Northern catalpa is a medium-sized tree with a short trunk and several large, ascending branches and a narrow, rounded crown.
Leaves are simple, opposite or in threes, egg-shaped with an abruptly pointed tip, 6–12 inches long, margin mostly smooth; upper surface dark green, smooth; lower surface paler, hairy; mildly scented when crushed; turn yellow in fall.
Bark is thick, with irregular, short ridges and deep grooves, not scaly.
Twigs are stout, brittle, green to purplish, hairy; becoming light orange or brown and smooth with age.
Flowers May–June; in upright, pyramidal clusters 4–8 inches long; flowers bell-shaped, 2 inches long, white, showy, attractive, and fragrant; throat with yellow spots and dark lines; lower lobe with a notch in the center.
Fruits October, single or 2 or 3 together, a beanlike pod 8–20 inches long, ¼–½ inch wide, light brown, splitting into 2 halves; seeds numerous, flattened, about 1 inch long. Pods persist on branches all winter, opening in spring before falling.
Similar species: Southern catalpa (Catalpa bignonoiodes) is not native to our state but has been planted widely. Northern catalpa, however, has bigger flowers (1½–2 inches long); each flower’s lower lip is shallowly notched; the fruits are fairly thick-walled (the sides remain concave after the pod splits open); and the bark is reddish brown and divided into thick (not thin) scaly plates on older trunks.