N.H. Rev. Stat. Ann. § 231:155

On all class I and III highways, and state maintained portions of class II highways, the commissioner of transportation shall have under his supervision the planting, acquisition, maintenance and removal of all trees and vegetation and the same powers relative thereto as conferred by this subdivision or any other law upon the cities and towns on highways under their jurisdiction. The commissioner shall make such rules and regulations for the purposes hereof as shall, in his judgment, seem for the best interests of the state.