The USDA has finally released an update to the 1990 plant hardiness zone map. This new map incorporates a longer data period (1976-2005) and more accurately takes into account how elevation affects climate in the western United States. Most of the Las Vegas is now listed as zones 9a/9b except for some of the western parts of the valley which are zone 8b. For outlying areas, Pahrump is zone 8a/8b, Mesquite is 8b/9a and Laughlin is 9b/10a.
Las Vegas Valley
Although the new map is a huge step forward, please use it only as a starting point. Most yards have micro-climates which temperature can vary as much as 5-10 degrees depending which side of the house you're on, how much sun the area gets and if there is any low spots where cold air can collect. If you would like additional information, please visit the new USDA plant zone map website which can be found at the link below.