The greenhouses

The Hogwarts greenhouses were the location for the school's Herbology lessons and were full of wild and rare magical plants, including Mandrakes. Depending on what year the students were in, they would frequent different greenhouses corresponding to the level of Herbology they were learning. During Harry Potter's time at school, Professor Sprout was the school's Herbology teacher and could often be found tending to the plants and various magical flora and fungi growing there.