Flagging In Trees – What Causes Tree Branch Flagging

tree flagging
tree flagging
(Image credit: Robert L. Anderson, USDA Forest Service, Bugwood.org)

Tree branch flagging is not a pretty sight. What is branch flagging? It’s a condition when tree branches scattered throughout the tree’s crown turn brown and die. Various pests can cause flagging. If you want more information about tree branch flagging, including different causes of flagging damage to trees, read on.

What is Branch Flagging?

The condition called tree branch flagging occurs when a tree’s branches turn brown, wilt, or die. Usually, the branches are not all grouped together. Rather, you may see them scattered around the tree’s crown. Flagging in trees can be due to cicada insects. The females use a sharp appendage on their abdomens to break open the bark of small, new tree branches to deposit eggs. The damaged young branches can then break off in the wind and fall to the ground. Although cicada-caused flagging in trees can drop large amounts of tree litter in your backyard, the tree branch flagging won’t kill vigorous specimens. Healthy branches will recover and keep on growing. If you want to treat cicada-caused flagging damage to trees, prune out the affected branches. Do this when the tree is dormant and burn the detritus.

Flagging Damage to Trees from Other Causes

Cicadas are not the only causes of tree branch flagging. Flagging in trees, like oaks, can also result from Kermes scales, sap-feeding insects that damage many kinds of oak. Tan or brown, these scale bugs look like small globes attached to twigs. Treat with appropriate insecticides. Flagging damage to trees can also be caused by twig girdlers and twig pruners. These are both types of beetles that attack oak, hickory, and other hardwood trees. You can limit flagging damage to trees from these beetles by raking up all fallen twigs and branches and burning them. Another cause of flagging in trees is botryosphaeria canker, caused by a fungus. Botryosphaeria canker generally affects oak twigs, bending the leaves inward toward the twig. Usually, the leaves stay on the twig, but they turn brown. This cause of flagging in trees is not serious and requires no treatment. Thousand cankers disease is another invasive pest that damages black walnut. This is a more serious condition and may require special treatment. Take a sample of the flagging to your garden store and ask them for suggestions.

Teo Spengler

Teo Spengler has been gardening for 30 years. She is a docent at the San Francisco Botanical Garden. Her passion is trees, 250 of which she has planted on her land in France.