the Cascades from Northern California to Bri­tish Columbia. To 40 to 80 ft. tall, 30 to 50 ft. wide. Leaves 6 to 12 in. long, divided into five to seven oblong to oval, light green, hairy or smooth leaflets; end leaflet to 4 in. long, larger than side leaflets. Male and female flowers on separate trees. Will grow in standing water during winter months. Needs no dry-season irrigation. Subject to many pests and diseases; not a first-rate tree.

Fraxinus pennsylvanica

Native to eastern U.S. Grows 30 to 40 ft. tall and wide, with a compact, oval crown. Gray-brown bark; dense, twiggy structure. Leaves 10 to 12 in. long, divided into five to nine bright green, rather ­narrow, 4- to 6-in.-long leaflets. Male and female flowers on separate trees. Takes wet soil and severe cold, but foliage burns in hot, dry winds.

Fraxinus uhdei

Native to Mexico and a favorite in South­ern California and low-elevation deserts. Grows fast to 25–30 ft. tall in 10 years; reaches 40 ft. in 20 years and may eventually attain 70–80 ft. or taller. Upright, narrow tree about 15 ft. wide when young; eventually takes on a spreading form as it grows older (may reach 60 ft. wide at maturity). Leaves divided into five to nine glossy dark green, finely tooth-edged leaflets about 4 in. long. Foliage may burn if subjected to hot winds.

In mildest climates, leaves hold on through winter; in colder areas, tree loses most or all of its foliage, but leafless period often lasts only for a short time. Sharp frosts may kill branch tips; tree will suffer serious damage at 15°F/ –9°C or lower.

Fraxinus velutina

Native to southwestern U.S. and Mexico. Withstands hot, dry conditions; cold hardy to about -10°F/-23°C. Grows about 30 ft. (possibly to 50 ft.) tall. Pyramidal when young; spreading to 30 to 40 ft. wide when mature, with more open form. Leaves divided into three to five narrow to oval, 3-in.-long leaflets; turn bright yellow in fall. Male and female flowers on separate trees.

Fraxinus velutina coriacea

Native mostly to Southern California. Broader, more leathery leaves than the species.