The zones are US Department of Agriculture zones, so they really only apply to the US. The Zones are based on minimum winter temperatures.

On the basis of minimum winter temperatures, we fit in a Zone 8 or 9, but we can't grow their Zone 8 or 9 plants because of our lower summer temperatures and shorter growing season.

Our temperatures are governed more by the Gulf Stream which keeps us warmer than we would otherwise be this far north. We also have lower light levels than most places in the US.

There is another system which works on temperatures, and I think that makes us a Zone 4 or something.

When we were allowed to swap seeds with people in the US, I often found they wouldn't want to swap if they were in a Zone 4, or if they were in a Zone 8, they wanted to send me palms and citrus. So I don't use the zone system any more.

Almost all of the UK is zone 8. Upland areas of the Pennines and Scotland and are zone 7. SW England, parts of the South coast, the centres of major urban areas of England, Western Wales, lowland Western Scotland including the Western Isles, most of Northern Island, NW England between the hills and the coast, and some parts of East England right on the coast, are all zone 9, average minimum winter temperature above 20F or -6.7C.

Here's a UK zone map.

As to Crazylady's points, the zones are based on winter minima, and apply everywhere, not just the US. The problems she details refer to the much hotter summers of the eastern US; the zones in Britain correspond very closely in growing season conditions to the same zones in the Pacific Northwest (Oregon, Washington, southern Alaska, and also British Columbia in Canada).

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Resin

Thanks for all the info guys, it will make it a lot easier for me to work out whether I can grow the plants that our American members are talking about.

"Here's a UK zone map"

That should of course read "Here's a UK and Ireland zone map"

Resin

What american members?

On the subject of zones, would any Londoners out there know how far the "zone 9" area extends? I'm in Streatham, south London, and it's several degrees colder than central London. We got several hard frosts last winter. (Apologies for asking this question to the vast majority of you who live nowhere near London and couldn't care less about its climate - I do appreciate that it's not the hub of the universe!)

Its really easy to know whether you are zone 9 or not. Did it get down below 20F (-6.7C) last winter, even on just one night? What about the winter before? And before that? If it more often than not stays above 20F for the whole winter then you are zone 9. If it more often than not gets below 20F at least once during the winter then you are zone 8. If you are on the margins of zone 9, as you probably are, the results might vary depending on just which years you include in your measurements.

Of course as they always say: "past performance is no guarantee of future performance"! Most of the country has had warmer than average winters over the last ten years, and that trend may or may not continue. Last year's "cold" winter was the coldest for ten years (for most places by most measures) yet was still warmer than the longterm average. Next winter may be like 1995/6 or 1981/2, or god forbid 1962/3, and all your lovely zone 9 plants will be zapped into a big plant graveyard.

And of course there are other factors to take into consideration, such as siting of plants, and microclimates. The conditions within one garden can vary considerably. The shelter of a warm house wall, or indeed of other plants, can often enable more tender plants to thrive.

You're absolutely right Ornata. Americans tend to be obsessed with zones to the exclusion of all other factors important to planting success. I've had online conversations where basic stuff like acid/alkali soil was ignored because 'hey, I got the right zone'.

I'd hate to see the UK get as zone-centric as the US. It's a half-decent idea but local micro-climates are SO much more significant.

Cytania, how cold it gets in winter is critical when you live in a country with both frost free locations and tundra. One number gives you 90% of the information you need to know if a plant will live through your winter. Why do people always have to knock that? Its information and as such is useful.

Since you live in a country with only a few degrees difference in winter temperatures for all main population centres, with microclimates perhaps more important even than which end of the country you live at, you may not ever feel the need to define how cold your winters are. We are spoilt by the simplicity that "frost hardy" will survive, "not frost hardy" will die. But please spare a thought for people who need to know, by a simple widely understood method, whether a particular plant will freeze to death or not in their back garden. Do you know whether a Brunnera is cold-hardy in Cleveland?

Well-said, shrubs. Folks are a bit obsessed with zones over here because there is need to be. I don't suppose any gardener anywhere imagines zone to be the only factor in gardening. However, it is true in my experience that micro-climate variation is poorly noted.

Not only do I not know if a brunnera is cold-hardy in cleveland, I don't know what a brunnera is. I do know that the imported white cabbage-moth (for instance) runs rampant in the mid-atlantic, probably from about zones 8 up to about 4.5. The beastie can't survive northern Vermont winters and does not establish in most of Florida. A great many other bits of useful info can be associated by zone.

I should think it might be useful to increase the zone-scale for an area such as the UK. How about adding decimals? Perhaps northern scotland or the pennine elevations would be high sevens, the midlands mid eights, cornwall mid nines, for example. 7.5 thru 9.5 gives twenty graduations - twenty zones.

To respond to ornata, I moved last year from from inner north London (Kilburn) to slightly more outer north London (Muswell Hill) and I strongly suspect that I have moved from zone 9 to zone 8. Indeed, in anticipation I left some of my more tender plants behind with the new owners of our old house. Yet the agaves and yuccas that I did move survived perfectly well in a supposedly "hard" winter - because it remained dry. As I understand it, the zoning system is not so helpful here in Britain because the key factor for many marginal plants in our climate is the damp more than the cold.

I'm not sure that zone decimals are helpful. Certainly they could be calculated, the climate data exists to assign a zone based on 1F intervals rather than 10F intervals. But the natural variation in winters, the duration or otherwise of the cold, the presence or absence of snow cover, etc. mean that the data would be more precise than useful. Even the existing a/b zone tags for 5F intervals are only partially used. I like to think that this is because people on the whole come up with the right answer in their own various ways and that their answer is that a 10F zone is a useful indicator while anything finer is not so helpful.

I do wonder if many people in the UK really appreciate how much climate variation does exist across our small country, even across just England. Rainfall varies from barely 20" to about 40" in lowland populated areas, and to more than 100" in west-facing hilly areas. Winter lows vary from barely freezing in extreme western areas to -10C in central lowland areas, and far below that in upland areas. Summer highs are 5C-10C less in northern England than in inland areas of the south, and even less in Scotland.

"well in a supposedly "hard" winter"

Worth noting that the 2005/06 winter was the coldest in most of Britain for over 10 years, but was also slightly warmer than the long-term average.

Whether we will ever get another really cold winter like 1981/82 in northern Britain, or 1986/87 in southern England (-7°C on Scilly), or 1962/63 for old-timers, again must be open to significant doubt.

Resin

One hundred inches of rain! Wow. That would make it temperate rain-forest, wouldn't it?

I was always surprised at the relatively small annual precips recorded in Gilbert White's journal (low 30's seemed to be average), as england is generally thought of as a wet climate. Those numbers would be considered drought years in the eastern US. Southern england must also have been much colder then, because his records of winter temps are very similar to what we experience now in southern new england.

Parts of the western highlands of Scotland and the Lake District get rainfall of over 400cm/year; yes, they do count as temperate rainforest (well, deforested temperate rainforest, sadly).

Conversely, Cambridge in the east of England only gets about 65cm/year.

Gilbert White was living during the 'Little Ice Age'; southern England was probably zone 7 back then. A particularly severe winter in 1740 was recorded as killing nearly all the Cedrus libani in Britain, which probably means a fairly widespread -27 to -30°C (as that's the temperature this species suffers significant mortality). That winter "the Thames was frozen for about eight weeks, and Thames shipping and London Bridge were damaged considerably by the ice"; "deep snow fell about Christmas in Norwich, which remained on the ground until March".

The 1715/16 winter was similarly brutal, also with the Thames frozen for 2 months with ice thick enough to hold a 'frost fair' on the river; "ice on Thames in London was lifted by 4.3m by a flood tide but did not break".

Resin

low 30's seemed to be average

That's still a fairly standard rainfall for most lowland areas of England. The reason England is "wet" with such rainfall is down a number of factors: consistency of rainfall throughout the year with a completely dry week being somewhat unusual; moderate temperatures with the average high around 70F in the middle of summer; relatively low solar insolation in summer since the whole country is further north than anywhere in the contiguous states and cloud cover is high in the marine climate; cool humid winters with very little sun mean that all soils are generally saturated by the end of every winter; and aquifers, rivers, and lakes are well fed from winter rainfall and from extra rainfall on high ground.

Though the short winter days and lack of ensolaration in the winter can't be much fun, the consistent year-round moisture and generally fertile soils must make the british isles amongst the best places in the world for gardening.

I once read an amusing account of some of the first settlers on Cape Cod (it wasn't meant to be amusing, but is so to someone familiar with gardening in this area). It was their first spring here, and when one of the typical early spring heat-waves came along, they figured the climate was the same as back home. They planted all the crops in early march! The amazing part was that most of the stuff germinated and even produced (even the maize), because it happened to be one of those 1-out-of-50-year warm springs. In subsequent years they nearly starved to death.

I guess that was before the 'little ice age'.

Of course, shrubs_n_bulbs has nailed it--it's most important for growers/gardeners to know the lowest Winter temperature in their area, and for suppliers (who sell plant material) to note the lowest (Winter) temperature at which their offerings will survive. Voila--USDA zones. Their are companies in America (and, no doubt, other countries) that have disclaimers as to the (lowest) zone some plants will survive. I'm thinking specifically of Edible Landscaping, in Virginia, who sells Figs, but will not guarantee their Winter-hardiness in zones 7 and below.(Don't expect a replacement if the Fig you bought from them is Winter-killed in zone 7.) On the other hand, I have attended many hundreds of farming and gardening seminars in the U.S., where successful commercial growers spoke (generally) on crop-specific subjects,or, (sometimes), on their entire operation; after their talk, when I asked them their zone, and their average annual rainfall, the MAJORITY did not know their zone, and some did not know the average rainfall for their area. I found it rather amazing.
cytania: Think Americans are zone-obsessed? Google "Sunset Climate Zones"; (for those who think a simple lowest-Winter-temperature is not nearly enough information to be included in a zone designation).

P.S. Google "gardenerguidezonemap" (all one word, minus quotation marks), for an explanation of ALL U.S. zone maps, and the necessity for their creation. There is a "Heat Zone Map", (officially named American Horticultural Society Plant Heat Zone Map) commonly used (by growers) in the U.S., that notes the average number of days, annually, for a given region, when the temperature exceeds 86 degrees (F), or 30 degrees (C). This is an important consideration for growers/suppliers in the U.S., also: Example: I'm in USDA zone 7, Maryland, the mid-Atlantic region of the U.S. If our highest (Summer) temperature here exceeds 90 degrees (F), 32 degrees (C), for more than 5 days, it's termed a "heat wave". I have a grower friend in Arizona, 3,000 kilometers West of Maryland, smack dab in the middle of the desert, also in USDA zone 7. It is not unusual for his highest (Summer) temperatures to reach 120 degrees (F), 49 degrees (C)! Obviously, though many plants can tolerate our lowest (USDA zone 7) temperatures, few can tolerate his highest temperatures. I wonder how many plants in the U.K. could tolerate 49 degrees (C)? By the way, for those who have not experienced 49 degree (C) heat, or been in the "desert", I am reminded of my first excursion there, many years ago, west of Albuquerque, New Mexico. I took a Summer day-trip into an area to photograph Jack Rabbits (a large hare, nearly a meter in length, which, at the time, I had never seen). I was duly warned by locals to be extremely careful, not to remain for an extended period, and not to wander from sight of my vehicle (for fear of becoming disoriented, and lost). I listened, but disregarded the advice--and later, became somewhat disoriented and (temporarily) lost, when I lost sight of my vehicle. I was a smoker, at the time, and remember my cigarettes snapping like twigs, when the 49 degree (C) heat removed all moisture from the tobacco, and the cigarettes became as brittle as dried twigs. My tongue swelled to nearly twice its size when all saliva left my mouth, and I found it difficult to talk. I believe the extreme heat was the cause for my disorientation--that, and the fact that the landscape looks completely the same, North, East, South and West, and if one loses sight of a particular landmark (like a vehicle), one has no idea from which direction one came. The only plant life I remember were knee-to-waist-high shrubs that we call "tumbleweeds" (once they're dislodged from their roots and proceed to tumble across the desert floor). "Mirages" abounded--strange visions, caused by extreme heat. The only wildlife I saw were Jack Rabbits, Coyotes, and Sage Grouse, all of which I flushed in my wandering (searching for my vehicle, that I had lost sight of); I thank the Lord that I didn't stumble upon any Rattlesnakes--always a clear and present danger in the desert Southwest. Where desert dwellers find water to drink, I have no idea; I saw none.

Yes, it's true - north america is a good deal more varied than the british isles. Even the parts most comparable have much more extreme climate. And so perhaps the concept of zones is not very useful for britain.

I wonder how many plants in the U.K. could tolerate 49 degrees (C)?

I have some :)

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Goodonya', shrubs_n_bulbs, and a beautiful photo of it, too. Would you be so kind as to disclose its Latin (and common) name? There are so many neat cactii--and in this country, particularly, I am STAGGERED by their numbers (and sizes and shapes and beautiful flowers. Twenty years ago I wandered upon a farm nearby in Laurel, Maryland, that had about 30 greenhouses full of cactii (and other succulents). The man said he had the largest collection of succlents in the U.S., and he shipped them 'round the world. What an assortment! I am completely ignorant of most--is it possible to ever learn the names of them all?
(Getting "long in the tooth" here, still waiting for my Century Plant to bloom.) :=)

Its a Strombocactus disciformis, from Mexico. I'm not aware of any common names. There are perhaps 10,000 latin cactus names, now grouped into a couple of thousand species. I don't have them all :)