Robinia pseudoacacia
black locust
The deeply rugged bark, with its wonderfully sheared ridges, recalls that of honey locust, but in April the tree takes on another character altogether, as though wisteria had climbed into its crown: cascades of slightly fragrant, white pea flowers hang from the boughs. The oval leaflets on the pinnate leaves are slightly notched at the tips, immediately distinguishing them from the pagoda tree’s pointed leaflets. At the base of the leaves, downy pairs of stipules begin as ½-inch structures and mature into stout, persistent spines – slightly longer with age, but never branching as in honey locust; they are especially evident on young trees and suckers.
The flat pods, only about 3 inches long, may have reminded early English colonists of the Mediterranean “locust” (the carob tree). The wood is hard, durable and remarkably resistant to rot: corner posts set directly into the ground by the first English settlers at Jamestown in Virginia, were found a century later to be “perfectly sound.”
Introduced to Europe in the early 1600s by Jean Robin – for whom the genus is named – the tree quickly became one of the most widely planted American trees there. An ancient, propped-up specimen, said to have been planted by Robin (in 1602, per a sign at its base), still stands in the Square René-Viviani in Paris, near Notre-Dame. The flower clusters are dipped in batter and deep-fried as beignets in France and as tempura in Japan.
A rugged specimen on Mayfield Avenue at Pearce Mitchell Place appears to have seeded across the road, where young plants, already armed and in flower, stand 6 feet tall. Others have sprung up down the road, in the park opposite 624 Mayfield Avenue. More than a dozen full-sized trees have arisen around the moist drainage area at the Frenchman’s Road entrance to the Dish. A large tree on Memorial Way near Frost Amphitheater was removed around 2025. An attractive example is at 1281 Stanford Avenue in Palo Alto; others are nearby.
Variety ‘Frisia’ has reddish orange new twig growth in spring with cheerful yellow foliage through summer. Two tall specimens at the north end of Clark Center on campus are all that remain of a dozen once planted in and around the inner courtyard.
Pink hybrids
Ornamental crosses of R. pseudoacacia with pink-flowering species of Robinia abound on campus.
Eight venerable Robinia × ambigua ‘Decaisneana’ line the Lower Row (Mayfield Avenue just north of Campus Drive), their heavy flower clusters a haunting creamy-rose, quite unlike any other floral spectacle. They represent a long-cultivated garden form of a cross between black locust and clammy locust (R. viscosa), from which they get their pink and their mildly sticky (clammy) young twigs. Similar trees are on Barnes Court. Another Robinia with pale pink flowers on the slope west of the stage at Frost Amphitheater has been lost. (Some black locusts remain behind the upper tiers on the east side.)
A formal planting of 46 R. × ambigua ‘Idahoensis’, with bright pink flowers, brackets the east end of the Science and Engineering Quad. This area was briefly known as Stone Pine Plaza (1999–2008) and once held 60 locusts. ‘Idahoensis’ was also seen in the area around Mirada Avenue and Frenchman’s Road.
Locusts sold under the trade name Purple Robe – reported to be a cross between ‘Decaisneana’ and R. hispida ‘Monument’ – largely replaced later-blooming ‘Idahoensis’ in the nursery trade and account for many of the more recently planted, deep pink–flowering specimens on campus. It bears no seedpods and is supposed to be spineless, in contrast with ‘Idahoensis’; petals shrivel to a luminous pale violet. Several young trees stand on Mayfield Avenue opposite Lasuen (572); others extend along Mayfield Avenue all the way to Alvarado Court. More occur on the North-South Mall near the Keck Science Building.
Deep pink–flowering Robinia of uncertain provenance march down the protected crosswalk that bisects the Tresidder parking lot.
About this Entry: Authored Apr 2026 by Sairus Patel.
