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
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.
The authoritative modern Köppen-Geiger climate maps, freely licensed and kept current. More technical than the other links here — map downloads plus an interactive viewer — but it's the real source behind the Köppen classification described above.
International zone maps and converters
South African horticultural zones — SANBI
Zone map for South Africa published by the South African National Biodiversity Institute. Covers the country's diverse climate regions from subtropical KwaZulu-Natal to Mediterranean Western Cape.
Hardiness zone overview — Wikipedia
Comprehensive overview of hardiness zone history, methodology, and international systems. Good starting point for understanding how different countries developed their approaches independently.
Every hardiness system in the world — Succulentes.net
Ambitious attempt to document and compare every known hardiness zone system globally. Particularly useful for understanding how lesser-known regional systems relate to USDA and RHS.
Further reading
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Missouri Botanical Garden Plant Finder — multi-system plant hardiness data
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The Royal Botanic Gardens Kew's global plant database — authoritative botanical names, native distributions, images, and plant traits for the world's flora. The best free place to confirm a plant's correct name and where it grows wild.
The astronomical year
Plant Season anchors the year to the solstices, equinoxes and lunar cycle rather than to calendar dates — a framing closer to how plants and pollinators actually keep time. These sources cover the underlying astronomy and the long traditions of working with it.
Royal Museums Greenwich — seasons and calendars
Plain-language explainers from the home of Greenwich Mean Time on what the equinoxes and solstices actually are, why they happen, and how they shape the seasons — collected alongside related material on how cultures around the world divide the year.
Time and Date — moon phase calendars
Searchable moon-phase calendar for any year and any location. Useful for planning sowing, harvest and observation against full and new moons.
NASA — moon phases and the lunar cycle
Plain-language explainer of why the moon goes through phases, the length of the synodic month, and how the cycle is observed from Earth.
Bureau of Meteorology — Indigenous Weather Knowledge
Aboriginal seasonal calendars from across Australia, divided not into the four imported European seasons but into six, eight or more locally observed seasons keyed to plants, animals and weather. A richer and older way of dividing the Australian year.
USA National Phenology Network
A US scientific network that tracks phenology — the timing of leaf-out, flowering, fruiting and dormancy across the year. Background to the seasonal phases the Plant Season chart is built from. US-focused in its data, but the concepts apply everywhere.
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.