A plant's performance is governed by the total climate: length of growing season, timing and amount of rainfall, winter lows, summer highs, humidity. Sunset's climate zone maps take all these factors into account — unlike the familiar hardiness zone maps devised by the U.S. Department of Agriculture, which divide the U.S. and Canada into zones based strictly on winter lows. The U.S.D.A. maps tell you only where a plant may survive the winter; our climate zone maps let you see where that plant will thrive year-round.