How to Prune horse mint?
Cut back after flowering

horse mint · Mentha longifolia
Also known as: brook mint; long-leaved mint; water mint
Grow in moist but well drained soil in full or partial sun. May have to potential to become a nuisance; restrict the root run in a deep container and plunge into the ground, or plant in a well chosen site where it can grow unrestricted. See mint cultivation for more information
Cut back after flowering