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~ The Idaho Locust, Robinia x ambigua 'Idahoensis' ~

Robinia x ambigua 'Idahoensis' is believed to be a cross between the white-flowered black locust tree Robinia pseudoacacia and the pink-flowered bristly locust shrub Robinia hispida. Uncertain as its parentage may be, the tree certainly combines the tall habit of the black locust and the vivid coloring of the bristly locust.

This member of the pea family features bluish, fern-like, compound leaves made up of rounded oval leaflets. In late spring, when the foliage is still a fresh apple green color, wisteria-like clusters of blooms hang gracefully from the new branches. Their strong and sweet fragrance of honey and roses attracts insect pollinators in droves while the bright pink appeals to hummingbirds. Scarce, 4-7 cm knobby and leathery seeds pods persist on the trees through the winter, adding to interest.

The fast-growing tree has a semi-columnar habit, up to 15 m, and casts a light shade, which makes is ideal for planting in areas where grass or ornamental plants are to be grown beneath the canopy. These same qualities can be exploited when making a multiple-trunk planting.

Very tolerant of poor, dry and compacted soils once established, it is often used in xeriscaping. Extensively used as a tough street tree in the dry localities of California and the American SouthWest, the Idaho Locust has not made the inroads it deserves in the Canadian nursery trade, perhaps of customer resistance to novelty, or because its northermost hardiness is not fully determined. In my experience at least, it is completely hardy and very succesful in at least zone 5a. If you're looking for something spectacular and exotic that is extremely easy to grow, be the first one in your town to showcase this tree on your lawn.

May also be referred to as "Purple Robe Locust," there appears to be considerable confusion in the nursery trade regarding nomenclature.

Robinia pseudoacacia and its cultivars are exceedingly susceptible to the locust borer. For tips to control this inevitable and very destructive pest, consult this page on locust borer control.


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The Idaho Locust is reliably hardy to zone 5, where no die back was observed even following a hard winter in a very exposed location. Some sources report hardiness to USDA 3.

Site, Soil and Habit
The tree is very accomodating of soil types, is drought and pest resistant but will respond favorably to rich, well-drained, moist soil. Prefers a sunny location. Ideal where light shade is desired.

By cuttings. Mature speciments may send "suckers" that can be dug out for propagation purposes, or kept in place if a thicket is desired.

Idaho Locust Links

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Michigan State University - Brief outline.
Hortico - Canadian mail order supplier.
Richmond Nursery - Canadian supplier.

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