Sunday, January 11, 2009

Heat zones and hardiness zones


This was the week that temperatures on the balcony dropped to -3°C (26°F) and we had 40 cms (about 16 inches) of snow within 36 hours. That's more than I've seen in Milan for about 20 years - and the first time I've ever seen snow heaped up on the balcony. It is however well within the limits of our hardiness zone.

It seems that every US gardener knows what hardiness zone they're in. Yet the idea is much less common in Europe. It is possible to find out though, and
backyardgardener.com links to maps showing not only the US, but also Europe ( this one from gardenweb.com is particularly good - click on the different countries to get an enlarged view of wherever you are in Europe, Australia, China and the States.

Knowing your hardiness zone isn't always much help though. First of all, because the categories seem to me to be extremely wide. We're in zone 8, which means that temperatures might go as low as -12°C. Well, yes I suppose they might. But they almost never do. If we have a week below freezing with a couple of days at -5 or -6, that's an exceptionally cold winter. Most years we never see temperatures drop far below freezing at all. Even this week we've mostly been around 0-2°.


One reason for that however is that we're in the middle of a city - where temperatures are always 2-3°C higher than the surrounding countryside. And that's another problem with hardiness zones - you need to take into account the individual factors of your garden : how exposed is it? If you're south facing, if you have nice sheltering walls, or if like me you garden on a balcony that receives warmth from the house, the official temperatures and those which your plants experience may be quite different. I've said before that I can get so-called annuals like petunias to survive the winter on the balcony just by covering them and placing them against the walls of the house. The conditions in the micro-climate of your own garden may be quite different from the "official" conditions of your zone.

However, the other problem is that hardiness zones only take into consideration the low, winter temperatures. I've said we're in zone 8 here - but so is much of Britain, and the type of plants that grow well here and in Britain are often quite different. We may have similar winter conditions, but the summer heat is another story.

In the US, it's also possible to find out your heat zone. The zones are based on how many days the temperature rises above 30°C / 86°F, and you can see how the zones are labelled at the site of the American Horticultural Society. But I can find nothing for any other part of the world. And as I lose far more plants to heat than cold, I'd like to know.

But for now, we're still coping with the snow.














18 comments:

Rob (ourfrenchgarden) said...

Hi Sue

Funny thing hardiness zones. How many plants are sold as hardy down to minus 10 only to be killed off by night after night of minus 5.

It's a shame there's no heat map for europe, that would make for interesting reading

Rob

VP said...

Great minds think alike Sue!

I also wrote about this yesterday, though mine won't appear until Wednesday - ABC Wednesday will be at Z, which always needs yo uto be a bit more inventive with your posts!

chaiselongue said...

Yes, I agree. Hardiness zones are not very helpful - we're in zone 9, but so is west Wales where we used to live and the climates and suitable plants couldn't be more different. Basing zones on the lowest winter temperature doesn't take account of summer heat or rainfall (lots in west Wales, frequent droughts summer and winter in the Languedoc). We're just learning from other gardeners and from what works here!

Cameron (Defining Your Home) said...

It really looks COLD there! I never think of Milan being snowy, but now you've changed my winter image!

It's been almost 10 years since we've had a big snow.

Cameron

Sylvia (England) said...

I agree Sue, a lot of plants the USA grows in lower zones to us just don't like our cool summers. I find that some plants have only just started to grow by September and never really flower very well some years.

One of the things I like about blogs is getting the chance to understand how different climates work on a daily basis.

Enjoy your snow, as you say it isn't too cold. We have had our coldest temperatures (-10C last week) in the last 10 years I have lived in our present garden. I think the lowest prior to this winter was -3C, quite a difference in the plants that will survive. I am looking forward to replacing some plants! Now if only it killed the newer bugs like lily beetle.

Best wishes Sylvia (England)

Teza said...

There is something unique about gardening as most gardeners will try and 'push' their designated zone. I am a Zone 5a gardener, but am always trying for Z6-7 plants... sometimes successful, but in most cases realize that I have planted a somewhat expensive 'annual' instead.... but that's fine with me.
I was shocked to see the photos of snow.... but 2008 proved to be very unpredictable where weather was concerned. Thanks for the welcome to Blotanical... its so much fun!

gardenerprogress said...

Wow, I never thought about Milan and snow together! Our zone now is supposedly changed to 8. My daughter then asked why we can't have an orange tree because the tag said zone 8. Our summers are just not warm for long enough.

Yolanda Elizabet said...

Wow Sue, that is a lot of snow, much more than we had over here. BTW I garden in zone 8 too and so do the gardeners in Austin Texas, but their zone 8 is very different from ours. ;-)

Jan and Steve said...

That's interesting, as I haven't really thought about hardiness zones, but just observed what other people here grow. I seems that we're zone 9, but to allow for our height above sea level and the wind I perhaps ought to think zone 8.

That's an awful lot of snow you've had!

HelenJ said...

Hmmm, you have more snow in Milan than we have here in the south of Sweden (= none). The world is upside-down....
I think the summer temperature is more important than the winter temperature, since the plants assimilate the heat to cope with the winter. And you can make your micro-climate better using stone walls, ponds, wind shelter and so on. It is quite interesting =) /Helen

orchis said...

Nice photos! All that snow reminds me of Norway. Glad we don't get it here though.

I think it's fun to push the limit of my hardiness zone. From the map I can see that I am well within zone 10. Still I manage to grow a few zone 11 plants. Delonix regia is one example. It has become quite big but not flowered yet.

kompoStella said...

oh! you lucky you! i adore snow... the light, the clean feeling and the silence it brings. we've had only miserable little attempts so far like the last several winters. i want to take pretty snow pictures like yours! here's to hoping...

Jan (Thanks For 2 Day) said...

This subject is of interest to me, too...Sue. I live in VA where I'm 'between' zones 6 and 7. It really depends on the time of year and what the weather is that particular season. I can't grow things that people in the same zone a bit south of me can...so it's a bit confusing.
You have so much snow, it looks freezing cold to me...brrrr. I think I'll go wrap myself in a nice warm quilt now...
Take care!

Monica the Garden Faerie said...

Wow! Not that I'm any kind of expert on Italy, but that *does* seem like a lot of snow for you!!

VW said...

Hi Sue,
Looks like the crazy winter weather isn't confined to the US! Here in Spokane, WA, December was the snowiest month since record-keeping began in 1893 (61 inches). We passed 2 meters total in January. RE the microclimates, have you read Garden Chick's post about that? She makes me laugh: http://garden-chick.typepad.com/garden_chicks_design_tips/2009/01/before-and-after.html
Regards, VW

VW said...

Oops, this is the link I meant to leave: http://garden-chick.typepad.com/garden_chicks_design_tips/2009/01/when-did-microclimates-become-a-fashion-statement.html
Sorry! - VW

LadyLuz said...

It's been good to stop by after a long gap of doing almost nothing (inertia has set in in this house, sadly). My, what a winter you've had.

We're supposed to be Zone 9, but have had the coldest, wettest winter yet, leaving us impotent to do anything out there.

Extraordinarily, things are blooming - nasturtium, clematis, lavender and the little bulbs are coming up.....so are the weeds, like crazy.

I know it'll dry up soon and we'll be cursing the heat.

Ellen Kirby said...

Great blog...I really enjoyed it and think you've got a real niche with the balcony garden. I worked in Brooklyn, NY and didn't see nearly as many balcony gardens as I would have liked. Great potential. and you're doing great to spread the word.

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