Some gardeners rely on plastic mulch for its ability to both battle weeds and insulate garden beds. But the inorganic material isn't right for all gardens, and should never be used with cool-season crops like root vegetables and lettuce. The plastic creates a shield over the soil, blocking water and air from reaching the crops. The trapped soil may become too hot during the summertime, causing plants to send out roots close to the soil's surface in search of nutrients, which eventually leads the plant to wither and die. Keep your cool-season crops healthy by choosing organic mulches like wood chips or bark instead.