Wednesday, 14 January 2009

ABC Wednesday - Z is for...


The United States Department for Agriculture (USDA) plant hardiness zone map, 1990

... Zones - a You Ask, We Answer Guide

If you blog about gardening here in Britain, it's not too long before you encounter your American and Canadian cousins who all talk about which zone they garden in as if it was as natural as breathing. Strike up a conversation with any of these gardeners and it's not long before you're asked what zone is your garden? Luckily when I was first asked this last year, I had just been looking at the above map in The Essential Design Workbook by Rosemary Alexander.

The USDA map is based on winter low temperature records and divides the continent into 11 hardiness zones - zone 1 is the coldest and zone 11 the warmest (plants surviving lows of -45.6 degrees centigrade or below, and lows of just 4.5 degrees centigrade respectively).

Whilst knowing their garden's zone is a useful tool for your American and Canadian friends to understand what they can grow in their gardens, there can be differences based on altitude, soil type, rainfall, day length, rural or city location etc etc. For example, Austin (Texas) and Portland (Oregon) are both nominally in Zone 8, but their local climates are very different. This also holds true when trying to place a British garden into these zones. Based on latitude alone, we should be gardening in relatively chilly zones 2 to 4 like they do in Canada. However, our maritime climate and proximity to the warming Gulf Stream puts us in the warmer zones 7 to 9.

The limitations of the USDA classification are recognised and consequently a couple of other maps were developed. The American Horticultural Society (AHS) uses highest summer temperatures for their Plant-Heat-Zone map and for western US gardeners there's the 24 zone Sunset map, named after the magazine in which it was first published. This factors in the length of growing season, humidity and rainfall patterns in addition to the winter lows and the summer highs used by the USDA and AHS. I believe this information is used for plant labelling and by plant suppliers - perhaps one of my American or Canadian readers can confirm this? I haven't been able to find AHS or Sunset equivalent maps for outside North America, but this link shows the USDA equivalents for Europe, if you're interested.

I've put my garden into zone 8a (temperatures down to -12 degrees centigrade) and whilst I'm happy with my conclusion, I'm still a bit cautious in quoting it too extensively. My garden rarely goes below -10 degrees centigrade (even this year!), but my south-western England winters do tend to be wet and I also garden on clay, a relatively cold soil. In my experience this combination is lethal for both tender plants and cold tolerant alpines, neither of which like a wet winter. Therefore I grow these kinds of plants much closer to the house - at the top of the slope (drier) and by my garden or house walls (warmer). Thus I'm using my garden's microclimates to ensure I can grow a wider variety of plants without having to give them much in the way of special treatment. That said, the recent weeks of very cold weather will surely test the antarctica portion of my tree fern's latin name!

Of course we don't have the USDA, AHS or Sunset information on our plant labels or at the garden centre. Here in Britain we're more used to the terms tender or half-hardy (protect from frost), hardy and fully hardy (tough as old boots) when choosing which plants to buy and I suspect many of us make some allowances when buying. For example, I shouldn't be able to overwinter half-hardy plants in my garden, but I know that by planting half-hardy fuchsias in the walled beds or keeping them by the house, most of them will survive. I also know this doesn't apply to my front garden as it's north facing. The RHS has devised a more detailed hardiness categorisation for the UK, which can be found here and you'll see it quoted in the RHS Plant Finder and on some plant labels.

I'm off now to have a look at my tree fern to see if I was a bit optimistic in not wrapping it up for the winter. Fingers crossed my dahlia duvet has worked too...

Do check out the ABC Wednesday blog for more entries on the theme of Z. I hope this piece hasn't been too much of a Zzzzzzzzzz for you!

29 comments:

  1. I have seen those zone things lots of times in my Daddy's seed catalogs. Very clever to use that for z! Way to go!

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  2. VP, you've outdone yourself again with this very informative post. When I first started reading blogs from the UK, I was amazed by the plants you could grow--like your fuschias. It took me awhile to realize the difference in climate. Although I can grow many plants in my zone 5 garden, anything labelled for higher zones would never survive our winters, especially this week!

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  3. I'm in Zone 7;) which sometimes acts like zone 8 or 11 this past summer. And on Friday, it's going to be a zone 5. I can be assured of one thing here in NC. It will never be predictable.

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  4. You explained our hardiness zone map as well as a U.S. master gardener. Hey, guess what, we are both zone 8a. I have another Z for you. Zexmenia. Yes, you can grow it. Great post.

    PS, Anna is right, our whole country is pushing their hardiness zone designations this week.

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  5. Thanks for that, I refer to these zones too but didn't always understand what they were. Very useful.

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  6. thanks for the education. i learned something about gardening today! so it is important to know your garden zone so that you can grow the right plants in your garden. i do love plants and i am hoping to grow mine one day!

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  7. VP I hate to contradict you, but I'm sure there is a nursery in West Scotland that does use zones. I have bought some plants there but I'm not dressed for dashing out in -1C to have a look (and neither plant is close to the house) I'm also sure that Jekka McVicar uses zones in her herb book and I'm sure she has a map of UK zones - again can't get access someone is asleep in that room and he's just finished a night shift. So I'll get back to you with answers. (I'm sure I figured that I'm zone 5 or 6 - although that isn't taking account of North facing, shade etc. I can see I'm going to have to look at this again! But I'm on the edge of the Highlands of Scotland so I think I've got this wrong - may have to go out and cover some plants :-(
    Michelle in Scotland

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  8. Such an informative post VP thanks.
    I could never really get my head around the Zones - because I also garden in a "micro-climate" living as I do on the (wet) west coast.
    But your post has helped.
    K

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  9. Michelle - well spotted. And it's great you've given extra information, that's what blogging and commenting's all about.

    I've just had a look at Jekka's book and she does indeed give the USDA zones for each herb for American readers, but I didn't find a map there. If I remember corectly some of the RHS books get republished by the AHS in the States, so it would be interesting to see if other RHS books have zonal information in them too - I'll have a look.

    Jekka also gives a good explanation of the hardiness terms we're more used to:

    Half-hardy (or tender) - can withstand short term periods of temperatures down to freezing point

    Hardy - -5 to -15 degrees centigrade (23 to 5 degrees fahrenheit)

    Fully hardy - below -15 degrees centigrade

    She also makes the point that frost dates are a very important factor in deciding when to plant out annuals and for harvesting perennials and annuals. I'll see if I can find some information on this - I think one of Caroline Foley's allotment books has the information, but it's buried in an enormous pile of books at the moment.

    Scotland does have the widest range of zones in the UK - 7 to 9 according to the map. I suspect summer daylight hours also makes a lot of difference as will frosts and slope aspect too. So maybe the range of microclimates is even wider in Scotland?

    So thanks Michelle, for making me go and find out and think a bit more!

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  10. This is a great post. I've experienced the same problem in trying to relate my garden to a US zone. As you say, while it's relatively mild in the southern UK, we don't really get sustained high temperatures and that makes a big difference. And then we get a winter like this one and it all goes out the window...

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  11. LOL! Zone 7a/b here. ;-)

    I wonder if by trying to place things in such tidy categories we don't lose a lot of local information. As you rightly point out, the zones are only a jumping-off point, and there are so many factors others than just the temperature of the overall climate. Not sticking to zones might help you become more tuned in to a specific place.

    I ignore a lot of the zone info, just as I tend to ignore the "full sun"-type info found on labels. Our sun is so bright that often things labeled F/S need to be in the shade...

    On the other hand, if you are a beginning gardener, then this type of info is enormously helpful in understanding why you can't expect some plants to overwinter, or why those flowers never bloomed...

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  12. Hi VP,
    Excellent choice and very informative. I now know that I'm in zone 6.
    Thank you.
    I have now decided to have a go with this ABC thing. I start in the very end Z. It's in the greenhouse blog
    xoxo Tyra

    THE GREENHOUSE IN TYRA'S GARDEN

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  13. Having looked at the link you gave to zones for the UK, I'd say from experience of gardening in both the north and south of Scotland that we have many more micro zones than the map shows.

    I first discovered the north American zones through the Marth Stewart magazine, of all things, and I began to notice how the gardening articles referred to the zone of the featured garden as a matter of course. I suppose in such a vast area it gives a better frame of reference than using place name.

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  14. thanks for this wonderful post. while microclimates are vital when planting, i found that your & michelle's zone information of immense help in placing garden conversations in context--of giving roots to conversations, if you will :)

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  15. VP,

    What Anna said only I am in Nashville, TN~

    Have a delicious day! (it's my breakfast time, can you tell!) Gail

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  16. We have zones, but you have RHS Colour Charts! I can step outside and know the temperature, but I can't look at a flower color and know what to call the color.

    Cameron

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  17. A brilliant use for your letter "Z" and a wonderful zone map -- thanks!

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  18. Hi VP, not a snorer at all! Quite the opposite in fact. The zone maps here are a broad outline for most of us, we can grow things in microclimates and need to know that crucial wet or dry info too. Sometimes the wet is worse than the cold for many plants, or sometimes they need wet that my slope cannot supply even though the zone is right. Lots of good thoughts here, VP, thanks for combining abc with yawa!
    Frances

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  19. great, I've been meaning to look up the hardiness zone for my area for ages - like you say, you do get asked from time to time.

    I put my garden in zone 8, borderline 9 - we rarely have frosts below -7C, and I garden on free-draining sandy soil so things don't end up soggy and wet all winter (thank goodness). It means things which really ought to keel over and die in winter, like cannas and dahlias, generally make it through without a problem. I haven't quite dared to put my olive tree outside yet - though they're meant to withstand -5C in freely draining soil. Maybe I'll get a sacrificial one and have a try!

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  20. Interesting post and good choice for Z. My country is so small that I can truly say that we have only one zone. So we can use the same plants in Groningen and Friesland like we have in the south of the country. Great Britain is so large that the climate in Scotland differs a lot with that of Cornwall for instance.

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  21. What an interesting post! I didn't know about the American zones, though it makes a lot of sense.

    It would be useful if more information was put on plant labels in garden centres though. Often it's the bare minimum, which is not helpful for those like me without green fingers!

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  22. Thank you so much for this blog! I agree this is not something that we ever talk about here in the UK, but our blogging friends over the pond talk about it as if everyone knows!

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  23. A fascinating post VP. I remember doing a lot of searching for information when I first came across the term 'zone'. It's amazing how much variation there is in weather and climate just in our little island. I imagine although I am to the north of you that you probably have colder winters, as we are more sheltered. On the plus side I am sure that you get less rain. I do hope that your tree fern has come through unscathed.

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  24. As no one answered the question you asked, I'll answer it. Some plant labels do list the USDA Zone information, but most do not. Gardeners must rely on their local nurseries to make the right choices for plants to offer for sale as "perennials." Here in Zone 5, many plants which you would consider perennial are offered for sale as annuals.
    The Sunset zones and the AHS zones are not commonly used in the Midwest. I don't know my zone in either of those measures. I have been tempted to grow things hardy to Zone 6 here, as we've had some warm (by our standards) winters, but this year is demonstrating the folly of that for trees and shrubs. Brr...

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  25. VP
    Sorry - took me longer than I intended to get back to you. Okay the nursery in Scotland that uses zones is garden cottage nursery (www.gcnursery.co.uk) and they even have a map of the uk showing hardiness zones http://www.gcnursery.co.uk/pics/hardchrt.jpg

    The book I have that has the hardiness zones in is Plant Propagation A to Z Geoff Bryant - very useful book for beginners like me!

    Naturally enough the two maps don't agree!

    Michelle in Scotland

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  26. VP
    Sorry I'm trying to work out how to use my google account to comment - it works on other blogs but not yours, I must be doing something wrong!

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  27. Hello - glad you enjoyed this post and thank you for taking the trouble to comment. I've just got some general remarks today rather than individual replies, so I'll be heading over to your places for a visit and comment as well.

    Judging by what most of you have said in the comments (and hinted at in the post itself), the zones are just a starting point, your own local conditions, knowledge and advice from local growers will determine what you really can grow in your garden.

    Thanks to those of you who've provided further information, especially Michelle and MMD - it's much appreciated.

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  28. I was trying to work out what the equivalent zone for me was over Christmas and came up with something like 8 but I think that makes us equivalent to Florida which seemed abit weird. I agree that other things like the never ending rain and our clay soil needs to be taken into account. My Geranium madrense is labelled as tender but has with stood the recent very cold weather and I gave up brining in my Chocolate Cosmos years ago which is also meant to the tender.

    I have noticed more and more gardening books using zones presumably for their US market but it would be good to have a UK equivalent as well

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  29. PG - Florida's a lot warmer than Zone 8. I agree they're a quite hard to get the hang of, especially as we're talking about completely different climate types anyway. Local knowledge is the key to all of this!

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