Mulches, with a few exceptions, are good for plants. Straw, wood chips, grass clippings, bark chips, sawdust, leaves, crushed rock and plastic can all be used as a mulch. Mulches, in general, help improve soil structure, and help conserve water. Also, as the mulch decomposes, it adds some nutrients to the soil. A three- to four-inch mulch will also prevent weed growth in the mulched area. And wood chip mulches have been shown to have some root disease suppression activity. There are also some downsides to mulch. It has been shown that plastic mulch reduces soil aeration and also allows the soil to stay moist right to the surface. As a result, trees and shrubs tend to develop a very shallow root system, some roots growing on the soil surface right under the plastic. Such plants are often not well anchored in the soil and are subject to wind throws in a windstorm. A study done on wood chips at an eastern university found that termites can colonize wood chip mulches. This was especially true if the chips were never disturbed after being spread. Finally, some mulches like sawdust and some wood and bark chips should be composted before application. Fresh wood chips, and especially sawdust, may release toxic substances in their early stages of decomposition. Sawdust could release sufficient quantities of these substances to be toxic to roots under the mulch. Composting sawdust or mixing sawdust with other mulch materials to dilute its effects will help prevent the above problem.