Good air drainage is important during frosty nights. Gentle slopes secure such drainage and are also likely to have good soil drainage. Steep hills should be avoided, because they increase the cost of production and harvesting and are subject to erosion and are usually not retentive of soil moisture. It is almost impossible to grow good crops of onions, celery, beets and many other small vegetables upon steep hillsides. Level lands or gentle slopes are nearly always used by gardeners who follow intensive methods.