A species of plant that lives longer than two years, perennials are divided into two large groups, those that are woody and those that are herbaceous. These plants are usually easy to maintain and thrive well in most environments. Perennial plants also benefit agriculture and soil health in general because they typically have deep, extensive root systems which can hold soil to prevent erosion; and capture dissolved nitrogen to prevent contamination of ground water. Perennials can also outgrow weeds, reducing the needs for herbicides.