Zone 3b is much like Zone 3a, but with slightly milder winter averages of 19 to 29°F (–7 to –2°C) and extremes that usually bottom out between –2 and –15°F (–19 to –26°C). Summer temperatures are a bit higher than in Zone 3a—mostly in the high 80s and low- to mid-90s. Zone 3b offers one of the longest growing seasons of the intermountain climates. Gardeners here count on 180 to 210 frost-free days with plenty of heat. However, it’s one of the smallest zones. Most of it lies in the warmest parts of eastern Washington’s Columbia Basin,with bits in Lewiston, Idaho, and parts of the Southwest. This is fabulous country for annual vegetables and flowers and a long list of perennials, trees, shrubs, and vines.
