Plantations are farms specializing in cash crops, usually mainly planting a single crop, with perhaps ancillary areas for vegetables for eating and so on. Plantations, centered on a plantation house, grow crops including cotton, cannabis, tobacco, coffee, tea, cocoa, sugar cane, opium, sisal, oil seeds, oil palms, fruits, rubber trees and forest trees. Plantation crops are grown on large lands called estates. These crops are grown with scientific methods and a large, highly skilled labor force. Crops such as tea and coffee are grown on hillslopes as they cannot withstand stagnant water. Protectionist policies and natural comparative advantage have sometimes contributed to determining where plantations are located.