Gravel consists of pea-sized stones that are laid across the top of a planting bed to help improve growing conditions. While gravel doesn't add nutrients to the soil, it does help to control moisture levels and impede evaporation. Thicker layers of gravel will reduce the amount of water that reaches plants, while even thin layers of gravel can reduce soil erosion and slow evaporation.
One of the most beneficial properties of gravel is its ability to expand the heat zone of a garden. Because these rocks collect sunlight during the day and release it at night, they can allow gardeners to start planting earlier in the season without worrying about low temperatures and frost. Using gravel may even allow for a wider range of plants to be grown, including those that would normally not survive in a given climate. Gravel also serves a decorative function in the garden; it's available in many different shades and textures to complement the visual appeal of each plant.