Green ash (Fraxinus pennsylvanica, Marshall 1785)

Identity

Green ash (Fraxinus pennsylvanica Marshall, 1785) is an accepted species of Fraxinus in the olive family, Oleaceae. American botanist Humphry Marshall described it in 1785 from Pennsylvania specimens, the place commemorated in its scientific name. ^{1 4}

Botanists formerly separated two forms: one with hairless twigs and glossy leaflets, called green ash, and another with hairy twigs, called red ash. That distinction involved Fraxinus pennsylvanica var. subintegerrima, now treated as a synonym of the species. Both English names remain in use for the same plant. ^{5 3}

Among the six North American ash species compared here, Fraxinus pennsylvanica has the broadest continental distribution. Emerald ash borer, an insect introduced from Asia, nevertheless threatens its survival throughout that native range. ^{6 7}

Recognition

Typically reaching 15–21 m (49–69 ft) tall and 11–15 m (36–49 ft) wide, the tree begins with a pyramidal crown that becomes rounded and irregular with age. The largest recorded specimen, a forest tree in Missouri, stands 32 m (105 ft) tall with a trunk 1.34 m (4.4 ft) in diameter, nearly half as tall again as the usual maximum. Grayish-brown bark develops intersecting diamond-shaped ridges early; European ash stays smooth longer. ^{2 3}

Compared with European ash, two features aid identification throughout the year: chestnut-brown buds, never black, and hairy current-season twigs, which become hairless with age. The 5–9 leaflets have short individual stalks, unlike the stalkless leaflets of European ash. Semicircular leaf scars have straight upper edges, with the bud immediately above. ⁶

Petal-free, greenish to purplish flowers open in April–May in the native Northern Hemisphere range, with or just before the leaves. Male and female flowers occur on separate trees. The winged fruits, or samaras, measure 4–5 cm (1.6–2 in), turning from green to light brown and often notched at the wing tip. Hanging clusters persist through winter on female trees. ^{3 2}

Useful taxonomy

Taxonomic references accept Fraxinus pennsylvanica Marshall in the Oleaceae. Synonyms reflect former morphological divisions: Fraxinus lanceolata Borkh., Fraxinus pennsylvanica var. subintegerrima (Vahl) Fernald and Fraxinus pennsylvanica f. viridis C.K.Schneid.. The last name recalls the common name green ash. ¹

Forestry identification keys compare several other North American ashes: Fraxinus americana, with entire leaflet margins and crescent-shaped leaf scars; Fraxinus nigra, with brown-black buds and rusty veins; and Fraxinus latifolia, with broader leaflets and rounded samaras. Scientific names are especially useful in France, where these introduced species are absent from traditional floras. ⁶

Habitat and range

From Nova Scotia, green ash extends west to southeastern Alberta and eastern Colorado, south to northern Florida, and southwest to Oklahoma and eastern Texas. This covers much of eastern and central North America. It has also naturalized across much of the western United States, Argentina and Europe, from Spain to Russia. ¹

French introductions for forestry or ornament date to the late nineteenth or early twentieth century, probably during the German annexation of Alsace, when foresters tested exotic trees in Rhine forests. Long confused with white ash or narrow-leaved ash, green ash was positively identified in 2003, at Schoenau and then Daubensand, using a stump with more than 90 growth rings. It regenerates naturally there but is outcompeted for light by European ash. The discovery’s authors considered an invasive designation disproportionate. ^{6 4}

Within its native range, green ash grows almost exclusively in alluvial bottomlands, along streams and on periodically flooded ground. Established trees tolerate both temporary waterlogging and pronounced dry spells. ³

Life cycle and reproduction

As a dioecious species, each tree bears only male or only female flowers in small panicles, before or during leaf emergence. Wind carries the pollen. On female trees, samaras ripen from August to October in North America and disperse throughout winter, carried by wind and, beside rivers, water. ³

Seeds germinate after natural winter stratification on the ground. Once established in full light and moist soil, young trees grow rapidly with a pyramidal outline. Beneath a dense canopy, limited light sharply slows growth; trees lean and are crowded out by more competitive species. ^{2 6}

Wildlife uses many parts of this tree. Seeds feed birds and small mammals, while rabbits, porcupines and beavers eat the bark. Its foliage supports caterpillars of Papilio glaucus, among other moths and butterflies. ³

Uses and risks

Hard, elastic wood, comparable to European ash, has long supplied furniture, tool handles and sporting goods. North American cities planted green ash extensively for shade and street trees after Dutch elm disease devastated American elms in the mid-twentieth century. This second urban monoculture repeated the earlier mistake: mass planting of one tree species leaves an entire city exposed to the same insect or fungus. ⁹

Ingestion toxicity is not reported for green ash in common domestic animals or humans, unlike many other ornamental street trees. ³

Tree health is the overriding concern. Emerald ash borer (Agrilus planipennis), an Asian beetle accidentally introduced to North America in the late 1990s, kills trees within three to five years of infestation, with mortality approaching 100% and almost no natural regeneration afterward. In 2017, the IUCN listed green ash as Critically Endangered, alongside four other North American ashes out of six assessed. It projected a decline of at least 80% over a century from a population near nine billion trees. New planting is no longer recommended in affected areas. ^{7 8 2}

Varieties, look-alikes and related plants

Before emerald ash borer became widespread, North American nurseries offered urban selections including 'Marshall's Seedless', fruitless to reduce self-seeding; 'Summit', with a regular pyramidal crown; 'Patmore', a dense-crowned male selection; and 'Cimmaron', valued for orange to purple fall color. These cultivars retain the species’ botanical identity and are now rarely recommended for new planting. ²

European look-alikes include European ash (Fraxinus excelsior), with distinctly black buds and stalkless leaflets, and Fraxinus angustifolia subsp. oxycarpa, with small brown buds and narrow, sharply toothed leaflets. In North America, white ash (Fraxinus americana) is the closest comparison. Its leaflets have entire margins, and crescent-shaped leaf scars surround the bud rather than sitting below it. ⁶

Main sources

• Plants of the World Online / Kew, Fraxinus pennsylvanica Marshall
• Missouri Botanical Garden Plant Finder, Fraxinus pennsylvanica
• NC State Extension Gardener Plant Toolbox, Fraxinus pennsylvanica
• INPN / MNHN, Fraxinus pennsylvanica Marshall, 1785
• Tela Botanica / eFlore BDTFX, Fraxinus pennsylvanica
• Boeuf R., 2003, An overlooked exotic ash in Alsace, Bulletin SBCO 34
• IUCN Red List, Fraxinus pennsylvanica assessment 2017
• IUCN, Once-abundant ash tree and antelope species face extinction
• University of Missouri Extension, Green Horizons, ash as elm replacement