Unless opaquely screened by existing vegetation cover, mobile home parks shall be screened by a temporary planting of fast-growing material capable of reaching a height of 15 feet or more, such as hybrid poplar, and a permanent evergreen planting such as Norway pine, the individual trees to be such a number and so arranged that within 10 years they will have formed a screen equivalent in opaqueness to a solid fence or wall. Such permanent planting shall be grown or maintained to a height of not less than 15 feet.