Much like the veins in our body, a tree's vascular system transports sap. The appearance of tree sap bleeding outside of the tree trunk, however, can help determine the health of a tree. It may not be an issue, depending on the type of tree or time of year you see it. Quite possibly a bleeding tree is in distress due to a disease. Or it could be that you pruned the tree too early or late to cause it to bleed sap. Learn when to be concerned about a tree bleeding sap and whether or not to stem the oozing.
Why Do Trees Bleed Sap?
Sap is a sugary liquid filled with water and nutrients that are carried throughout the tree via two types of tissue called phloem and xylem. Phloem carries nutrients from the leaves to other parts of the plant, while xylem carries nutrients upwards from the roots. When something (pruning or disease) interferes with the tree's system of tissue, sap can bleed.
Sap is produced in the leaves (or needles) of a tree and is distributed throughout the tree through the phloem, which runs vertically from top to bottom on the tree. If a cut is made in the trunk or a branch of a living tree, the cut severs some of the phloem, allowing the sap to ooze out.
The amount of sap in the tree varies based on the time of year. In some varieties, the sap levels are especially high in early spring. If you make pruning cuts at that time, the tree may bleed sap. This isn't usually too much of an issue, but it's best to avoid it so you can prevent problems like gummosis and tree decline.
When to Prune
There are some trees that you should wait to prune until later in the spring because they tend to bleed sap. The best way to control bleeding sap is to prune at the right time for that particular species of tree.
In general, you should prune deciduous trees in late winter and early spring (February, March, and April). Though they may bleed, the trees will not be hurt as the flow of sap will slow and finally stop. If pruning a deciduous tree results in bleeding, just leave it alone. Don't paint over it or cover the wound.
Protect Trees from Diseases
There is one important exception to the rule of leaving deciduous tree wounds uncovered and left to bleed. If you have an elm or oak tree that is bleeding, you will need to tend to it. These trees are highly susceptible to Dutch elm disease and oak wilt, respectively, and pruning paint may be a critical step to help save the tree and keep it healthy, according to the U.S. Forest Service. In a Dutch elm, for instance, painting the pruned location will prevent elm bark beetles from being attracted to the tree.
A tree may also bleed sap if it has bacterial wetwood (also called slime flux), which is a disease that rarely kills a tree, but can contribute to its decline. The disease causes sap to bleed through cracks in the bark. It's best to consult an arborist to confirm this problem, and the most you can likely do is alleviate the tree's stress with fertilization and proper pruning.
Trees That Tend to Bleed Sap
If you have any of the following trees, they will likely bleed sap when pruned. To avoid this, be sure to research the proper times to prune each tree before you make the first cut.
- Beech (Fagus)
- Birch (Betula)
- Elm (Ulmus)
- Grape (Vitis)
- Linden (Tilia)
- Maple (Acer)
- Mulberry (Morus)
- Poplar (Populus)
- Walnut (Juglans)
Tree sap and tree resin are often confused. Sap is a more watery, sugary substance. Resin is thicker and stickier. In fact, pine resin is often used by hikers in an emergency to treat wounds.
Making Use of Tree Sap
Bleeding sap isn't always detrimental, and sometimes we can even put it to good use. One of the best examples is when the sap is collected from maple trees each year, which is done by tapping the trunk of sugar maples. It can take up to 50 gallons of sap to make one gallon of maple syrup. The sap of birch trees also can be used to make syrup as well as homemade beverages, such as birch wine, birch mead, and birch beer (similar to root beer).