The first time I ordered plants from an American catalog, I was completely lost when faced with labels like “Hardy to Zone 7” or “Zones 5–9.” I knew my garden in southern France had cold winters, but how was I supposed to translate those mysterious numbers into actual temperatures? This experience, which every gardener has had at least once, reveals a fundamental need: a universal way of communicating plant cold hardiness.
Imagine discovering a stunning agave in an Australian nursery catalog. The seller says it “grows easily here.” But “here” in Australia looks nothing like “here” in your garden. How do you know whether the plant will survive your winters? This is precisely the problem that hardiness zone systems were developed to solve.
These systems are really common languages that allow gardeners around the world to discuss cold hardiness in a standardized way. Rather than saying “this plant tolerates frost” — which is vague and subjective — you can say “this plant is hardy to zone 7,” which gives precise, comparable information.
But not all zone systems are equal. Some focus solely on minimum winter temperatures. Others factor in summer heat, humidity, and the length of the growing season. Some are simple and universal; others are complex and regional. Understanding the differences between these systems is essential for making the right choices in your garden.
The USDA system: understanding hardiness zones 1 through 13
History and principle
The USDA (United States Department of Agriculture) system was developed in the 1960s to help American gardeners and landscapers choose appropriate plants. It is now the most widely used system in the world — the universal language of gardening.
The principle is remarkably simple: the territory is divided into zones based on the average annual minimum temperature. Each zone spans a range of 10°F (about 5.5°C), and each zone is subdivided into two half-zones, “a” and “b,” representing 5°F (about 2.8°C).
The latest version of the USDA map, published in November 2023, defines 13 zones ranging from zone 1 (the coldest, below −51°C / −60°F) to zone 13 (the warmest, above 15.6°C / 60°F). This map is based on 30 years of weather data (1991–2020) from more than 13,400 weather stations, making it a statistically robust tool.
The USDA zone table
Zone 1: below −45.6°C (−50°F). Zone 2: −45.6 to −40°C (−50 to −40°F). Zone 3: −40 to −34.4°C (−40 to −30°F). Zone 4: −34.4 to −28.9°C (−30 to −20°F). Zone 5: −28.9 to −23.3°C (−20 to −10°F). Zone 6: −23.3 to −17.8°C (−10 to 0°F). Zone 7: −17.8 to −12.2°C (0 to 10°F). Zone 8: −12.2 to −6.7°C (10 to 20°F). Zone 9: −6.7 to −1.1°C (20 to 30°F). Zone 10: −1.1 to 4.4°C (30 to 40°F). Zone 11: 4.4 to 10°C (40 to 50°F). Zone 12: 10 to 15.6°C (50 to 60°F). Zone 13: above 15.6°C (above 60°F).
Strengths of the USDA system
The strength of the USDA system lies in its simplicity and universality. A single number communicates essential information. When an American, British, or Japanese catalog states “Hardy to Zone 7,” any gardener worldwide understands the plant tolerates temperatures down to about 0°F to 10°F (−12°C to −17°C).
This standardization enormously facilitates international horticultural exchange. Nurseries, garden book authors, and botanical researchers all use this system as a common reference. It has become the Latin of modern gardening.
Moreover, the USDA system has been adapted in many countries. Canada uses the same system (with local variants). Europe, Australia, and New Zealand have created their own maps using the same methodology, enabling easy comparisons across regions of the globe.
Why the USDA system is not enough: its fundamental limitations
But the USDA system is far from perfect. Its main limitation is that it focuses on a single variable: the average minimum winter temperature. Yet plant survival depends on many other factors.
Winter humidity is a perfect example. An agave can survive 5°F / −15°C in Arizona where the air is dry, but die at 18°F / −8°C in Normandy where the winter is damp. Both regions may fall in the same USDA zone. The system does not capture this critical difference.
Summer heat is another factor the USDA ignores. The United Kingdom and parts of California are both in zone 8 or 9. Yet many plants that thrive in California fail in England because British summers are too cool and not sunny enough.
The length and structure of the growing season, the reliability of snow cover, and local microclimates are all factors the USDA system does not capture.
USDA zones in France: city-by-city table
Most of mainland France falls between zones 7 and 9, with zone 6 in the mountains and a few pockets of zone 10 along the most sheltered coastline.
| City | Region | USDA Zone | Avg min temp | Notes |
|---|---|---|---|---|
| Lille | Northern France | 8b | 16 to 21°F / −9 to −6°C | Mild continental, wet winters |
| Rouen | Normandy | 8b | 16 to 21°F / −9 to −6°C | Oceanic, very wet |
| Brest | Brittany | 9a | 21 to 27°F / −6 to −3°C | Strongly oceanic, mild winters |
| Nantes | Loire Valley | 9a | 21 to 27°F / −6 to −3°C | Mild, sheltered |
| Paris | Île-de-France | 8b | 16 to 21°F / −9 to −6°C | Urban heat island +2–3°F |
| Strasbourg | Alsace | 7b | 5 to 10°F / −15 to −12°C | Continental, cold winters |
| Dijon | Burgundy | 7b | 5 to 10°F / −15 to −12°C | Semi-continental |
| Lyon | Rhône Valley | 8a | 10 to 16°F / −12 to −9°C | Rhône Valley, sheltered |
| Bordeaux | Southwest | 9a | 21 to 27°F / −6 to −3°C | Mild oceanic |
| Toulouse | Southwest | 9a | 21 to 27°F / −6 to −3°C | Fairly mild |
| Montpellier | Mediterranean | 9b | 27 to 30°F / −3 to −1°C | Mediterranean |
| Avignon | Provence | 9a | 21 to 27°F / −6 to −3°C | Rhône Valley, mistral |
| Marseille | Provence | 9b | 27 to 30°F / −3 to −1°C | Mediterranean, coastal |
| Nice | French Riviera | 10a | 30 to 39°F / −1 to +4°C | Very sheltered |
| Toulon | Provence | 9b | 27 to 30°F / −3 to −1°C | Mediterranean, sheltered bay |
| Ajaccio (Corsica) | Corsica | 10a | 30 to 39°F / −1 to +4°C | Insular Mediterranean, very mild |
| Gap | Southern Alps | 7a | 0 to 5°F / −17 to −15°C | Alpine, 2,460 ft / 750 m |
Important note: these zones are averages based on winter minimum temperatures over the last 30 years. Urban microclimates, sheltered gardens, or exposed situations can shift these zones locally by ±1 full zone. The last truly severe winter in France (January 1985) is no longer within this 30-year window, which affects the averages.
How to calculate your USDA zone
The USDA system relies on a simple climatological method: for each location, the average of the annual minimum temperatures is calculated over a 30-year period. The most recent map (2023) uses 1991–2020 data from over 13,400 weather stations. The 30-year span is the standard duration for defining a “climate normal,” as it smooths out single-year anomalies while capturing multi-year cycles (El Niño, La Niña, the North Atlantic Oscillation).
At the garden scale, you can refine this reading: recording your winter minimums for at least 5 years already gives a solid indication, reveals your microclimate, and allows you to adjust for exposure (often by about half a zone). A south-facing wall: +1 half-zone. A cold hollow at the bottom of a slope: −1 half-zone. An urban center: +1 half-zone.
The Sunset system: Californian precision
Faced with the USDA’s limitations, the American magazine Sunset developed in the 1970s a far more sophisticated system, specifically designed for the thirteen western US states. Rather than focusing on minimum temperatures alone, the Sunset system integrates minimum and maximum temperatures, growing season length, rainfall, humidity, oceanic influence, continental influence, elevation, and latitude.
This comprehensive approach captures the real complexity of climate. Instead of 13 zones like the USDA, Sunset defines 24 zones for the western US with detailed narrative descriptions of each.
San Francisco is the perfect example. In USDA terms, it is mostly zone 10a. But Sunset distinguishes three different zones within the city: zone 15 (Noe Valley — sheltered, warm, sunny), zone 16 (downtown — foggy but mild), and zone 17 (the Sunset district — constantly bathed in cold ocean fog). All three have similar winter minimums (hence the same USDA classification), but radically different summer climates. A Mediterranean plant will thrive in zone 15, struggle in zone 16, and likely die in zone 17 from lack of summer heat.
The main limitation of the Sunset system is that it is geographically restricted to the western United States. However, its methodological principle is universal: to truly understand whether a plant will prosper somewhere, you must consider the entire annual climate, not just the winter minimum.
The RHS system: the British approach
The Royal Horticultural Society of the United Kingdom developed its own system in 2012, radically different from the American approach. Rather than dividing the territory into geographic zones, the RHS assigns a hardiness rating directly to plants, from H1 to H7. The USDA says “your garden is in zone 7.” The RHS says “this plant is rated H4.” It is a plant-centered approach rather than a location-centered one.
H1a: above 59°F / 15°C — tropical plants. H1b: 50–59°F / 10–15°C — subtropical. H1c: 41–50°F / 5–10°C — tender. H2: 34–41°F / 1–5°C — frost-sensitive. H3: 23–34°F / −5 to 1°C — half-hardy. H4: 14–23°F / −10 to −5°C — hardy, survives average British winters. H5: 5–14°F / −15 to −10°C — very hardy. H6: −4 to 5°F / −20 to −15°C — hardy in continental Europe. H7: below −4°F / −20°C — fully hardy.
Approximate correspondence: H7 ≈ USDA zone 5 and below. H6 ≈ zone 6. H5 ≈ zone 7. H4 ≈ zone 8. H3 ≈ zone 9. H2 ≈ zone 10. Caution: the RHS system runs in reverse from the USDA — a higher number means hardier (i.e. colder), which is a constant source of confusion.
Traditional cultivation zones: olive, citrus, and indicator plants
The olive zone: marker of the Mediterranean climate
“Where the olive tree gives up, the Mediterranean ends.” This quotation from Georges Duhamel perfectly illustrates the role of Olea europaea as a climatic indicator. The French botanist Charles Flahault proposed at the start of the 20th century that the Mediterranean region could be delimited by the area within which olive cultivation is feasible — an approach that has proven remarkably pertinent.
USDA correspondence: zones 8b to 9 (typically zone 9). Cold tolerance: −10 to −12°C / 10 to 14°F for hardy varieties (‘Aglandau’, ‘Cipressino’); −8 to −10°C / 14 to 18°F for standard varieties. In France: the core zone covers Provence, Languedoc, the Rhône Valley south of Montélimar, and Corsica.
The citrus zone: subtropical Mediterranean
The citrus zone is more restrictive than the olive zone. It corresponds to areas where citrus can fruit reliably and produce commercial-quality crops. USDA correspondence: mainly zone 9b and zone 10. In metropolitan France: Menton, Nice, the Alpes-Maritimes coast, and coastal Corsica.
The vine zone and the chestnut zone
The grapevine (Vitis vinifera) tolerates much colder winters than the olive (down to 5°F / −15°C to −20°C for the vine alone), spanning USDA zones 6 through 9. But its ripening requirements vary enormously: full-bodied reds need heat and sunshine, while white wines can be produced further north (Champagne, Alsace, Loire Valley). The vine is therefore not a good indicator of cold hardiness, but rather of cumulative summer heat — exactly what Kira’s Warmth Index measures.
The sweet chestnut (Castanea sativa) marks the transition between oceanic/continental climate and Mediterranean climate (USDA zones 6–8). It indicates acidic soils, regular rainfall above 28 inches / 700 mm annually, and moderate winters. The chestnut “gives up” where summers become too dry (interior Provence) or winters too cold (mountain limits).
Why indicator plants remain relevant
Zone systems based on indicator plants offer advantages the USDA cannot. First, a holistic approach: an indicator plant automatically integrates winter cold + summer heat + humidity + season length. Second, direct observation: you can see immediately whether you are in the “olive zone” — just look around. Third, ecological relevance: if the olive grows naturally, other Mediterranean plants are very likely to succeed too. These zones have been anchored in Mediterranean agricultural tradition for centuries.
The most effective approach combines both: determine your USDA zone, then identify local indicator plants. Avignon and Nantes are both USDA zone 9a — but Avignon is in the true “olive zone,” and Nantes is not. The indicator plants reveal two radically different climates that the USDA zone alone cannot distinguish.
The Kira system: Japan’s thermal sum approach
A visionary ecologist in wartime Japan
In 1945, a young ecologist at Kyoto University published a 23-page monograph that would revolutionize vegetation climatology across East Asia. Tatsuo Kira (吉良竜夫, 1919–2011) proposed an entirely new climatic classification for East Asia, explicitly designed as a foundation for agricultural geography.
What makes the Kira system fundamentally different from the USDA is that it does not merely measure the winter minimum. It measures the total thermal energy available for plant growth, during both the warm and cold seasons. It is a two-dimensional approach where the USDA is one-dimensional.
The two Kira indices: WI and CI
The Warmth Index (WI) is the annual sum of the difference between each month’s mean temperature and a biological threshold of 41°F / 5°C, for all months where the mean exceeds that threshold: WI = Σ(Ti − 5) for each month where Ti > 5°C. The result is expressed in “°C·months.”
The Coldness Index (CI) works in reverse: CI = −Σ(Ti − 5) for each month where Ti < 5°C. It measures cumulative winter cold intensity.
The 5°C threshold is the temperature below which most temperate plants cease active growth — a biological threshold, not an administrative one.
A worked example
At La Londe-les-Maures, southern France (USDA zone 9b), all 12 months exceed 41°F / 5°C. The WI reaches about 129°C·months, with a CI of zero (no month averages below 5°C). At Sapporo, Hokkaido, Japan (USDA zone 7a), only 7 months exceed the threshold: the WI drops to 75°C·months, while the CI rises to 26 (long, cold winters).
The USDA tells us La Londe is “warmer” than Sapporo. Kira tells us why and by how much: the thermal energy available for growth is almost double in La Londe. That is why a Cycas revoluta thrives in Provence but merely survives in Hokkaido.
Kira’s vegetation zones
- Arctic / alpine — WI = 0 to 15. Tundra, alpine meadows, treeless.
- Boreal / subalpine — WI = 15 to 55. Coniferous forests (spruce, fir).
- Cool-temperate — WI = 55 to 90. Deciduous broadleaf forests (beech, oak).
- Warm-temperate — WI = 90 to 180. Evergreen broadleaf forests. This is the zone where cycads, fan palms, and most temperate succulents thrive.
- Subtropical — WI > 180. Subtropical forests.
- Tropical — WI > 240, CI = 0.
Reference table: Kira’s Warmth Index (WI) for 40 cities worldwide
We have calculated Kira’s index for some forty representative cities from climate normal data. This table is, to our knowledge, the first publication of its kind in the English-language horticultural press aimed at gardeners. It allows you to instantly place your garden on the scale of available thermal energy.
| City | Country | USDA Zone | WI (°C·mo) | CI | Kira zone |
|---|---|---|---|---|---|
| Boreal / subalpine (WI 15–55) | |||||
| Reykjavik | Iceland | 7a | 24 | 26 | Boreal |
| Helsinki | Finland | 5b | 47 | 36 | Boreal |
| Cool-temperate — deciduous forest (WI 55–90) | |||||
| Moscow | Russia | 4b | 55 | 43 | Cool-temperate |
| Stockholm | Sweden | 6b | 56 | 24 | Cool-temperate |
| Berlin | Germany | 7b | 71 | 11 | Cool-temperate |
| London | UK | 9a | 73 | 0 | Cool-temperate |
| Seattle | USA | 8b | 73 | 1 | Cool-temperate |
| Sapporo | Japan | 7a | 75 | 26 | Cool-temperate |
| Strasbourg | France | 7b | 78 | 8 | Cool-temperate |
| Paris | France | 8b | 82 | 2 | Cool-temperate |
| Minneapolis | USA | 4b | 84 | 42 | Cool-temperate |
| Denver | USA | 5b | 85 | 15 | Cool-temperate |
| Nantes | France | 9a | 87 | 0 | Cool-temperate |
| Mild-temperate — transition to evergreen (WI 90–130) | |||||
| Chicago | USA | 6a | 96 | 21 | Transition |
| Bordeaux | France | 9a | 103 | 0 | Transition |
| New York | USA | 7b | 103 | 9 | Transition |
| Santiago | Chile | 9b | 107 | 0 | Transition |
| Seoul | South Korea | 6b | 107 | 17 | Transition |
| San Francisco | USA | 10a | 109 | 0 | Transition |
| Beijing | China | 6b | 115 | 19 | Transition |
| Madrid | Spain | 9a | 115 | 0 | Transition |
| Avignon | France | 9a | 117 | 0 | Transition |
| Montpellier | France | 9b | 119 | 0 | Transition |
| Rome | Italy | 9b | 124 | 0 | Transition |
| Nice | France | 10a | 127 | 0 | Transition |
| La Londe-les-Maures | France | 9b | 129 | 0 | Transition |
| Tokyo | Japan | 9a | 130 | 0 | Transition |
| Warm-temperate — evergreen (WI 130–180) | |||||
| Atlanta | USA | 8a | 136 | 0 | Warm-temperate |
| Cape Town | South Africa | 10b | 138 | 0 | Warm-temperate |
| Shanghai | China | 9a | 140 | 0 | Warm-temperate |
| Mexico City | Mexico | 10b | 144 | 0 | Warm-temperate |
| Lisbon | Portugal | 10a | 148 | 0 | Warm-temperate |
| Los Angeles | USA | 10b | 157 | 0 | Warm-temperate |
| Athens | Greece | 10a | 157 | 0 | Warm-temperate |
| Sydney | Australia | 10a | 158 | 0 | Warm-temperate |
| Marrakech | Morocco | 10a | 177 | 0 | Warm-temperate |
| Subtropical (WI 180–240) | |||||
| Houston | USA | 9a | 186 | 0 | Subtropical |
| Tucson | USA | 9a | 190 | 0 | Subtropical |
| Taipei | Taiwan | 10b | 216 | 0 | Subtropical |
| Phoenix | USA | 9b | 221 | 0 | Subtropical |
| Miami | USA | 11a | 237 | 0 | Subtropical |
| Tropical (WI > 240) | |||||
| New Delhi | India | 10b | 241 | 0 | Tropical |
| Bangkok | Thailand | 13 | 283 | 0 | Tropical |
Key observations from this table:
- London and Nantes (USDA 9a) have WI values of 73 and 87 — Kira’s cool-temperate zone, the realm of beeches and deciduous oaks. The USDA places them in the same zone as Bordeaux (WI = 103) or Madrid (WI = 115), which is misleading for exotic plants.
- Minneapolis and Sapporo have similar WI values (84 and 75) but radically different CI values (42 vs. 26). Minneapolis is far more continental — colder winters but hotter summers. The WI alone is not enough: you need the CI to understand continentality.
- San Francisco (WI = 109) is in USDA 10a but has a lower WI than Bordeaux (103, USDA 9a). This is a perfect illustration of the USDA’s limits: San Francisco has very mild winters (hence zone 10a) but cool summers due to fog (hence a modest WI). Desert succulents struggle there for lack of summer heat.
- Beijing and Madrid have exactly the same WI (115) but radically different USDA zones (6b vs. 9a). The CI makes all the difference: 19 for Beijing (frigid winters), 0 for Madrid (mild winters).
- La Londe-les-Maures (WI = 129) sits right at the threshold of Kira’s warm-temperate zone, where evergreen vegetation begins to dominate. This is consistent with the natural presence of holm oak, mastic tree, and olive in the area — the classic markers of Mediterranean climate.
- Phoenix and Houston are both USDA 9a–9b but their WI values (221 vs. 186) show a major difference. Phoenix has intense dry desert heat; Houston has humid subtropical heat. Two different worlds for succulents.
Kira and succulent plants: an underestimated tool
Why does an Agave americana thrive in Provence (WI ≈ 129) but struggle in southern Brittany (WI ≈ 75), even though both regions are in USDA zone 9a? Because Provence accumulates 70% more summer heat. The agave does not lack cold hardiness — it lacks the energy to grow.
Kira’s WI captures exactly what the Californian Sunset system attempts to describe qualitatively: the amount of heat available during the growing season. The difference is that Kira provides a universal number, applicable anywhere in the world.
If you take away only one thing from the Kira system, let it be this: a plant’s winter survival depends not only on the minimum temperature, but also on the energy it was able to accumulate during the previous growing season. A yucca that grew through a hot, dry summer enters winter with full reserves and well-matured tissues. The same yucca, after a cool, wet summer, enters winter weakened. The January thermometer may read the same: the outcome will be radically different.
The Soviet system: a multifactorial approach centered on agricultural productivity
Why the USSR never adopted the USDA model
The USDA model answers the gardener’s question: “will this plant survive winter?” The Soviet model answers the state planner’s question: “which wheat, which sunflower should be sown here to maximize the collective farm’s yield?” The first focuses on survival; the second on productivity. One measures a minimum; the other integrates the entire annual cycle.
This difference is rooted in Russian scientific tradition. Vasily Dokuchaev (1846–1903), the founding father of soil science, established as early as the 1880s the principle that soil, climate, vegetation, and relief form an inseparable system.
Selyaninov’s Hydrothermal Coefficient (HTC, 1928)
The most emblematic tool of Soviet agroclimatology combines temperature and precipitation during the growing season into a single number: HTC = R / (ΣT≥10 / 10), where R is precipitation (mm) during the period when the mean daily temperature exceeds 50°F / 10°C, and ΣT≥10 is the sum of active temperatures above that threshold.
The scale: HTC ≤ 0.3 (very dry, desert). 0.3–0.6 (dry, steppe). 0.6–1.0 (moderately dry, forest-steppe). 1.0–1.4 (slightly humid, chernozem zone, agricultural optimum). 1.4–1.6 (humid, forest zone). > 1.6 (very humid, taiga). This coefficient is still actively used across the post-Soviet space and was included in the WMO Handbook of Drought Indicators in 2016.
The sum of active temperatures (ΣT10)
The sum of all mean daily temperatures above 50°F / 10°C throughout the year — the close cousin of Kira’s WI, but with a higher threshold more relevant for field crops. The Soviet climatologist F. F. Davitaya used this index in 1948 to define the USSR’s viticultural zones: grapevines require a ΣT10 above 2,500°C, a frost-free period of at least 150 days, a mean absolute minimum above 5°F / −15°C for unprotected cultivation, and a Selyaninov coefficient between 0.5 and 2.5. This combination of four simultaneous criteria is far more informative than a single USDA zone.
Soviet natural-agricultural zoning
The most ambitious system identified 64 agroclimatic zones across Russian territory, combining soil types, temperature sums, the hydrothermal coefficient, growing season length, and snow cover depth. The USDA tells you “you are in zone 5.” The Soviet system tells you “you are in the forest-steppe subzone of ordinary chernozem, ΣT10 = 2,200°C, HTC = 1.1, growing season 145 days: sow winter wheat Mironovskaya-808.”
The legacy of Vavilov and the shadow of Lysenko
Nikolai Vavilov (1887–1943), a brilliant botanist and geneticist, identified the centers of origin of cultivated plants and assembled the world’s first major seed bank. Trofim Lysenko, an ideologue-agronomist backed by Stalin, imposed for decades a pseudoscience that denied Mendelian genetics. Vavilov was arrested in 1940 and died in prison in 1943. Over 3,000 biologists were persecuted. Soviet agronomic science did not recover until Lysenko’s disgrace in 1964.
Relevance for succulent gardeners
The HTC formalizes what every agave grower knows instinctively: it is not temperature alone that matters, but the ratio of heat to moisture. Interior Provence has a summer HTC of about 0.4–0.7 — ideal for xerophytes. Southern Brittany records an HTC of 1.2–1.6 — too humid for most succulents without drainage work. The Selyaninov coefficient states this in a single number; the USDA does not state it at all.
The Chinese system: a three-level zoning inherited from Kira
China, stretching from the frozen steppes of Manchuria (USDA zone 1) to the tropical rainforests of Hainan (USDA zone 13), has never developed a hardiness zone system in the Western sense. Chinese agronomists adopted a more complex approach inherited from both the Soviet tradition and Kira’s Japanese ecology.
The official system uses a three-tier nested structure: 12 temperature zones, 24 moisture zones, and 56 climate zones combining both levels with altitude effects (Tibet), monsoon influence, and continentality. For vegetation ecology research, Chinese scientists have massively adopted Kira’s Warmth Index, tested on 671 stations nationwide.
For international horticultural trade, USDA-adapted maps show China covers the full USDA spectrum: zones 1–2 in northern Heilongjiang, zones 5–6 in Beijing, zones 8–9 in Shanghai, zone 10 in Guangzhou, zones 12–13 in Hainan. No other country on Earth spans the entire range.
The Indian system: when the monsoon replaces frost
India provides a spectacular illustration of the limitations of the hardiness zone concept itself. Across the vast majority of the subcontinent, frost simply does not occur. In USDA terms, virtually all of peninsular India lies in zones 10 through 13. The determining climatic factor is not cold but the monsoon: its arrival date, duration, intensity, and reliability.
India has developed multiple competing zoning systems: 15 agroclimatic regions (Planning Commission), 20 zones (ICAR), 131 operational district-level zones, and 21 agro-ecological regions (NBSS & LUP). These classifications cross rainfall, temperature, soil types, water resources, and cropping systems.
The only region where the USDA concept makes sense is the Himalaya, covering USDA zones 4 (high altitude, 13,000 ft / 4,000 m) to 10 (subtropical foothills). Ladakh, with winters reaching −22°F / −30°C and short but intense summers, is India’s zone 4–5 — a world apart in a predominantly tropical country.
India teaches something fundamental: cold is not the only limiting factor, and for many plants it is not even the main one. For succulents of Indian origin, the question is never “will they survive the cold?” but “will they tolerate the winter humidity of my garden?”
Other systems around the world
The Canadian system
Canada uses the USDA system adapted with additional sophistication: growing season length, summer precipitation, July maximum temperature, and snow cover. Zones range from 0 (arctic north) to 9 (coastal southern British Columbia). The system explicitly recognizes the importance of reliable snow cover.
The Australian system
Australia developed a system that runs numerically in reverse from the USDA: Australian zone 1 ≈ USDA 13 (tropical hot); Australian zone 7 ≈ USDA 6–7. This inversion is a constant source of confusion.
European systems
Europe has no unified system. France has no official system (garden centers use adapted USDA or RHS). Germany uses adapted USDA. Scandinavian countries use national systems based on Nordic criteria (day length, snow duration, summer sun intensity).
Complementary systems
AHS Heat Zones
The American Horticultural Society created a system measuring the number of days per year when temperature exceeds 86°F / 30°C: 12 zones from zone 1 (less than 1 day) to zone 12 (more than 210 days). This information is crucial for alpine plants that die from heat stress and for Mediterranean plants that need intense heat. Unfortunately, the system remains poorly adopted.
The Köppen-Geiger system
This scientific climate classification distinguishes five major categories (tropical, arid, temperate, continental, polar). For gardeners interested in ecology and botanical geography, it helps identify which natural climates correspond to your garden. If you are in a Mediterranean climate (Csa in Köppen), then plants from central Chile, California, the South African Cape, and southwestern Australia share the same climate type.
Comparative table: USDA, Kira, Selyaninov
| Criterion | USDA | Kira (WI/CI) | Selyaninov (HTC) + ΣT10 |
|---|---|---|---|
| Origin | USA, 1960s | Japan, 1945 | USSR, 1928–1948 |
| What it measures | Mean annual min winter temp | Cumulative heat (WI) + cold (CI) | Heat/moisture ratio (HTC) + cumulative heat (ΣT10) |
| Question it answers | “Will it survive winter?” | “Will it get enough heat to grow?” | “What yield can be expected here?” |
| Biological threshold | None (absolute minimum) | 41°F / 5°C (plant growth) | 50°F / 10°C (agricultural crops) |
| Accounts for moisture | No | No | Yes (HTC) |
| Geographic scope | Worldwide (international standard) | East Asia (tested elsewhere) | Post-Soviet space |
| Usefulness for succulents | High (winter survival filter) | High (summer energy) | Medium-high (heat/moisture ratio) |
Combining all three systems: the best strategy
- USDA → How cold can my garden get? The winter survival filter.
- Kira’s WI → Does my garden accumulate enough heat for the plant to grow and mature? The summer vigor filter.
- Selyaninov’s HTC → Is the heat-to-moisture ratio compatible with the plant? The drainage and seasonal humidity filter.
Concrete example: Avignon and Nantes are both USDA zone 9a. But Avignon has a WI of about 117 and a summer HTC of 0.5 (hot and dry). Nantes has a WI of about 87 and an HTC of 1.3 (mild and wet). The first garden suits Agave americana perfectly; the second is risky without drainage work.
The USDA tells you whether the plant will survive January. Kira tells you whether it will thrive in July. Selyaninov tells you whether it will tolerate the rain in November. All three together paint the complete portrait of your climate.
How climate change is shifting hardiness zones
Between the 2012 and 2023 USDA maps, roughly half the United States shifted half a zone warmer. The trend is clear: winters are warming. For gardeners, this means previously impossible plants are becoming possible — olives in the Loire Valley, citrus in southern Brittany, agaves in the Paris region, hardy palms as far north as Burgundy.
But warming does not mean the end of severe winters. Extreme events are becoming more unpredictable. A mild winter can be followed by a brutal cold snap in March. For long-term plants (trees, palms, cycads), plant species rated for your current zone minus half a zone. For short-cycle plants (fast agaves, succulents), experiment half a zone above. Diversify your genetic sources: an Agave parryi from New Mexico may be hardier than one from Arizona.
Specific applications for Mediterranean and succulent plants
For exotic plant enthusiasts, hardiness zones are only part of the equation. A desert agave needs three things: tolerable cold, sufficient summer heat, and winter dryness. The USDA only covers the first.
Think in climate equivalents: Mediterranean climate (mild wet winters, hot dry summers) — Provence, central California, central Chile, South African Cape. Subtropical humid (cool winters, hot humid summers) — the American Southeast, southwest France. Oceanic temperate (mild winters, cool summers, year-round rain) — Brittany, western UK, New Zealand. Moderate continental (cold winters, hot summers) — Burgundy, central Europe, the northeastern US.
For succulents specifically, use a three-dimensional mental model: USDA zone (minimum temperature) + drainage quality (excellent / good / fair / poor) + winter humidity (dry / moderate / wet). An Agave parryi is easy in zone 7 + excellent drainage + dry winters. It will fail in zone 7 + fair drainage + wet winters.
Practical tips for gardeners
- Know your approximate USDA zone. It is your starting point for reading international catalogs.
- Subtract half a zone to be conservative. If you are in zone 8, buy plants rated for zone 7.
- Add half a zone for favorable microclimates. A south-facing wall or urban garden can gain a full zone.
- Consider winter humidity. In maritime climates, choose plants that tolerate cold moisture.
- For exotic plants, estimate your Kira WI. If your WI is below 100, strict Mediterranean plants will struggle even if your winter is mild.
- Assess your Selyaninov HTC (or at minimum, observe whether your summers are dry or wet). This is the third decisive filter for succulents.
Frequently asked questions
My plant is labeled “Zone 8” but I am in zone 7. Can I try it anyway?
Yes, with precautions. Plant it in your warmest microclimate (south-facing wall, well-drained soil). Protect it for the first two winters with fleece and thick mulch. Many plants survive half a zone below their official rating if local conditions are optimal.
Is the USDA zone a guarantee of success?
No — it is solely an indication of winter survival, not of thriving. A plant may survive 14°F / −10°C but never bloom if summers are too cool, or decline if the soil stays too wet. That is exactly why Kira (summer heat) and Selyaninov (heat/moisture ratio) are essential complements.
What is the difference between 8a and 8b?
Exactly 5°F / 2.8°C in average minimum temperature. Zone 8a: 10 to 16°F / −12 to −9°C. Zone 8b: 16 to 21°F / −9 to −6°C.
Should I use USDA, Sunset, RHS, or Kira?
USDA for reading international catalogs and as a universal reference. Sunset if you garden in the western US. RHS if you consult British sources. Kira if you grow exotic plants and need to understand the “summer heat” dimension. The USDA remains the most practical first filter, but Kira is a valuable complement for succulents and cycads.
Have zones changed with climate change?
Yes. Between 2012 and 2023, roughly half the US gained half a zone. In Europe, the trend is similar: winter minimums are on average 2–3°F / 1–2°C higher than 30 years ago. But extreme events remain possible.
Glossary of essential terms
Half-zone (a/b) — Subdivision of each USDA zone representing 5°F (about 2.8°C). The “a” is always colder than the “b.”
Hardiness — A plant’s ability to withstand winter cold and survive from year to year in a given climate.
Microclimate — A local area whose climate differs from the surrounding environment (south-facing wall, valley bottom, urban garden).
Warmth Index (WI) — Kira’s index measuring cumulative annual heat above 41°F / 5°C, expressed in °C·months.
Coldness Index (CI) — Kira’s index measuring cumulative annual cold below 41°F / 5°C.
Hydrothermal Coefficient (HTC) — Selyaninov’s index measuring the ratio of precipitation to heat during the growing season.
ΣT10 — Sum of active temperatures above 50°F / 10°C, the central tool of Soviet and post-Soviet agronomy.
Vernalization — A period of cold required by certain plants to induce flowering.
Drainage — A soil’s ability to evacuate excess water. Critical for desert and Mediterranean plants.
Continental / Oceanic / Mediterranean — Climate types. Continental: cold dry winters, hot summers. Oceanic: moderate temperatures, distributed rainfall. Mediterranean: mild wet winters, hot dry summers.
Urban heat island — The phenomenon where cities are 2–5°F / 1–3°C warmer than the surrounding countryside.
Conclusion: the art of nuance
Hardiness zone systems are precious tools, but they will never replace observation, experience, and common sense. A plant is not a machine that operates between 14°F and 5°F and shuts down abruptly at 4.9°F. It is a living organism influenced by dozens of interacting factors.
The USDA system is an excellent starting point — universal and simple. The Sunset system reminds us that climate is multidimensional. The RHS system teaches us that practical experience in a specific climate matters more than abstract numbers. The Kira system reveals the hidden dimension of summer heat. The Soviet Selyaninov system forces us to think about the heat-to-moisture ratio. The Chinese and Indian systems show us that half of humanity does not think about climatology in terms of winter minimums.
For the gardener growing exotic plants, the most effective strategy is to combine:
- The USDA as a common language and first winter filter
- Kira to understand available summer energy
- Selyaninov to assess the heat-to-moisture ratio
- Indicator plants to read the landscape around you
- And above all, your own experience in your own garden — the best laboratory there is
Zones give you a probability of success. But gardening is not pure statistics. It is an art that blends science, intuition, and acceptance that nature always retains an element of mystery and unpredictability. And that is precisely what makes gardening so fascinating.