The season is turning toward spring, with warmer days outnumbering colder ones. Sap has begun to flow, swelling the buds of poplars and turning willow stems deep pink and orange. My friend Roni recently asked me about why her amur maples were “leaking.” They had been pruned last fall and because they hadn’t yet developed the calluses that close pruning wounds, they were weeping. Some trees are more prone to excessive outflow of sap than others. Maples, birches, beeches, poplars, elms and willows are known to be weepy. All are best pruned in late summer, rather than in the spring to avoid as much potential leakage as possible. Sap loss doesn’t usually damage or kill a healthy tree if it has been well-irrigated throughout the year and if there is soil moisture available to replenish what is being lost. However, drought-stressed trees could be weakened. One good thing is that sap doesn’t leak for long. In some instances, a bacterial disease called wetwood might be causing the trees to ooze. This could be a problem for the health of a tree. Wetwood is not the case here. First, Roni’s trees are healthy with no signs of decline, dieback, cracking bark or previous discolored oozing that could be signs of bacterial wetwood. They are not stressed and get plenty of water through the seasons. Secondly, they were pruned properly, which means the trees will soon compartmentalize and seal the pruning wounds off from the rest of the branch or trunk. Unless there are new wounds, they shouldn’t leak anymore after compartmentalization. What’s happening is that the trees are coming out of dormancy and their vascular systems are becoming active. The warmer temperatures during the day generate a pressure buildup within the vascular system. A pruning cut provides the perfect pressure release valve to this flow. The maples are simply leaking out the watery sap that is rising up from the roots through the trunk and into the branches. It’s similar to the rise and flow of maple syrup out of a sugar maple. While you might find information online or in books for other parts of the country that suggest sealing a pruning wound with some commercially available product or tree paint, that is not recommended here. Sealants are usually petroleum-based products and can actually impede compartmentalization and callus development. These products dry out in our arid environment allowing cracks and openings that provide reservoirs for moisture and access to insects and disease organisms. The natural compartmentalization process of the tree that has been pruned correctly is the best sealant. JoAnne Skelly is Associate Professor & Extension Educator Emerita University of Nevada Cooperative Extension. She can be reached at skellyj@unr.edu.
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