Unwither instantly revitalizes your crops and makes them harvest ready again once they have withered due to taking too long to harvest (usually three days after crops have become ready to harvest)
How to get it? One or two quests may grant you Unwither Unwither. You can also buy Unwither Unwither for Farm Bucks Farm Bucks, see below. Neighbors can unwither your crops for free, 5 per 18 hours max. Leveling up might get your crops back up again.