‘TIS THE SEASON? TO PRUNE?

Most tree species will “bleed” for days or even weeks following an injury due to the interior movement of sugar and starch.  This “bleeding” is the sap or resin that you see secreting from a tree.  Some tree species, such as maple and birch, are prone to bleed more than other species due to higher sap pressure.  However, ash trees have less sap pressure and tend not to bleed.  We call the period when sap pressure is high the mobilization phase.  The parenchymal cells of a tree are highly active during this mobilization phase and the subsequent growing phase.  During the colder winter months, a tree’s reaction to wounds is significantly weaker in comparison to other seasons.  If you live in an area with mild winters, like San Diego, then the fluctuations in wound reactions due to seasonal change may not be as pronounced in certain species.  

The cambium, which is the living layer of cells that lies just underneath the bark of a tree, plays a large role in repairing wounds.  It too has varying reactions to wounds throughout the year, as it dries out in the winter, causing cambial necrosis to form.  In other words, the living cambium cells in the affected zone die off.  During the months of October through November, these cambial necroses tend to be larger.  As a result, the size of the wound is increased due to the necroses at its edge, causing the callusing and encapsulation of the wound to require more time.  On the other hand, necroses tend to be smaller during the growing season and smallest in the spring, allowing the tree to quickly seal up its wound, forming the woundwood, which studies have shown to be strongest when formed in the spring in certain tree species.