Global warming of the last 50 years has not been uniform across the globe, nor has the trend been constant. Easterling (2002) reports that the number of frost-free days in the US over the last 50 years has increased, more so in the western half of the US than in the eastern half. He finds that California and the Pacific Northwest have gained 4-5 additional frost-free days per decade over this period. The Great Plains region has gained 1-2 frost free days per decade over 1948-1999, totaling 8.5 more frost-free days than in the mid-20th century. Even the southeastern US, which has been undergoing a cooling trend in recent decades, has a weak increase in length of the frost-free period.
Reference
Easterling, D. R., 2002: Recent changes in frost days and the frost-free season in the United States. Bull. Amer. Meteor. Soc., 9, 1327-1332.