One of the expected effects of possible anthropogenic “global warming” is a lengthening of the growing season. Climate models suggest that the greatest temperature rise from an enhanced greenhouse would occur in the cold season; this might cause the dates of late spring frosts to occur earlier, and late autumn frosts to occur later, thus lengthening the “freeze free” season. According to the National Research Council (2001), “A lengthening of the growing season also has been documented in many areas, along with an earlier plant flowering season and earlier arrival and breeding of migratory birds.” To investigate possible growing season changes in the Pacific Northwest, the authors used data from the Historical Climatological Network for Washington, Oregon, Montana and Idaho and generated time series of the last spring and first autumn occurrences of 24, 28, and 32 degree F temperatures to determine trends for each station and each temperature threshold.