Sap carries nutrients to the parts of the tree. While it isn’t exactly like blood to the tree, an overly heavy flow out of a pruning or other wound removes nutrients the tree might have otherwise used. But if you’ve pruned the tree properly, the sap flow won’t hurt the tree at all.
Excessive Sap Flow
Some species of trees, such as maple and elm, normally exude a large amount of sap after being trimmed. These and other heavy-sap-flow trees are nicknamed “bleeders” because of the sheer amount they let out. Of course, a heavy flow of sap is an advantage for those tapping maple trees, as that sap is what’s boiled down into maple syrup.
Best Time to Prune
Schedule pruning to minimize sap flow, both to minimize cosmetic issues and to conserve nutrients for the tree. Prune “bleeders” either during the winter, when they are dormant, or during summer after much of the trees’ growth for the season has finished. At that point, the sap flow within the tree will not be as strong as it was during the spring growth period. Other trees are best pruned either in early spring, when the energy of the new season allows them to heal more quickly, or in summer, again after the season’s growth is done.
If pruning a “bleeder” during winter, ensure that you do it when the nighttime temperatures will not fall below 20 degrees Fahrenheit. Kansas State University Research and Extension notes that temperatures less than 20 F will hurt the tissue in the wound.
Seeing sap flow out of a beloved tree may be uncomfortable, but according to Texas A&M’s AgriLife Extension, the extra sap acts as a barrier against germs, helping to keep them out of the wounds from pruning. Older advice was to apply some sort of wound dressing after pruning, but this is unnecessary—and potentially harmful—to many trees, encouraging decay instead of healing. There are very few circumstances that call for a wound dressing.