Its called herbicide persistence and could last for years. Weather (wetter the better) and concentrations of herbicide both play a big part in how long the herbicide will last. One remedy is to plant a cover crop of some sort, let it mature, then pull it out and toss it in the garbage. Keep doing that until the herbicide is neutralized. Its called biodegradation - the idea is to get enough microorganisms into the soil to 'digest' the herbicide.
