What causes a long vertical split opened up in my trunk over the winter?
A hard, fast temperature drop — bright winter sun on a dark trunk followed by a night crash — cools the outer wood faster than the inner wood. The outer layers contract against inner wood that hasn't yet, tension exceeds what the fibers can take along the grain, and it lets go. Cracks almost always start at an existing flaw: an old pruning stub, a mower or string-trimmer wound, a branch scar, a lightning path. Thin-barked species carrying a lot of water in the wood — maple, London planetree, sycamore, linden, young oak, horse chestnut — crack most. Once formed, that line stays the weak point and reopens on every hard cold snap.
What you'd see
A straight vertical crack in the bark, usually on the south or southwest face, often several feet long. It typically shows up in the coldest stretch of winter, sometimes with an audible bang. In summer it closes nearly shut; the next winter it reopens in the same line. Over years a raised, lipped ridge of callus wood — a frost rib — builds along both edges. On maples it may weep dark sap in spring.
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