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Average Date of Last Killing Frost

Most planting and seed sowing instructions are based on the average last frost date, which is defined as "the last day in the spring that you might have a killing frost".
When making your plant selections, it is wise to consult more experienced gardening friends and neighbors for advice regarding possible local micro-climates or unusual conditions. Their knowledge can be invaluable.

Map showing the last expected frost dates in the United States         frost key

Temperature Conversion Table

Temperature conversion chart

Temperature ranges of Plant Hardiness and Gardening Zones

  Zone     Fahrenheit       Celsius       Example Cities  
1  Below -50 F   Below -45.6 C Fairbanks, Alaska;   Resolute, Northwest Territories (Canada) 
2a -50 to -45 F   -42.8 to -45.5 C Prudhoe Bay, Alaska;   Flin Flon, Manitoba (Canada) 
2b -45 to -40 F -40.0 to -42.7 C Unalakleet, Alaska;   Pinecreek, Minnesota 
3a -40 to -35 F -37.3 to -39.9 C International Falls, Minnesota;   St. Michael, Alaska
3b -35 to -30 F -34.5 to -37.2 C Tomahawk, Wisconsin;   Sidney, Montana
4a -30 to -25 F -31.7 to -34.4 C Minneapolis/St.Paul, Minnesota;   Lewistown, Montana
4b -25 to -20 F -28.9 to -31.6 C Northwood, Iowa; Nebraska
5a -20 to -15 F -26.2 to -28.8 C Des Moines, Iowa;   Illinois
5b -15 to -10 F -23.4 to -26.1 C Columbia, Missouri;   Mansfield, Pennsylvania
6a -10 to -5 F -20.6 to -23.3 C St. Louis, Missouri; Lebanon, Pennsylvania
6b -5 to 0 F   -17.8 to -20.5 C McMinnville, Tennessee;   Branson, Missouri 
7a 0 to 5 F -15.0 to -17.7 C Oklahoma City, Oklahoma;   South Boston, Virginia
7b 5 to 10 F -12.3 to -14.9 C Little Rock, Arkansas;   Griffin, Georgia
8a  10 to 15 F -9.5 to -12.2 C Tifton, Georgia;   Dallas, Texas 
8b 15 to 20 F -6.7 to -9.4 C Austin, Texas;   Gainesville, Florida
9a 20 to 25 F -3.9 to -6.6 C Houston, Texas;   St. Augustine, Florida
9b 25 to 30 F -1.2 to -3.8 C Brownsville, Texas;   Fort Pierce, Florida
10a 30 to 35 F 1.6 to -1.1 C Naples, Florida;   Victorville, California
10b 35 to 40 F 4.4 to 1.7 C Miami, Florida;   Coral Gables, Florida
11 above 40 F above 4.5 C Honolulu, Hawaii;   Mazatlan, Mexico