Idaho Locust is a fast growing, upright, flowering tree in the pea family that can reach a mature height of 30 to 40 feet and an equal width. It has a vase-like habit and the foliage is light green in spring, gradually turning blueish in summer, and yellow in fall. Fragrant, bright red blossoms appear in spring time. The genus name honors Jean Robin, botanist to Louis XIII of France, and Vespasian Robin his son . The species name is Latin meaning ‘ambiguous', referring to its parentage. Idaho Locust prefers moist, well-drained soil. Propagate by hardwood cuttings or sow seeds in the fall. Once established, it is drought tolerant. This is a small to medium sized, low maintenance tree that tolerates compacted or poor soils and is useful as a street tree. Idaho Locust is believed to be a cross between Robinia pseudoacacia and Robinia hispida . Its upright habit is similar to Robinia pseudoacacia and the red flowers are similar to those of Robinia hispida . Insects, Diseases, and Other Plant Problems: Susceptible to aphids. Fusarium, armillaria, and root rot can also be problems.
Curated pairwise relationships between this plant and others in the library — who feeds
whom, who competes, who confuses the pests. Green reads as a good pairing, red as one to avoid.
No curated interactions recorded for this species yet.
Also noted by NC State
Related species the NC State Plant Toolbox flags for this plant — look-alikes to tell
apart, and plants that play a similar role in a planting. Ones we carry in the library are linked.
Full sun (6 or more hours of direct sunlight a day) | Partial Shade (Direct sunlight only part of the day, 2-6 hours)
Soil texture
Clay; Sand; Shallow Rocky
Soil pH
Acid (<6.0); Alkaline (>8.0); Neutral (6.0-8.0)
Soil drainage
Good Drainage | Moist | Occasionally Dry
Available space
12-24 feet; 24-60 feet
NC region
Coastal; Mountains; Piedmont
Mature size
Height: 20 ft. 0 in. - 30 ft. 0 in. | Width: 20 ft. 0 in. - 30 ft. 0 in.
Growth rate
Rapid
Maintenance
Low
Canopy layer
2 - Understory (15-40 ft)
Form & Appearance
Habit / form
Columnar; Erect; Open; Oval; Vase
Texture
Medium
Foliage
Deciduous
Leaf color
Blue; Green
Leaf type
Compound (Pinnately , Bipinnately, Palmately)
Leaf value
Showy
Leaf size
< 1 inch long, < 1 inch wide
Hairs present
No
Leaf detail
Green leaves become blue green during the summer and yellow in the fall. Individual leaflets are 3/4 of an inch long and 1/2 an inch broad.
Fall color
Gold/Yellow
Flower color
Pink; Purple/Lavender
Flower value
Fragrant; Showy
Flower form
Raceme
Flower size
1-3 inches
Flower detail
Fragrant, pea-like, purple/ pink 2 inch flowers appear in clusters on pendant racemes. Both female and male parts are in the same flower (that is, perfect).