Whenever the lands adjoining any hard-surfaced public road or highway in this State shall be beautified by the planting or maintaining of flowers or ornamental trees or shrubs on the lands adjoining it by any agreement of the landowners adjoining such road or by their permission, such stretch of road shall be marked at either end by the road authorities of the county or counties wherein it lies by some suitable sign showing that the stretch of road between such signs has been beautified.