The Grove of Cenarius is a subzone of Val'sharah, home to Cenarius. During the Val'sharah storyline Cenarius is afflicted by the Emerald Nightmare and tended to by his thero'shan Malfurion Stormrage, who requests that several archdruids across Val'sharah be gathered to aid the Lord of the Forest.
Another Grove of Cenarius was located between Felwood and Mount Hyjal. It was shown on the maps of Warcraft III: Reign of Chaos Game Manual and during the loading screens, but neither visited nor mentioned in the game.
The Grove of Cenarius may be the grove near Hyjal where Rhonin and Krasus were taken in the War of the Ancients after Cenarius saved them from night elf soldiers that had captured them. Whether it is or is not the same location, the grove in the war was certainly still in Val'sharah, as Val'sharah is where Cenarius taught Malfurion the ways of the druid.