How long can I leave a long vertical split opened up in my trunk over the winter before it becomes dangerous?
Years to decades, and frequently never. Watch depth, not length: a crack closing over each summer with a thickening rib is winning, while one whose edges stay open, stay dark, and turn soft or punky is losing ground to decay. Tapping the trunk with a mallet and hearing a hollow tone beside the crack means the decay column is wide. It becomes a genuine hazard only when decay has eaten enough cross-section that the trunk cannot carry a wind load, and by that stage you will also be seeing crown thinning and conks near the base.
Whether it can still be saved
Yes, in nearly every case, and the most important treatment is what you do not do. Do not fill the crack with cement, foam, or caulk, do not bolt it shut, and do not paint it with wound sealer — sealing traps moisture and feeds decay while smothering the callus tissue that actually closes it. Protect the trunk from further wounding: a mulch ring to keep mowers and trimmers away, water in drought, and winter trunk protection on a young thin-barked tree. Get an assessment if the crack has opened into a cavity you can put a hand into, if it runs the full height of the trunk, if fungal conks are growing along it, or if there are matching cracks on opposite sides. The response then is usually crown reduction to lower the leverage on a weakened trunk, not removal.
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