Organizations around the world have developed different systems for describing climate, and each one approaches the question from a slightly different angle. Some focus on winter cold, others on summer heat, and still others take into account day length, humidity, wind, rainfall patterns, or even regional microclimates. No single system captures everything, but together they give gardeners a much clearer picture of the conditions their plants will face.
These climate-zone systems are not interchangeable — each one is designed for a specific purpose. For example, the USDA Hardiness Zones help you understand how cold your winters get, while AHS Heat Zones focus on how hot your summers feel. Sunset Zones layer in more nuance by including factors like ocean influence and elevation. International systems, such as those used in Canada or the UK, adapt climate-zone mapping to their own geographies.
Because each system highlights different environmental stresses, experienced gardeners often consult more than one. Knowing your zone(s) helps you choose plants better suited to your region, time your gardening tasks more accurately, and understand why certain plants succeed while others struggle.
Below are the major climate-zone systems used by gardeners, horticulturists, and climatologists. Each system offers its own lens for understanding your garden’s conditions:
- USDA Hardiness Zones (cold hardiness): https://planthardiness.ars.usda.gov/pages/map-downloads
- AHS Heat Zones (heat hardiness): The AHS removed their heat zone map page for some reason. You can still find it at this archival site for now: https://web.archive.org/web/20171122183751/http://ahsgardening.org/gardening-resources/gardening-maps/heat-zone-map
- Sunset Zones: https://www.sunsetwesterngardencollection.com/climate-zones/
- Köppen climate classification (global climate system used in geography and ecology, not plant hardiness): https://koppen.earth/ and https://en.wikipedia.org/wiki/K%C3%B6ppen_climate_classification
- Growing Degree Days (GDD; temperature-based measurement of seasonal heat units used to predict plant development): https://www.canr.msu.edu/news/understanding_growing_degree_days
- Heat Island Effect (urban heat effect that dramatically alters local microclimates): https://www.epa.gov/heatislands
- US Forest Service “Climate Change Pressures Heat Zones” Map: https://data-usfs.hub.arcgis.com/documents/usfs::climate-change-pressures-heat-zones-map-service/about
- Canadian Hardiness Zones: https://planthardiness.gc.ca/?m=1
- UK Hardiness Ratings (cold-hardiness scale used by the Royal Horticultural Society): https://www.rhs.org.uk/plants/pdfs/plant-labels/rhs-hardiness-rating
- Australian Climate Zones (Australian Bureau of Meteorology classification based on temperature, humidity, and rainfall patterns): http://www.bom.gov.au/info/climate-zone/
- New Zealand NIWA Climate Zones (New Zealand’s national climate classification used for horticulture and agriculture): https://niwa.co.nz/climate
- World Climate Zones (this map’s climate/hardiness zones were modeled using NAPPFAST (2012) climate data): https://evergreenbotany.com/blogs/news/plant-growing-zones
Additional climate and environmental classification systems:
- American Aridity Zones (US dryness index based on evapotranspiration and long-term moisture levels): https://www.climate.gov/maps-data/dataset/us-aridity-index
- USDA Major Land Resource Areas, MLRA (agricultural zones combining climate, soils, and landform characteristics): https://www.nrcs.usda.gov/programs-initiatives/mlra
- WUCOLS, Water Use Classification of Landscape Species (California system ranking landscape plants by water needs, VL–H): https://ucanr.edu/sites/WUCOLS/
- Energy Star Climate Zones (building energy–climate categories that help infer heat gain/loss and microclimate effects): https://www.energystar.gov/index.cfm?c=new_homes.hm_newhomes
- ASHRAE Climate Zones (architectural climate classification based on temperature and humidity patterns): https://www.energycodes.gov/prototype-building-models
- USDA Fire Regime Groups (climate-driven vegetation and fire-history zones used in wildfire-prone regions): https://www.fs.usda.gov/fire/fmod/fire-regime-groups
- CAL FIRE Fire Hazard Severity Zones (California’s climate–vegetation–weather fire-risk mapping): https://osfm.fire.ca.gov/divisions/community-wildfire-preparedness-and-mitigation/fire-hazard-severity-zones/
- Wikipedia has a few more listed if you want to keep digging: https://en.wikipedia.org/wiki/Climate_classification
Together, these systems give gardeners multiple tools for understanding their climate and choosing plants that will thrive in their unique growing conditions.