Hardiness zones explained: what your zone doesn't tell you

A plant label that reads “hardy to zone 8” tells you one thing and lets you assume the rest. The zone is a winter figure, the average of the coldest night each year, and the label says nothing about whether the summer that follows is a cool coastal drizzle or a desert furnace. Two gardens can sit in the same zone and grow almost nothing in common.

Phoenix and New Orleans make the point. Both land near the warm end of USDA zone 9, so both promise the same mild winter, and a catalogue sorted by zone alone would offer them the same plants. One is a low desert that sees under 250 mm of rain a year and bakes through weeks of extreme summer heat; the other is wet subtropics that takes 1,600 mm and sits under warm ocean air all summer. A date palm ripens its fruit in one and rots in the other. The zone they share is the half of the climate that happened to be easy to measure.

What a hardiness zone measures

The United States Department of Agriculture built its map around the average annual extreme minimum temperature, the coldest reading of a typical winter averaged over thirty years. It divides that figure into bands ten Fahrenheit degrees wide, numbered 1 at the frozen end to 13 in the frost-free tropics, and halves each band into an “a” and a “b” of five degrees. Zone 8 bottoms out around minus 12 to minus 7 deg C. Zone 5 reaches minus 29. The 2023 revision shifted a good part of the United States half a zone warmer than the previous map, which is the warming climate showing up in the one number gardeners already watch.

The Royal Horticultural Society runs a parallel scale for a European readership, rated H1a to H7, and it does the tender end better. Where the USDA folds everything above about 4 deg C into its top three zones, the RHS scale splits that range into a heated-greenhouse band, a cool-greenhouse band and a just-frost-free band, which matters far more to a grower of tropical plants than to a grower of oaks. At the hardy end the two agree closely, so an H5 plant good to around minus 15 deg C is a zone 7 plant, and an H7 plant is anything that laughs at minus 20.

Both scales measure winter cold and stop there. That is the right thing for them to measure, because for most plants in most temperate gardens the coldest night is what kills, and a single number that predicts survival is worth having. The mistake is reading it as a description of the climate rather than as one line of it.

What the number leaves out

Summer heat is the first omission, and it has its own map. The American Horticultural Society answered the gap with a heat-zone scheme, twelve zones counting the number of days a year the temperature climbs above 30 deg C, from under one in the coolest to more than 210 in the hottest.

A lettuce or a delphinium that sails through a zone 8 winter can still be cooked out of a zone 8 garden if that garden also sits in a high heat zone, and nothing on the cold map would warn you. Heat stops growth as surely as frost, it just does the damage at the other end of the year.

Water is the second, and the larger. Rainfall, when it falls and how fast it evaporates decide as much as temperature does, and a cold map is silent on all three. A Mediterranean hillside and a temperate meadow can share a winter minimum while one bakes dry from May to September and the other never dries out at all. The plants that suit them have opposite habits, and a gardener who reads only the zone plants the wrong half.

Then there is the length and shape of the season. A plant needs a run of warm days long enough to flower, set seed or ripen fruit, and two places at the same winter minimum can offer very different runs. The far side of this is winter chill. Many temperate fruit trees will not flower properly until they have banked a set number of hours below about 7 deg C, so an apple or a peach fails in a mild subtropical winter, not from cold but from the lack of it. Humidity and wind fill in the rest, drying a leaf out or keeping it wet long enough to rot.

Koppen, the map of the whole climate

Wladimir Koppen, a botanist before he was a climatologist, drew the boundaries that still carry his name by following the vegetation. Where the plants changed, he reasoned, the climate had changed, so his classes are cut to lines a grower already cares about. The modern version sorts every place on land into five broad groups by temperature and rainfall together.

Group A is the tropics, every month warm and the year defined by when the rain comes. Group B is the dry world, the deserts and the steppes, where evaporation outruns rainfall. Group C is the temperate middle, mild winters and warm summers, split by whether the dry season lands in summer or never comes at all. Group D is continental, cold enough for a real freezing winter but hot in summer. Group E is polar and alpine, too cold in the warmest month for trees. A second and third letter refine each one, marking a dry summer, a monsoon or the harshness of the winter. Read your zone for the winter and your Koppen letters for the broader climate, and together they tell you far more about what a garden can grow than either can alone.

The wet tropics

Every month is warm and wet, frost has never been recorded, and the limit on what grows is not cold at all. This is USDA zone 11 and warmer, RHS H1, the climate of lowland Malaysia, the Congo basin and the Amazon. The plants that built our fruit bowls and spice racks come from here, and almost none of them will take a single frost. Coconut, breadfruit and black pepper are the pattern, tropical to the core, killed outright the first time the temperature drops near freezing and slowed to a stop well before it gets there.

Banana, Musa acuminata, shows how narrow the margin is. Its growth stalls below 15 deg C, the leaves die in frost, and only the hardier types survive winter as an underground corm. Grown outside the tropics it is a foliage plant that pretends at fruit, because the season is never long or warm enough to ripen a bunch. What all these plants ask is heat without a pause, and the one thing a cool climate cannot fake is a winter that never happens.

The seasonal tropics

Move away from the equator and the tropics gain a dry season. The rain arrives with the monsoon as the tropical rain belt swings overhead in the warm months, then retreats, leaving months of clear dry weather. This is savanna and monsoon country, Koppen’s Aw and Am, and its plants are built around the pause in the rain rather than around cold they will still not tolerate. Mango and papaya fruit on this rhythm, and a mango in particular needs the dry spell to flower well, which is why it disappoints in a wet equatorial climate that never gives it one. Ginger does the opposite, dying back to a rhizome through the dry months and driving up new shoots when the rain returns.

Deserts and dry country

At around 30 degrees north and south the air of the tropics, having risen and dried at the equator, sinks back to the surface and warms as it falls. Sinking air makes no rain, so the world’s great hot deserts sit in a band at that latitude, from the Sahara to the interior of Australia. Koppen’s group B is defined by the shortfall of water against what the sun evaporates rather than by heat, which is why it takes in cold high steppe as well as the burning sand. The gardener’s version is a wide swing between a scorching day and a cold night, dry air, and cold that is only survivable while the plant stays dry at the root.

The plants read as tough because they are storage vessels. Agave, prickly pear and aloe vera hold water in swollen leaves or pads and spend it slowly, and all three take a few degrees of frost as long as the soil is bone dry, then rot in a wet cold that a hardier-looking plant would shrug off. The date palm is the classic desert crop, taking a light frost at the root while needing sustained heat above 30 deg C in the air to swell its fruit, which is exactly what a desert oasis provides and a mild damp climate does not. The pomegranate is the most forgiving of the group, dormant-hardy to about minus 12 deg C, though it wants the long hot dry summer to ripen all the same.

The dry-summer Mediterranean climate

The same sinking subtropical air that makes the deserts drifts a little further towards the poles in summer and parks over the western edges of the continents, shutting off the rain for months. In winter it retreats, the storm track slides back over, and the rain arrives. A climate with its wet season in the cool half of the year and its drought in the heat of summer is Koppen’s Cs, and it is rare, covering the Mediterranean basin, coastal California, central Chile, the western Cape and parts of southern Australia. The plants that evolved for it share a look, small tough grey leaves that cut water loss and often carry aromatic oils.

This is the home of the herb garden. Rosemary, thyme, lavender and bay laurel all come from these dry stony hillsides, and every one of them wants sharp drainage and hates a wet winter far more than a cold one. The olive, fig and grape vine are the crops of the same ground, deep-rooted enough to find water through the summer drought and hardy in the wood to around minus 10 to minus 15 deg C, though a late frost on the new growth undoes a year. Grown in a cool wet maritime climate they survive the winter easily and then sulk, because the summer they were built to bask in never turns up. The lemon marks the tender edge of the group, damaged below about minus 2 deg C and needing a frost-free winter, which is why it lives in a pot that comes indoors wherever the winters bite.

Humid subtropical

The eastern sides of the continents at the same latitude get a different deal. Warm ocean to the east feeds moist air inland all year, so the summers are hot and humid and the rain keeps coming, with only a mild winter to break it. This is Koppen’s Cfa, the climate of the American Southeast, southern China, southern Japan and much of eastern Australia, and it grows the broad-leaved evergreens that want warmth and moisture together. Camellia and the tea plant, Camellia sinensis, are natives of exactly this belt in East Asia, hardy to around minus 10 deg C and happiest where the summer is long, warm and reliably moist. Star jasmine fills the same climate niche as an evergreen climber. These are the plants a Mediterranean gardener struggles with, because the dry summer that suits an olive starves a camellia of the moisture it expects in its growing season.

The maritime temperate belt

Water heats and cools far more slowly than land, so a coast washed by a mild ocean current and a prevailing onshore wind gets a climate with the corners knocked off, cool summers, mild winters and rain spread through the year. This maritime temperate climate, Koppen’s Cfb, is the one Britain shares with the Pacific Northwest, coastal Chile, Tasmania and both islands of New Zealand. The winter is gentle for the latitude and the summer is modest, so the plants that thrive are the ones that ask for a long cool growing season rather than a burst of heat.

Rhubarb is the emblem, a plant that wants a cold winter to break dormancy and a cool moist summer to bulk up, and it grows better in this belt than almost anywhere. Apple crops reliably here for the same reason, taking the winter chill it needs and never being asked to survive a heat it cannot. The wild flora tells the same story: bluebell, foxglove, heather and lupin are woodland, heath and hillside plants of cool damp country, fully hardy and untroubled by a summer that never gets going. The catch in this climate is a winter wet that never lets the soil drain, which is why so much of the advice comes down to grit, raised beds and standing plants where the water runs off.

The continental interiors

Away from the moderating ocean, the swing returns. The interiors of North America and Eurasia, from the Canadian prairies through Ukraine to northern China, freeze hard in winter and turn hot in summer, because there is no nearby sea to hold the temperature steady. This is Koppen’s group D, USDA zones 3 to 6, and its plants are the ones that treat a savage winter as a normal part of the year and often depend on it.

Lilac, tulip and snowdrop are continental plants that need the cold to perform, the bulbs to trigger their spring flowering and the lilac to set its buds, so all three flower poorly in a mild winter that never delivers the chill. The apple belongs as much here as in the maritime belt, since its chill requirement is met with room to spare. What these gardens gain over the mild coasts is a hot reliable summer that ripens wood and fruit; what they lose is every tender thing, because a plant that dies at minus 10 deg C has no future where minus 30 is an ordinary January.

The far north and the high mountains

Push north into the boreal zone, or up a mountain, and the summer shrinks until only the toughest trees can complete a year. This is the taiga, Koppen’s Dfc, and the cold-hardiness on show is extreme. Silver birch, scots pine and rowan push further north and higher up than almost any other trees, the pine taking around minus 40 deg C and the birch growing at the very edge of where a broadleaf can live. Norway spruce is the timber tree of the same cold forests. Above the tree line the woody plants give out and the survivors are low, slow alpine cushions. Common houseleek and yellow gentian come from the screes and pastures of the Alps and Pyrenees, built to sit under snow all winter and pack a whole year of growth into a short bright summer. What defeats an alpine in a lowland garden is rarely the cold. It is the wet, mild, sunless winter that rots a plant adapted to a dry frozen one.

Day length, which no climate map shows

Latitude sets the length of the day, and temperature and rainfall have nothing to do with it. On the equator every day runs close to twelve hours the year round. Move towards either pole and the summer day stretches while the winter day shrinks, the gap widening the further you go, until a midsummer day at fifty degrees of latitude reaches around sixteen hours and its midwinter counterpart falls below eight. A plant that measures the day to time its own growth will behave differently at two places that share an identical climate but sit at different latitudes.

Onions are the textbook case. An onion starts to form its bulb only when the day passes a set length, and seed catalogues sort the varieties by that trigger. Long-day onions need days of about fourteen to sixteen hours, so they bulb only in the long midsummer of high latitudes. Short-day onions swing into bulbing once the day reaches ten to twelve hours, which in the subtropics comes over a mild winter. Plant a long-day variety near the equator and the day never grows long enough, so the plant makes leaf after leaf and no bulb; plant a short-day variety into a high-latitude summer and it bulbs while still tiny, on a handful of leaves. The onion that gets this wrong fails from the angle of the sun rather than from any heat or cold.

Garlic answers to cold rather than to light, and it is strict about it. A clove will not divide into a fresh head of cloves until the plant has sat through a cold spell, so garlic planted into a winter that stays warm swells into a single undivided round instead. That one requirement splits the crop in two. Hardneck garlic, the variety ophioscorodon, is built for a hard continental winter, throws up a curling flower stalk called a scape, and carries the boldest flavour while storing the worst. Softneck garlic, the variety sativum, asks for far less chill, suits a mild maritime or subtropical winter, plaits well and keeps for the best part of a year, which is why it fills the supermarket shelf. A grower in a warm region who plants hardneck lifts a crop of mean little single-clove bulbs, and a grower in a cold continent who plants softneck can lose the row to a winter it was never hardened against.

Day length pulls other levers too. Spinach and many lettuces bolt to seed as the days lengthen past their threshold in early summer, which is why they crop best from a spring or an autumn sowing and run straight up from a midsummer one. Chrysanthemums and poinsettias work the other way, holding their flowers back until the autumn nights draw out, and a nursery that wants them blooming for a fixed sale date fools them into it under blackout screens. None of this shows on a hardiness map or a rainfall chart, and all of it moves with the line of latitude you garden on.

The cultivar settles the rest

A species places a plant in a broad climate band, and the cultivar fits it to a region. Within one species a breeder can shift the very things the maps measure, so two plants that share a botanical name can want quite different worlds, and the name on the label is only half the instruction.

Chill is the clearest example. Low-chill cultivars of peach and apple have been bred for warm-winter regions where the standard sorts never bank the cold they need to flower. A low-chill peach may set fruit after only a hundred to two hundred hours below about 7 deg C, against eight hundred or more for an old northern variety, which lets it crop in a subtropical garden that would leave its parent standing bare in spring.

The same logic runs through every other limit. Onion seed sold as long-day, short-day or intermediate is a latitude instruction wearing a marketing label. Short-season cultivars of sweetcorn, tomato and maize ripen in sixty to seventy days, so they beat the frost in the brief summers of high latitudes and high altitudes where a full-season variety would be caught green. Others are picked for heat, like the slow-to-bolt lettuces and the tomatoes bred to set fruit through tropical nights, or for a disease that a particular region carries and a neighbouring one does not.

The seed catalogue spells all of this out in its own shorthand, the day-length class, the chill rating, the days to maturity, the heat rating. Read those the way you read a hardiness zone, because between them they decide whether a plant that suits your climate on paper will suit your garden in the ground.

Reading your own site

Find your USDA zone or RHS rating first, since the coldest night is still the line most plants die on, and a plant rated below your zone is a gamble every winter. Then find your Koppen type from any of the high-resolution maps, and let it answer what the zone cannot, whether your summer is wet or dry, hot or mild, or long or short. A plant suits your garden when it clears both, and most disappointments come from a plant that passed the cold test and failed a heat, drought, chill or day-length test nobody ran, or that was the wrong cultivar for the region.

Your own plot then edits the map. A south wall in the northern hemisphere, or a north wall in the southern, holds enough stored heat to carry a plant a zone warmer than the open garden. A frost pocket at the bottom of a slope runs a zone colder, because cold air is dense and flows downhill to pool. A city runs warmer than the countryside around it, a raised bed drains where heavy ground drowns, and a hedge that grows up over a few summers turns a bright border into a shady one without anyone connecting the change to the plants that started to fail. Match the plant to the zone, the climate type and the corner of the garden you have, choose the cultivar bred for your region, and the label that promised one thing starts telling you the rest.