Hardiness zone

United States Hardiness Zones

• USDA system aids gardeners and landscapers in the U.S.
• State-by-state maps available on USDA-ARS website
• Zones range from -65°F to +65°F
• Zones help determine plant survival based on temperature
• Zones divided into a and b subdivisions for finer distinctions
• Warmest and coldest zones in the U.S. are specified
• 2023 update of the USDA Plant Hardiness Zone Map
• New zones added for tropical plants
• Higher resolution map shows local variations

History of Hardiness Zones

• First hardiness zone system attempts in 1927 and 1938
• Modern USDA system started in 1960
• USDA map revised in 1990 with a and b subdivisions
• USDA updated map in 2012 based on 1976-2005 weather data
• USDA released updated map in 2023 based on 1991-2020 weather data

Global Plant Hardiness Zones

• USDA system initially developed for U.S. gardeners
• Zones range from -65°F to +65°F
• USDA maps available for different countries
• Different countries have adapted their own hardiness rating systems

Alternative Hardiness Systems

• Indicator plants can be used to describe plant hardiness
• Sunset publishes a series with 45 distinct climate zones in the US
• Köppen climate classification system can be used for general growing conditions
• Trewartha climate classification provides a real-world concept of climates
• Different systems may offer more detailed or broader information compared to USDA zones

International Hardiness Zones

• Australian hardiness zones defined by steps of 5 degrees Celsius
• Canadian hardiness zones mapped to USDA zones for practical purposes
• European hardiness zones with specific considerations for different regions
• American Horticultural Society heat zones complement USDA Hardiness zones
• South Africa has five horticultural or climatic zones

Hardiness zone Data Sources