The Purple robe locust, a.k.a. Idaho locust or Robinia x ambigua 'Idahoensis' is a hybrid locust with some very confusing parentage, hence "ambigua". Nonetheless, it is one of the most beautiful locusts available that combines the tall columnar form of the Black locust (Robinia pseudoacacia) and the brightly colored purple-pink flowers of the Bristly locust (Robinia hispida).
Beginning in late spring, with the beautiful ferny foliage a bright chartreuse, lovely wisteria-like flower clusters hang elegantly from the branch tips. In addition to this visual display there is also a sweet rosy perfume that attracts bees. The foliage, in mid-summer, takes on a bluish tone and then finally, with the cooler weather, fades to a pretty yellow. The foliage admits a dappled light, which keeps it from being a shade tree, but that very quality means it's wonderful for growing grass or ornamental plants beneath the canopy.
The form is also attractive with a vase-shaped to columnar habit, up to 40 feet in height, but only 20 feet wide, a spread that most gardens could accommodate.
In addition to its many aesthetic qualities it is a fast-growing tree that is very tolerant of summer heat and winter cold as well as poor, dry and compacted soils. It is an ideal choice for xeriscaping which is why it has been successful as a street tree in the dry southwest of the United States. But it has a vast range of adaptability growing from southern Ontario in Canada to the US- Mexican border. It can be bothered by borers but otherwise it is tough and easy to grow. It can also be grown in the UK and New Zealand and is very well-suited to Australia.

Another notable cultivar is Golden locust, Robinia pseudoacacia ‘Frisia’.

Purple Robe locust


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