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Wednesday, March 10, 2010

Do Hardiness Zones Really Matter?

I often find, when I’m browsing through books or catalogues, that the zone recommendations for a plant are wrong. I have spent my gardening life in either zone 2 or zone 3. When I come across plants that are recommended for zone 4 or 5 and I have them growing happily in my zone 2 gardens, I wonder what is going on.

After reading the section on hardiness zones in, Flora a Gardeners Encyclopaedia, I realized that hardiness zones span the globe, and not all areas in the same zone have the same ecosystem let alone the same climate.

cherry blossom

Tony Avent over on Plant Delights Nursery, points out that the zone maps do not tell us how long an area receives the cold temperature only that the area has received that cold temperature. He says the length of time may differ from a few hours to a few days to a few weeks. A plant may be able to survive a temperature drop over night but it will die if that same low temperature goes on for a week.

As well, Avent talks about the concept of provenance, where the parents live. Seed taken from a hardy tree in a northern area will be hardier than seed taken from the same tree in a more southern region.

The instructor who taught the master gardener course here in Prince George is all for pushing the zone barriers, and trying to grow whatever she feels like in the clay soil of her zone 3 garden. For her it's a challenge and a triumph to succeed with a plant, rated as being hardy in a much warmer zone.

I’m beginning to wonder how relevant hardiness zones are, as a guide to what will grow in my garden.

Magnolia blossom

Kathy Purdy, over on Cold Climate Gardening, says the reason plants get labelled with the wrong zone information is because of statistics. Zones 2, has fewer gardeners than a more temperate zone 7 climate so less is known about what grows there. The only way to find out what grows in your garden is to experiment and document you successes and failures.

Flower bed 2

As my master gardener instructor said, it all comes down to knowing the plants requirements and realizing that in experimenting with different plants you will kill some of them.

Differences in soil, drainage, predators, snow cover or lack of snow cover, and the amount of wind may have more effect on the plants survival than how cold it gets. You may have to compensate, ie wrap a plants susceptible branches in burlap to protect it from drying winds, when it is dormant. Mulch or pile on snow over the roots of herbaceous perennials, or, accept the fact that a plant will never fully reach its potential, since it winterkills to the ground every year and can only grow so much from its roots before it is killed back yet again.

Microclimate and soil are two big factors determining plant survival. Clay soil is harder for plants to adapt to than a sandier soil. A plant grown next to a brick wall will be less affected by temperature change and drying winds than the same plant grown in the open garden.

Keeping the plant healthy, giving it adequate amounts of sunshine or lack of sunshine, fertilizer and water will definitely help its survival.

If you are a plant collector enthusiast then go ahead and experiment. I am past the wide eyed, fervent beginner stage. I have had may share of disappointment when plants die, even after I thought they were well established. Nowadays I just plant the tried and true not because I want to avoid disappointment but rather I don't want the work it takes to coddle a plant into survival.


  1. I push to the limits rather than go by what the books say. It sure is easier when you have a lighter soil and a sheltered position where the worst of the winter wet can't get near a plant. I think most winters I have got away with a few borderline plants but not sure if they will have overcome this harsh winter. I'll find out very soon though!

  2. I think if there weren't zone reccommendations, I would spend a lot more money on zone 7 things, whereas now I just spend money on those zone 4 and 5 things. Pushing the zone boundaries is fun, but when it's an expensive tree that I had shipped up from WA or OR, not so much. But like you said, plant hardiness in the cold zones is not as well known so for herbaceous materials I definitely am an experimenter.

    Christine in Alaska

  3. I say push the limits and experiment for sure. It is true that this is the best of life's exercises and the last time I checked plants don't read Zone maps:) Good thing too.

  4. Great point about zones, I totally agree.

    I too have stretched the boundaries a few times and have gotten away with it. Other times, not. I now stick to trusted plants for the very same reason, not a fan of coddling.

  5. I enjoyed reading your blogs. I'm not sure what zone Ireland is in but I know there are lots of things that won't survive the midlands where I am living at the moment, but will do fine along the East Coast where I am moving to soon!

  6. I have lived in different states and find some things growing where the zone tags say they will not. I am more of a trial and error gardener and if I really like something and find it at a bargain price but not suited for my zone, well, I may go for it once. But only once as I have learned my lesson if it fails. But some things survive and I am happy with giving them a try...

  7. I will give almost any plant a 'shot' at living in my yard if I like it enough, although I've lost many in the past. I am 'less' likely to put something that is clearly not meant for my area. But the things that are 'iffy' I always try. I have a wide range of zone-levels growing in my yard;-) I think the labels are usually of some help and guidance but not an 'end all' to whether a plant will like your environment or not.

  8. Great timing on this post - I was just wondering how all my spring plants who have shot up and started blooming will cope with the wet sleety snow dump we got overnight! I do hope that my plants survive but if they don't, well, I suppose then they just aren't hardy for my area.

  9. Rosie- hopefully your borderline plants will pull through.

    Christine- I know what you mean about the expense of plants that may or may not survive. It's another reason I've curtailed my experimentation.

    Tina- interesting isn't it, that we have never asked the plants about where they would like to grow :)

    miss m- Hey, we have similar gardening philosophies :)

    Ali- Thanks! I look forward to reading about your new garden.

    Skeeter- Go for it! as long is it doesn't break the bank it is fun to try growing iffy plants.

    Jan-That's great! I'm looking forward to seeing more of your garden especially those iffy plants that are doing well.

    Stevie- I'm sure everything will survive. The great thing about spring plants is that they are built to survive setbacks like wet sleety snow.

  10. Nice work, your awsome content have forced me to to leave some positive feedback BioBizz Fish-Mix

  11. Hi Melanie
    A good post - I love experimenting but I take the easy way out mostly, by getting advice from the garden centre before I buy. Sometimes they work and sometimes they don't :(. I have thrown away quite a few I must admit...

    Re my post about banksias, here's a good page with info if you are interested: http://www.anbg.gov.au/banksia/. I quote from the page "The fruits of banksias (called follicles) are hard and woody and are often grouped together to resemble cones (which they are not ­ true cones are produced only by conifers)."


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