At this time of year the increase in day length and more frequent sunny days trigger new growth in our indoor plants. Outside, temperate flora remains dormant, waiting for the trigger of warmer temperatures to initiate growth once again. The month of February and early March are dangerous times for our specimen landscape trees, and even for exposed woodland trees.
Southwest injury happens when tree trunks are bathed in sunlight during a cold, late winter day.
At this time of year the most direct rays of the sun strike the southwest face of a tree and, especially in the afternoon, the trees experience a substantial localized temperature increase on the exposed side. As the woody tissues become warmer, they expand. Though plant cells are naturally equipped with ample sap to keep them flexible and resilient, months earlier the tree evacuated much of this lubricating liquid to the roots. Here, sap is protected from freezing during the winter, beneath an insulating blanket of soil, and thereby escapes the formation of cell-damaging ice crystals. But with sap now at a premium in a tree's above-ground parts, cells stressed by heat expansion are in greater danger of separating.