"Lucky you pay attention in Herbology, Hermione."
-- Ron Weasley after they escaped the Devil's Snare (PS16)
Herbology class at Hogwarts is the study of magical plants and fungi, including their care and their magical properties and uses. Some magical plants form important ingredients in potions, others have magical effects in their own right.
At Hogwarts, all students are required to attempt an O.W.L. in Herbology, so all first through fifth year students take the class, which is taught by Professor Sprout. Herbology classes are held in the greenhouses and include plenty of hands-on activities, including pruning alihotsy bushes, repotting mandrakes, and harvesting bubotuber pus. Students are also assigned essays as homework (OP13).
For the Gryffindors, Herbology is a double class with the Hufflepuffs. Neville Longbottom is particularly good at Herbology. It's usually the only class in which he raises his hand.
Used by students to protect themselves from plants like the Mandrake which can harm with the sounds they make; these ear muffs are apparently magical as they completely block all sound (CS6).
Flesh-Eating Slug repellent
Occasionally required to keep Flesh Eating Slugs out of Hogwarts' cabbages, these must be bought on Knockturn Alley, which suggests that they are somewhat Dark in nature (CS4)
Students begin working in greenhouse 3 with more dangerous plants that require protective gear for safe handling, such as mandrakes, which when fully grown can kill the unwary (CS6). Among the practical lessons in Herbology this year students are taught how to prune Abyssinian shrivelfigs CS15).
Students have practical lessons in learning to harvest puffapods (PA8).
Students learn the properties of bubotuber pus and have practical lessons in how to collect it safely (GF13).
Like most of the other teachers, Professor Sprout begins her fifth year classes by lecturing the students briefly on the importance of their O.W.L. before beginning a year of loading them down with homework, which even in this class involves essays (OP13). At the end of this year students take the O.W.L., which includes a practical portion that requires each student to work with some selection of plants. Harry, for example, was bitten by a Fanged Geranium during his exam (OP31).
Students who have achieved an O.W.L. pass grade in Herbology that meets Professor Sprout's standards may continue taking the subject this year in preparation for the N.E.W.T. . Harry, Ron, Hermione and Neville all took Herbology in 6th year, working with "more dangerous" plants (HBP11).
Students prepare to take the N.E.W.T. in Herbology, the highest level of qualification offered by Hogwarts.