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Why Do Trees Produce Sap?

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Trees add beauty to the landscape and especially to your property when well-placed and well-cared-for. In fact, we’re willing to put up with some of the inconvenient aspects of trees to enjoy their beauty—like raking leaves, cleaning up fallen branches, and dealing with sticky sap that gets on hands or clothing after close encounters with trees.

Sap is a vital component of a tree’s health. Sap is a tree’s lifeblood, so to speak. It carries water enriched with nutrients and hormones throughout the tree, from roots to leaves. The flow of sap spurs a tree’s growth, sending energy along the branches to the tips, where new buds produce needles or unfurl into leaves. Sap feeds the layers of the tree, expanding the girth of the trunk and branches.

Certain species of trees naturally produce more sap than others, like the sugar maple, for example, well-known for the distinctly flavored maple syrup made from its sap. Other trees that produce edible sap include species of walnut and birch. Though, the sap from these trees has a much lower sugar content than the maple. Birch sap can be consumed as a hot tea rich in nutrients and antioxidants.

What Is Tree Sap?

Tree sap contains two separate substances, known as xylem and phloem. The xylem cells are responsible for transporting the water, minerals, and hormones throughout the tree system. Xylem channels die-off each year and new growth is produced. This is how the tree grows—when you see the rings of a felled tree, you’re seeing the annual growth of the xylem layers.

Phloem is the familiar sticky sap we come into contact with outside the tree. It’s made up of the sugars created when photosynthesis takes place and is fed to the tree as needed during active growth periods.

How Does Sap Flow?vernon-imel-why-do-trees-produce-sap (1)

Tree sap flows through the sapwood layer via living xylem cells. The process produces carbon dioxide, causing pressure to build up in the tree. If there are any wounds or openings in the bark, broken or pruned branches, or areas of removed bark, the pressure causes sap to ooze from the tree.

Temperature also causes similar pressure within the tree system. When temperatures drop below freezing, trees pull water up through the roots and replenish the tree sap. This is why sugar maple trees are tapped in the spring when night temperatures drop and the sun then warms the tree during the day, causing the sap to flow freely.

Why Do Trees Produce Sap?

Clearly, sap is important to the growth of trees. All tress produce sap, some more than others. However, healthy trees don’t spontaneously ooze sap. Maple trees have to be tapped with a spile—which looks like a spigot—in order to collect the sugary liquid in buckets. Tapping a tree is essentially causing an injury to the tree. The tree sends sap flowing to the injury for the purpose of sealing over the wound.

Often, the appearance of tree sap on the outside of a tree is a clear indication of injury or something else amiss, whether a pest infestation or disease. Any unusual or unexpected oozing and blistering of sap is a signal that your tree made need attention and treatment. Vernon Imel Tree Service can help you assess any potential problems and keep your trees in the best health possible.


One common pest that damages trees and forces the tree to produce sap as a protective measure is the bark beetle. These wood-boring insects burrow into the tree to lay eggs beneath the outer bark layer. When they hatch, the larvae start to carve deep burrows into the xylem, feeding on the nutrient-rich cells. The tree’s natural defense mechanism fills the holes made by the adult beetles with sticky sap in an effort to prevent the insect from laying eggs and possibly even trap the offending beetle.

If you notice a gummy-like sap oozing at the top of dying bark and sawdust at the base of the tree, it’s time to check for borers. Trees showing evidence of beetle boring usually require chemical treatments to save the tree.


A bacterial canker infection is another reason why trees produce sap in noticeable amounts. These cankers afflict trees that have been previously injured by allowing the bacteria Pseudomonas syringae to penetrate the tree through openings. The bacteria causes unusually high pressure in the tree, forcing fermented sap to flow out from cracks or other openings.

Sunken, watery, or gummy lesions on the trunk or branches are a sign of bacterial canker. You may also notice wilt or dieback on the branches. Cankers occur most often during cool weather and are spread by rain, watering, and pruning tools, so be sure to sanitize your tools before you prune each tree.

Trees suffering from root rot fungus may produce similar cankers, causing the tree to ooze a deep-red to black-colored sap. Root rot is caused by soil that is too wet. Avoid overwatering, create adequate drainage, and pull back on soaking the soil to allow the moisture to evaporate.

Slime flux is another bacterial infection, which produces a sour-smelling, slimy-looking sap. Sap produced from slime flux is usually gray, has a foamy appearance, and darkens as it dries, possibly staining the tree bark. This sap is toxic to the bark and eats into the subsequent layers of the tree. It also attracts insects like flies, ants, and maggots. Disinfecting the wound with alcohol or a very weak household bleach solution can stem the infection.

The occurrence of oozing sap can cause secondary threats when it attracts insects such as mealybugs and aphids. These pests feed on the substance and, in turn, produce their own sticky substance known as honeydew, which increases sooty mildew.

If you’re concerned about the appearance of sap on your trees, contact one of our professional arborists at Vernon Imel Tree Service for a consultation.