The Oxford Companion to Beer definition of
Idaho (southern hop region)
Idaho (Southern Hop Region) yields notably different results than hop growing in the northern part of the state. See idaho (northern hop region). In southern Idaho, hop cultivation is concentrated in the Treasure Valley, in the flood plain of the Boise River, some 65 km (40 miles) west of the state capitol of Boise. In contrast to the more moderate climate in northern Idaho, the Treasure Valley is hot and dry, similar to the Yakima Valley in Washington State. See yakima valley hop region. The first hop yard in southern Idaho was established in 1934, a year after the end of Prohibition. Production remained modest until the end of World War II, when a shortage of hops inspired many more farmers to get into the business. Until the late 1970s and early 1980s, Cluster, Talisman, and Cascade were the dominant varieties planted in southern Idaho. Subsequently, these were largely replaced by Galena. See galena (hop). When higher-yielding, high-alpha varieties began to be planted in the Yakima Valley in the 1990s, these were also found to do well in the Treasure Valley. Although some aroma hops such as Willamette and Cascade are being cultivated in the southern Idaho region as well, bittering varieties, as well as the dual-purpose Chinook, are much better suited to the hot climate. However, because summers in the Treasure Valley tend to be even hotter than in the Yakima Valley, identical varieties often yield lower alpha acid values and a bit less aroma in southern Idaho than they do in Washington State.
See also cascade (hop), chinook (hop), galena (hop), and willamette (hop).