Tree of the Week: Purple Robe Locust

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The Purple Robe Locust –

Robinia x ambigua

Plant breeders keep hoping for a big success. The tough Black Locust, Robinia pseudoacacia, has been crossed several times with the shrub-like Clammy Locust, Robinia viscosa. The tall and graceful Black Locust, an eastern U.S. native, tolerates climate extremes and neglect; has annoying thorns and surface roots; is often planted around farmlands; and has become naturalized in some California locales. The Pink Locust, also an Easterner, is better behaved and smaller, with more desirable flowers. The hope was that the resulting ambigua hybrid species would combine the best characteristics of each parent, and yield mild-mannered cultivars with beautiful flowers. Opinions vary as to the qualities of the Descainea, Idaho and Purple Robe Locusts offspring. Sometimes even the Purple Robe parentage is suspect: Some people consider it only one of many cultivars (selections) of the Black Locust itself or a hybrid of other species.

The deciduous Purple Robe Locust grows moderately fast to a fairly open, attractive, rounded 40-foot-tall by 30-foot-wide tree. Brown, rope-like ridges with intervening furrows define the bark. Branches and bark may carry thorns. Wood is rot resistant, but branches are brittle; structural pruning in youth helps. Nitrogen-fixing bacteria inhabit the aggressive roots; the trunk has a broad, fluted base. Dark green, 10-inch-long compound leaves are pinnate feathered, with as many as 20 ovate leaflets. Showy, fragrant, deep pink to purple pendulous flowers resemble those of Wisteria and yield excellent honey. Flat, purple-brown seed pods, to 4 inches long, follow the flowers. The Purple Robe Locust likes full sun or dappled shade and will take most any soil, drought or lawn watering. The tree is susceptible to several pests and diseases, including borers and leaf miners. Still its faults are considered minor enough that several communities use it as a street tree.

Locust trees are named Robinia pseudoacacia after Jean and Vespasian Robin, French 16th and 17th century botanists.

-- Pieter Severynen

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