According to the 1974 U.S. Census of Agriculture, $31.7 million worth of greenhouse vegetables were produced on a covered area of 37.2 million square feet. Though this represents less than 2 percent of total fresh market vegetable production in the country, traditionally greenhouse vegetables have provided high quality produce during months when field production is at a low level. This has been especially the case in the Northcentral and Northeast regions where a relatively large and affluent population of the metropolitan areas demands a year round supply of fresh vegetables. Much of the greenhouse production relates to the growing of salad items. Tomatoes are the single most important crop accounting for roughly two-thirds of the covered area and value of all greenhouse vegetables. In 1974, 63 percent of the U.S. covered area for tomatoes was located in the Northcentral region, 10 percent in the Northeast and the rest in the South and the West.