Zone systems — official sources
USDA Plant Hardiness Zone Map
Measures average annual extreme minimum winter temperature only. 13 zones in 10 °F (5.6 °C) increments. The most widely cited system globally, but captures cold tolerance only — says nothing about heat, humidity, or rainfall.
RHS Hardiness Ratings (UK)
Seven ratings (H1a–H7) based on minimum winter temperature. Applied per-plant rather than per-location, which means you look up the plant's rating and compare it to your conditions rather than zoning a map. More nuanced than USDA for UK and European climates.
Canadian Plant Hardiness Zones
The most complex system — uses seven variables: minimum winter temperature, frost-free period, summer rainfall, maximum summer temperature, January mean temperature, maximum wind speed, and snow cover. Published by Agriculture and Agri-Food Canada.
Australian National Botanic Gardens Zones (ANBG)
Seven zones based on minimum winter temperature, structured similarly to USDA but calibrated for Australian conditions. Cold tolerance is rarely the limiting factor in Australian gardens — heat, drought, and humidity matter more, so treat these zones as a rough guide only.
Supplementary systems
AHS Plant Heat Zone Map (American Horticultural Society)
Measures number of days per year above 30 °C (86 °F). Complements USDA zones for heat-sensitive plants. 12 zones.
Sunset Climate Zones
24 zones covering the western US, originally developed for Sunset Magazine. Incorporates rainfall, humidity, summer heat, and length of growing season — more practically useful than USDA for that region. Sometimes referenced in Australian gardening literature.
Köppen Climate Classification
Academic system using letter codes (e.g. Cfa, Cfb, BSk) based on temperature and rainfall patterns. Used in ecological and botanical literature. Sydney = Cfa, London = Cfb, Perth = Csa, Vancouver = Cfb.
Further reading
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Missouri Botanical Garden Plant Finder — multi-system plant hardiness data
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Gardenia.net zone lookup — broad international zone cross-reference
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Royal Botanic Gardens Kew — global plant database with distribution data
Hardiness zones are guidelines, not guarantees. Microclimates within a single garden can differ by a zone or more — a south-facing wall may overwinter plants that fail in an exposed border ten metres away. Local knowledge from nurseries, gardening clubs, and a careful look at neighbouring gardens remains the most reliable guide to what will actually thrive in a specific spot.