5. mulch: mulch keeps plant roots cool, prevents soil from crusting, minimizes evaporation and reduces weed growth. organic mulches, such as bark chips, pole peelings or wood grindings, should be applied 2 to 4 inches deep. fiber mulches create a web that is more resistant to wind and rain washout. inorganic mulches, such as rocks and gravel, should be applied 2 to 3 inches deep. surrounding plants with rock makes the area hotter; limit this practice.