Weed management on organic farms consists of cultural and mechanical techniques such as the rotation of crops that suppress weeds, mulching, tillage, cultivation, water management, and manual weeding. Weeds often help to conserve soil, improve organic matter, and provide beneficial habitat for natural enemies on organic farms. Plastic mulches are permitted provide they are removed at the end of the season. Insects and diseases can help to keep certain weed populations in check. There are a few natural substances that are also used to manage weeds, but the efficacy of these substances is still subject to question.