White ash is a medium to large tree with a straight, tall trunk and narrow, rounded or pyramidal crown.
Leaves are opposite, feather-compound, 8–12 inches long, with 5–9 (usually 7) leaflets; leaflets broadest near the base or middle, 3–5 inches long, 1½–3 inches wide, margin often with rounded teeth, tip pointed, upper surface dark green, dull to somewhat shiny; lower surface paler, whitish, smooth; leaf stalk smooth.
Bark is light gray to dark brown, grooves deep, with narrow, interlacing ridges that are flat-topped, forming diamond or X-shaped patterns.
Twigs are stout, rigid, brittle, green to brown, or gray, smooth; pores pale; bud at tip about ¼ inch long.
Flowers April–May, with male and female flowers in clusters on different trees. Male flowers small, green to red, with no petals; female flowers similar to male flowers.
Fruit matures in August–September, in dense clusters up to 8 inches long; fruit is a samara, with the wing partially around the seed; yellowish-brown, 1–2 inches long, smooth, flat.
Similar species: Missouri has 6 native ash (Fraxinus) species. In addition to white ash, there is Biltmore ash (F. biltmoreana), green ash (F. pennsylvanica), pumpkin ash (F. profunda), blue ash (F. quadrangulata), and Sullivan's ash (F. smallii). Biltmore and Sullivan's ash have sometimes been considered subspecies of white ash. A Eurasian species, flowering or manna ash (F. ornus), is sometimes cultivated as a landscaping tree. All will likely be eradicated by the invasive emerald ash borer, a wood-boring beetle that kills ash trees before they can reproduce.