Low tunnels, high tunnels, and greenhouses are structures used to grow plants under protected conditions. The progression of terms shows the level (low to high) of technical sophistication in the plant-growing systems. Low tunnels, also called row covers, primarily advance the growing season for outdoor crops (for example, tomatoes, melons, strawberries, and sweet corn). Low tunnels are created using long, narrow strips of transparent plastic material (often polyethylene) buried in the ground along their outer edges to cover one or several adjacent rows of plants grown directly in the soil. French cloches and traditional hot caps are variations of this technology. High tunnels are large versions of low tunnels, raised sufficiently above the ground that people can walk within them.