Tree sap is a watery, nutrient-rich fluid that circulates within the vascular system of trees, supporting all metabolic processes from root growth to leaf development. Seeing this normally internal substance oozing or dripping down the trunk or branches is often the first sign that something has breached the tree’s protective layers. The leakage is essentially the tree’s internal transport system spilling out, which can be a temporary, benign event or a symptom of a significant underlying health issue. The specific cause of the leakage determines the severity of the problem and the appropriate response.
What Sap Does Inside the Tree
The movement of sap is controlled by two distinct vascular systems, which function much like a plant’s internal plumbing to move different materials where they are needed. One system, the xylem, is primarily responsible for transporting water and dissolved minerals from the roots up to the leaves and canopy. This flow is largely one-directional and contains the watery sap that is most noticeable in early spring when deciduous trees are actively drawing moisture from the ground.
The second system, the phloem, acts as the tree’s food distribution network, moving sugars produced during photosynthesis from the leaves to all other areas of the tree for energy or storage. Phloem sap is thicker and rich in sucrose, moving in a multi-directional flow to supply energy to growing tips, fruits, and roots. A breach in either of these transport systems can lead to a fluid leak, with the composition of the leaked sap providing clues about the damaged internal structure.
Leakage from Physical Damage
The most common reasons for sap leakage are mechanical injuries that break through the protective outer bark and cambium layers. A fresh cut from pruning, scrape marks from a lawnmower, or a limb torn off by a windstorm all create an open wound. The resulting sap flow, often called “bleeding,” is the tree’s immediate, localized response to flush the wound and inhibit the entry of microbes or insects.
For some species like maples, birches, and walnuts, this leakage is particularly heavy during the late winter and early spring when the tree is transitioning out of dormancy. The high internal pressure from the flow of watery xylem sap causes it to seep readily from any opening, a phenomenon utilized for maple syrup harvesting. While cosmetically unappealing, sap leakage from a clean physical wound is generally not detrimental to the tree’s overall health, and the wound will slowly seal itself with new tissue formation. The use of wound dressings or tree paint is not recommended, as it can trap moisture and impede the tree’s natural healing process.
Leakage Caused by Pests and Disease
Sap leakage can serve as a visible symptom of a serious internal problem caused by biological invaders. Wood-boring insects, such as certain beetles or moths in their larval stage, tunnel beneath the bark to feed on the water and nutrient-conducting tissues. The tree responds to this invasion by pushing sap out of the insect’s entry or exit holes in an attempt to drown or flush out the pest. Observing the leakage can reveal other signs, such as a mixture of sap and fine sawdust-like material called frass, which is the waste product of the tunneling insect.
Fungal or bacterial infections can cause localized lesions on the bark known as cankers, which disrupt the vascular flow and cause sap to ooze out. These cankers appear as sunken, discolored, or cracked areas on the trunk or branches, indicating the pathogen is actively killing the tree’s living tissue. The sap may exhibit a specific color; for example, a dark brown or reddish-black sap can indicate certain fungal diseases like Phytophthora. Certain fruit trees, like plums and cherries, often exhibit a condition called gummosis, where a thick, amber-colored gum leaks from the bark in response to infection or stress, signifying an attempt to seal off the damaged area.
Understanding Sap and Resin
A common source of confusion is the distinction between true tree sap and the substance known as resin or pitch, especially when observed on conifers. True sap is the metabolic fluid, composed primarily of water, sugars, and minerals, that moves through the vascular tissues. It is generally clear, watery, or slightly milky in appearance and is the fluid involved in transport and growth.
Resin, conversely, is a thick, sticky, hydrocarbon-based substance produced by specialized cells, primarily in pine, fir, and spruce trees. It is not part of the tree’s transport system but functions as a defense mechanism. When a conifer is wounded, the resin flows out to seal the injury, hardening into a protective, antimicrobial barrier that traps and deters bark-boring insects and fungal spores. Resin is noticeably more viscous, stickier, and often has a strong pine scent, making its composition and defensive purpose distinct from sap.