Section 13. State highways shall be maintained and kept in good repair and condition by the department at the expense of the commonwealth. The department shall keep all state highways reasonably clear of brush and shall cause suitable shade trees to be planted thereon if practicable. As used in this chapter, the term ''state highways'' includes such public roads in state forests, parks and reservations outside of the metropolitan parks district, and such public roads within the limits of any property under the control of any department, board or commission of the commonwealth, as may from time to time be designated by the department as roads for general public use and approved for such use by the executive head of the department, board or commission controlling such property. The department shall, subject to appropriation, construct, improve and maintain all roads on such property.