Locust Tree With Purple Blooms

Robinia Pseudoacacia

• The Robinia pseudoacacia, or black locust, species bears white pendulous flowers. However, at least one cultivar bears purple flowers. "Purple Robe" is a tree with a round canopy that grows to about 40 feet tall, and it bears bronzy-red foliage in spring. It blooms slightly earlier in spring than the species, and the flowers are deep pinkish-purple. It is also known as "Pink Cascade," "Rouge Cascade" and "Casque Rouge." Black locusts are hardy in U.S. Department of Agriculture plant hardiness zones 3 through 8. The straight species, Robinia pseusdoacacia, is invasive in some areas.

Robinia Neomexicana

• Robinia neomexicana, or New Mexico locust, is a species with few or no named cultivars. It's a small thorny tree reaching 15 to 25 feet tall under ideal conditions. It tolerates alkaline soils and tends to grow at high elevations. It is rhizomaceous and will form thickets if new growth from roots is not trimmed off. For this reason it's often considered a great plant for erosion control. New Mexico locust bears white to lavender drooping flowers that emit a light fragrance. They grow well in USDA zones 4 through 9.

Robinia Hispida

• Robinia hispida is called bristly locust because the branches and petioles are covered with a reddish-brown, bristly pubescence. It's also known as rose acacia or rose locust. It's a shrub to small tree reaching only 10 feet tall, and it doesn't set fruit. The pale pink to deep lavender flower clusters are 2 to 4 inches long, and it's hardy in USDA zones 5 through 8. In some areas of the U.S. this tree is considered invasive.

Robinia x Ambigua

• Robinia x ambigua is thought to be a cross between Robinia pseudoacacia and Robinia hispida. The straight species bears pale lavender flowers, but the cultivar "Idaho" (sometimes called "Idahoensis") bears vivid pinkish-purple fragrant flowers. Many nurseries consider this tree the same as Robinia pseudoacacia "Purple Robe," and there remains much confusion throughout the trade. It's hardy in USDA zones 3 through 8.

Robinia x Slavinii

• Robinia x slavinii is also known as Robinia x margaretta and is a cross between a type of Robinia hispida and Robinia pseudoacacia. The straight species bears pale lavender flowers, as does the variety "Hillieri," whose lavender blooms are lightly scented. This tree reaches 15 to 30 feet in height, flowers heavily and is not resistant to windy locations. However, it tolerates wet soils and is drought-resistant. It's hardy in UDDA zones 5 through 9.