How long do you have to wait before replanting after using rock salt?

Rock salt can destroy the soil for a long time. Possibly years. If you want to kill the Ivy, use "Roundup". Paint a concentrated solution of the Roundup on a cut section of Ivy vine.

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In ancient times it was common for the conqueror to salt the soil of the farming fields around the defeated peoples cities, to make sure that the city could never rise again. In many of these cases these soils are still incapable of growing plants several thousand years later!

Never use salt to kill undesired vegetation.