Hair-like structures are shared by most living organisms. The hairs on plant surfaces, commonly referred to as trichomes, form diverse types to sense and protect against various stresses. However, it is unclear how trichomes differentiate into highly variable forms. Here, we show that a homeodomain leucine zipper (HD-ZIP) transcription factor named Woolly controls the fates of distinct trichomes in tomato via a dosage-dependent mechanism. The autocatalytic reinforcement of Woolly is counteracted by an autoregulatory negative feedback loop, creating a circuit with a high or low Woolly level. This biases the transcriptional activation of separate antagonistic cascades that lead to different trichome types. Our results identify the developmental switch of trichome formation and provide mechanistic insights into the progressive fate specification in plants, as well as a path to enhancing plant stress resistance and the production of beneficial chemicals.