a method of promoting the growth of plants which comprises applying to the surface of a plant growth medium to be exposed to a source of ultraviolet radiation a light - degradable mulching film for agricultural use , comprising a resin composition , whose resin components consist essentially of from 5 to 50 percent by weight of polyisobutylene oxide resin , the balance of a second resin component selected from the group consisting of at least one polyolefin resin and a mixture of at least one polyolefin resin and at least one polydiene resin . the resin composition can be made into a mulching film which , when exposed to the sunlight , photolytically degrades into a pulverized condition within a selected period of time .