Black locust (Robinia pseudoacacia) is a fast-growing tree that quickly reaches a height of 65 feet. In many areas of the United States, it is considered invasive. Black locust is hardy in U.S. Department of Agriculture plant hardiness zones 4 to 8 where it will grow in nearly any type of soil. This tree is susceptible to attack from beetle borers and two diseases that will cause the branches to die. These issues rarely kill black locust trees, but steps should be taken to minimize the damage and help the tree recover.
Fall Colors Any Time of Year
Canker disease will cause foliage on affected branches to wilt and turn yellow or brown. Bark on the infected branches usually develops odd coloration and may ooze resin. Fungal growths may also appear on the branch and foliage. The cankers may girdle the branch, killing it and eventually the whole tree. Infected branches should be pruned off. Make the pruning cuts just above the branch collar. Disinfect the loppers or pruning saw with household disinfectant between cuts to avoid spreading the disease. If the trunk becomes infected, the tree will not survive and should be removed. Provide proper care to keep the tree as healthy as possible and better able to withstand the disease. Water the tree every 10 days when it does not rain. Spray the water beneath the entire tree canopy, giving it 6 gallons of water per 10 square feet. Give the tree 10-10-10 fertilizer in early spring each year. Measure the diameter of the tree trunk 4 feet up and spread 1 pound of fertilizer per inch of trunk diameter over the soil beneath the tree canopy.
Wood rots cause the wood within an infected branch or trunk to rot, which will eventually kill the branch. The rotted area becomes soft, moist and spongy and turns yellow or white. Large fungal growths may also form on the rotted wood. The fungal spores enter the wood through wounds in the bark. The infected branches should be cut off and the trunk inspected closely for signs of rot. Water the tree every 10 days and give it fertilizer once each year to keep it growing vigorously.
Don’t Wave That Flag
Bark beetles and roundheaded borers may bore into black locust tree branches. Bark beetles can cause the outer tips of branches to die, which is a condition called “flagging.” They mine into the branches up to 6 inches from the tip, causing the end of the branch to die. The beetles may also attack the trunk of the tree. Frass, which resembles sawdust, and sap may be found around the small entry holes in the bark. Adult beetles and damage to the inner bark may be found under the bark when it is removed. Branches infested with beetles should be pruned off and burned immediately to keep the beetles from infesting other trees. Roundheaded or longhorned borers commonly attack diseased or damaged trees that are already dying. The leaves on branches that have been attacked wilt and the branch dies. Holes can be seen in the bark as well as dark oozing sap. They may also attack the trunk of the tree. The dying branches should be removed and burned. If they have weakened the trunk, the tree may need to be removed.
That’s Not a Pretty Caterpillar
Carpenterworm larvae mine into the wood of large branches, creating tunnels and eventually killing the branch. Spots of dark sap are the first sign of carpenterworm infestation, accompanied by frass and sawdust at the point of entry. The galleries eventually reach a diameter of 1/2 inch and length of 6 to 10 inches. Infested branches should be pruned off. Trees kept in good health by watering during dry periods and fertilizing in early spring should survive a light infestation. If the infestation is severe, however, and the trunk has been weakened, it should be inspected and removed if there is danger of the tree being blown over.
- Cal Poly: Urban Forest Ecosystems Institute: Black Locust: Robinia Pseudoacacia
- University of California Agriculture & Natural Resources: UC IPM Online: Pests in Gardens and Landscapes: Locust, Black Locust -- Robinia spp.*: Family Fabaceae (Pea Family)
- University of California Agriculture & Natural Resources: UC IPM Online: Pests in Gardens and Landscapes: Canker Diseases
- Washington State University: Puyallup Research and Extension Center: The Myth of Cloroxed Clippers
- Invasive Plant Atlas of the United States: Black Locust
- North Carolina State University: Watering Shrubs
- North Carolina State University: A Gardener’s Guide to Fertilizing Trees and Shrubs
- University of California Agriculture & Natural Resources: UC IPM Online: Pests in Gardens and Landscapes: Wood Decay Fungi in Landscape Trees
- University of California Agriculture & Natural Resources: UC IPM Online: Pests in Gardens and Landscapes: Bark Beetles
- University of California Agriculture & Natural Resources: UC IPM Online: Pests in Gardens and Landscapes: Roundheaded (Longhorned) Borers