Sap flow from a tree sometimes referred to people as ‘Bleeding’ is most commonly caused by an injury or stress which can lead to insect feeding. This can cause sap flow but also attract insects to a flow. Additionally, trees under stress can be attacked by insects causing another source of injury.
Similar to people, when trees sustain and injury, they can bleed. So, let’s start with the obvious. Some species of trees will exude sap when pruned.
A good example is this Honeylocust. Not all the pruning cuts are bleeding but the one on the main truck was putting out sap. When I smelled what was dripping from the tree, it smelled sweet. Overall, the tree was healthy and seems to be fine. Deciding on when to prune trees can be species specific. For example, with Honeylocust, it is best to trim in late fall after the leaves have fallen or in the early spring, before the sap starts flowing.
This is just the opposite for Maples. Sugar Maples are tapped in late winter when their sap is flowing for maple syrup. If Sugar Maples are pruned in the winter or early spring, they are sure to bleed.
As for this Pin Oak, it was oozing from an old wound. When I smelled the sap, it had a fermented fragrance which attracted flies and ants. Pruning oaks in the summer can cause them to bleed. This can potentially attract picnic beetles which can carry Oak Wilt disease, that can quickly kill the oak. The vascular tissue becomes clogged, causing the leaves to turn brow and fall, even in the middle of summer.
More on Oak Wilt can be found in the following post: https://bygl.osu.edu/node/1668
So, the best time to prune Oaks, November 1 – March 31st to be sure the sap will not be flowing. See the attached article: https://bygl.osu.edu/node/2081
Pruning at the proper time can prevent bleeding, but trees can be damaged in other ways besides pruning injury.
In the case of this Sugar Maple, a bird (Sapsucker) drilled holes in the tree looking for insects. Since it was late winter, the tree released sap, as if it had been tapped for maple syrup.
Other insect can cause trees to ooze.
A Weeping Flowering Cherry shows signs of Peach Tree Borer infestation. This clear oozing gum is being release by the tree as a defense against the borer, but the borer is already present in the tree trunk.
A fruiting Peach Tree also shows evidence of Peach Tree Borer.
Some trees will bleed sap when damaged by Ambrosia Beetle. But this beetle tends to attack trees that are under stress. A healthy growing tree tends to be a less desirable target. Dogwood, Lilacs, and Redbud seem to be favored targets.
So “Why do Trees Bleed?” the short answer is a response to an injury, just like us!
Before you prune, do some research to determine if the timing is right!