Is it religiously forbidden to water plants with Zamzam water that has been drunk from, instead of using it (for drinking)?

It is disliked to use Zamzam water for removing impurities or to pour it into impure places, because it is blessed and revered water. Many hadiths have been narrated explaining its virtue, among them: "The best water on the face of the earth is Zamzam water; it is a food that satisfies and a cure for illness," and "Zamzam water is for what it is drunk for." Ibn al-Qayyim described it as "the master of waters, the most noble and exalted in status, and the most beloved to souls." Accordingly, if the plants watered with it were impure, then that is disliked; otherwise, no. As for what you did with the water because you thought it had been drunk from directly, there is no harm in that, for the prohibition against blowing into drinks has been reported. The wisdom behind this prohibition is to ensure the safety of the drink from contamination by harmful carbon dioxide.