Global warming and plant hardiness

Garden Making

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Laburnum tree in flower. Garden Making photo.
Laburnum tree in flower. Garden Making photo.
Laburnum tree in flower. (Garden Making photo)

I’m confused. Current winter weather predictions (2010-11) from Environment Canada, the Weather Network and the Farmers’ Almanac all agree that my southern Ontario garden will have normal temperatures and normal snowfall this winter. But does anyone know what normal is anymore?

Over the past decade winters have grown gradually warmer with less snow, and 2010 was the warmest year since Canada began keeping weather statistics. It leads me to think these conditions are the result of global warming, and hardly anyone is still skeptical about that premise. But despite the warming trend, borderline hardy plants in my garden continue to suffer winter damage. The apparent message for gardeners is that global warming hasn’t altered plant hardiness zone designations. Plants with Zone 6 hardiness ratings, such as golden chain tree (Laburnum x watereri), fragrant winterhazel (Corylopsis glabrescens), or ‘Red Baron’ Japanese blood grass (Imperata cylindrica), have all suffered substantial wood and root loss in my Zone 6 location. This does nothing to support a theory that global warming will allow us to grow more tender plants than in past decades.

It does seem that global warming has reduced snowfall, with more precipitation falling in warm months as rain, than in cold months as frozen snow. Last winter my garden had a snowfall on Dec. 13 (2009), and insignificant dustings of snow a few times thereafter. This winter the prediction is for normal amounts of snow, and I hope that means a substantial ground covering that will insulate perennial plants and remain in place until spring.

Gardeners and farmers always complain about the weather, and with good reason. Global warming is dangerous for plants. It causes erratic weather patterns with extremes of all kinds—intense heat waves, deep cold snaps, torrential rain and blasting wind storms. These weather phenomena stress plants and strain their growing conditions. Moderation is gone, and we are gardening in a climate of unpredictability. Yes, sometimes spring comes early, and sometimes summer gets very hot. But there are days in winter when the temperature dips low enough, for long enough, to still cause winter injury to plants in my garden, growing at the edge of their hardiness zone.

Ginger: still green in the garden

Looking about for anything that might still be green and standing, I notice how well the European wild ginger (Asarum europaeum, Zone 4) is soldiering on. The wide cushions of ginger continue to reflect light off their glossy foliage. This is a useful plant for shade to part sun places with moist soil; it makes a most attractive ground cover with dense cushions of kidney-shaped leaves, three inches (8 cm) wide and on five-inch (13-cm) stems, all overlapping each other in an artful way.

European wild ginger is so attractive it shouldn’t be hidden behind anything, but set visibly between taller shrubs and plants at the front of the border. It’s also a great partner with woodland plants, and brings style and distinction to informal garden corners.

Perennial plant authority Allen Paterson recommends setting it among Virginia bluebells (Mertensia virginica), trilliums and Mayapple (Podophyllum peltatum), which would be very appropriate. Paterson is enthusiastic about Asarum species and grows several kinds, all with bell-shaped brown flowers hidden beneath the leaves in spring. He recommends that gardeners “crawl around on hands and knees and peek under the leaves to find these elusive flowers—it’s great fun.” I have to admire a fellow who doesn’t mind getting dirty for an ant’s-eye view of ginger flowers in bloom.

My original three small pots of this ginger started out next to hellebores, primulas and Japanese painted ferns (Athyrium niponicum ‘Pictum’); but now have seeded about and make cushions everywhere. They’re beautiful, can be moved around and are entirely useful. I also have spreading clumps of the North American wild ginger (Asarum canadense), with larger matte leaves (no glossy shine) and otherwise very similar to the European kind. But A. canadense is already dormant and without foliage this late in the season. Aside from style and beauty, the added value of late season display makes European wild ginger a winner.

Seed catalogues rolling in

Seed catalogues have started arriving and I’ll be spending long, pleasant hours over these. So far I have Stokes Seeds ( and Veseys Seeds (, and they feel nice and fat. I’m surveying zinnias this winter, looking for the tall husky types with five-inch (13-cm) flowers I remember from my grandmother’s garden. Already I see some new varieties, but I’ll wait to give a better report when more catalogues have arrived. I really want to see some startling zinnia colours next summer, so you can be sure I’m on it!

Thanks for visiting Making a Garden. See you next week.

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4 thoughts on “Global warming and plant hardiness”

  1. To LM Sullivan, Dec. 8

    Yes, there is interesting and so far inconclusive information about potential hardiness zone change. It takes decades of temperature and plant data to accurately identify a trend or real change. I only write about my own garden, and the plants I'm growing myself — and I hope it will be understood that way.

  2. Dear Judith, Scientific research shows evidence that native forests show signs of a northward migration. It is not possible to assess the impact of global warming through the observations of one person, especially based on the hardiness of a few ornamental or non-native plants. Statistically valid sampling and sound statistical methods are required.

    As gardeners, we need to be prudent in making generalizations about broad trends such as climate.

  3. Morning Judith. From Winter Wonderland.

    After seeing your hellebores in bloom the other day, I came home and checked mine. Lots of fat buds still nestled in the foliage, on two inch stems. Maybe if there's a Janurary thaw they might arise. At the moment they are covered in about eight inches of fresh snow. One of the joys of living here, that much further north of the lake, is that when snow arrives, its doesn't immediately melt, as in the city. Last winter, gray in T.O., we had a dusting of snow three or four nights a week. Resulting in a freshening up of the beauty of winter throughout the season.

    Take care. I'm off to shovel the driveway, then into the woods to go snowshoeing. Give me a call if you'd like to join me sometime! (I won't sit by the phone and wait!)



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