While commercial greenhouses often consist of a double layer of air-inflated polyethylene plastic that remains installed year around, the High-Tunnel Greenhouse (or hoop-house) usually has only a single layer of greenhouse film or woven fabric that's applied and removed as needed. Commonly, high tunnels have no heating systems, although solar or space heaters are sometimes used to extend the growing season. Also, because high tunnels sit directly on the ground (not on foundations), the crops are usually planted in-ground (not in containers).