How long can I leave the side of my tree facing the road is brown and dying before it becomes dangerous?
Seasonal and cumulative, not sudden — no one gets hurt by salt damage. Injury happens all winter and shows in April and May, each winter stacking on the last, and it is typically the third or fourth salted winter, or the first summer drought layered on salted soil, when a street tree crosses from ugly to dying. The action window is narrow and specific: leaching has to happen in early spring, before the tree is fully leafed out and while the salt is still high enough in the profile to move. Miss it and the tree carries the load all summer. Meanwhile sodium keeps degrading soil structure, the crown thins from the tips inward, and borers and cankers find the stressed tree.
Whether it can still be saved
Usually yes for spray damage — the tree flushes new growth from behind the killed buds, and a spring prune of dead tips plus an early-spring rinse of evergreen foliage with plain water handles most of it. Soil damage is harder, and the treatment is leaching: heavy irrigation in early spring, applied slowly across several sessions, to move chloride below the root zone. That only works if the soil actually drains, so on tight clay it may need aeration or compaction relief first. Gypsum on the root zone can help sodium-damaged soil by supplying calcium to displace sodium, but it is not a general-purpose fix and deserves a soil test rather than a guess. The durable fixes are upstream: switch your own pavement to calcium magnesium acetate or plain sand, stop piling plowed snow on beds, screen the road side with burlap over winter, and replant with tolerant species where the salt is not going to stop. Remove when more than about half the crown is dead and dieback has progressed several consecutive years.
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