David C. Zlesak, University of Minnesota Extension Educator
Rose acacia (Robinia hispida) is in full flower across Minnesota. It is a plant many people are unfamiliar with and are asking what it is. The abundant pendulous clusters of rosey pink, pea-shaped flowers makes it especially showy this time of year. Although it is native to the Southeastern United States and is often listed as hardy to zones 5 or 6, forms of it are perfectly adapted across Minnesota and do not suffer dieback. Although not readily for sale in the garden centers, once planted (typically shared among friends) it tends to persist. The amount of it therefore continues to increase across Minnesota landscapes.
Photo 1: Rose acacia flowers are showy and produced over an extended period. David Zlesak
It is an easy to grow plant because of its tolerance to poor soil and drought. Often it is seen growing on old farmsteads or along the roadside. The plant habit is somewhat open and airy because of coarse stems and its compound leaves. It typically reaches 8-10 feet in height. It suckers and spreads over time into thickets, which makes appropriate placement very important. Young shoots can easily be pulled if one is persistent, but if cultivated it is best to place it in a confined area or area where it can be permitted to spread. Stands of it in Minnesota typically do not set viable seed. Because of its vigor, it is recommended not to let it escape cultivation.
Photo 2: Rose acacia can grow into an open, spreading thicket. David Zlesak
Other common names for rose acacia include hairy or bristly locust. This is because the stems are covered with soft bristles. It can set multiple clusters of flowers on each stem resulting in an extended bloom time.
Photo 3: This locust growing in Excelsior, MN is likely a hybrid of black locust and rose acacia. David Zlesak
Rose acacia is a species of locust and is closely related to black locust (Robinia psuedoacacia). Black locust is a full-sized tree that has clusters of white, pea shaped flowers. Crosses have been made between the two species and we can periodically find the hybrids for sale in garden centers such as 'Casque Rouge' and ‘Purple Crown’. The hybrids typically grow into smaller trees than black locust and have flowers that are soft to medium pink. The hybrids have typically been selected to have limited suckering. Both locust species and the hybrids can sucker, especially when the roots have been disturbed. Because of this they tend to be used sparingly in typical landscapes.