Uncomfortably warm

We had record high temperatures last weekend, up to 74 degrees at Logan Airport on Sunday, the warmest January temperature ever recorded in Boston. For us humans, it was a pleasant break from winter cold. Scientists in town for the American Meteorological Society’s annual meeting linked the freakish weather to the trend toward global warming.

    After my friend Amy noticed that some blue vinca flowers (Vinca minor) had opened, I checked and found some in my front yard too. 

Vinca flower looking peaked after Friday's cold

My Asian witch hazel (Hamamelis mollis) is in full bloom in the backyard. The danger is that these early bloomers started up their spring metabolism too soon, and they’ll suffer for it. By Friday, we were back to a windchill of 5°F.

So far the witch hazel flowers are holding up

    I’m hoping that despite the White House’s attempts to deny what everyone else knows, we’re finally squaring up to serious action to address global warming. Meanwhile, though, we’re warned that even if we immediately reduced carbon pollution radically, we’d still have to expect climate effects that have already been set in motion.

    In the garden, I’m afraid this doesn’t mean I’ll be picking mangoes and pineapples. 

We could end up with waterfront property due to sea level rise, but the palm trees are unlikely

Instead, I’m expecting more winter warm spells alternating with vicious cold without snow cover. That’s already been happening in recent winters, and it’s especially hard on my shrubs and vines.

    I particularly notice the effects of uneven winter temperatures on a climbing rose that grows near the house on the garage and an adjacent pergola over the garden gate. 

    When I started the garden thirty years ago, I planted Rose ‘New Dawn,’ in this spot. I learned later that some garden designers regarded ‘New Dawn’ as a boring cliché, but to me it was the epitome of rose charm. I pictured a romantic cloud of blush pink roses spilling over the gateway. 

'New Dawn' is my idea of a romantic climber

After a few years, that did happen—sometimes. A winter warm spell followed by a cold snap could kill back even this tough survivor.

Rose canes exposed to chilling winds. Cross-vine leaves are still clinging on in January.

    At first the vine bounced back, quickly sending up new canes to replace killed ones. In the last couple of years, it seems to be struggling just to survive, and flowering has been sparse. ‘New Dawn’ is rated as hardy to Zone 5, meaning that it can bear the cold down to -20 F, lower than what we get in this area. I think it’s the cold, drying wind when there’s no insulating snow that’s depleting the vine’s reserves as the assault is repeated year after year. It’s got only so much stored energy for repairs.

    To help ‘New Dawn’ and my other favorites survive the new climate reality, I’m resolving to surround their bases with some compost and composted cow manure next spring to help them store up energy during the warm season. To me, shrubs wrapped in burlap through the winter are an eyesore. 

Poor thing!

But I might break down next fall and pile up some leaves or soil around shrub and vine crowns for a little extra insulation. As I post this, there's snow on the ground again, thank goodness!

Missing snow

The vinca groundcover in our front yard emerged from this year’s mild winter looking ragged around the edges. Vinca (Vinca minor) leaves typically stay shiny and green through the winter, but this year some turned an unsightly brown. I’m guessing that this was due to lack of snow cover. With no snow to insulate them, the usually evergreen leaves dried up and died.

Last winter brought less than our usual snow accumulation

    Meanwhile, back in the vegetable bed, we lost two 21-year-old lavenders (Lavandula angustifolia). Despite their Mediterranean ancestry, they’d proven their ability to survive cold winters. What did them in, I think, was too little snow.

All that's left of a lamented lavender

    A blanket of snow keeps the temperature around plants relatively stable and protects them from desiccation. Plants that survive New England winters have adaptations that enable them to endure the cold. But frigid, drying winds can overwhelm their defenses when there’s no snow cover.

Consistent snow cover insulates plants

    I know we shouldn’t confuse weather with climate. One mild winter with little snow doesn’t prove a trend. In winter 2017-18 the nearby town of Milford recorded 87 inches of snow, in contrast to 47 inches this year. But we know there’s a change underway due to global warming. The Fourth National Climate Assessment, released in 2018, predicted that by 2035, the Northeast will be more than 3.6°F (2°C) warmer on average than during the preindustrial era.

Cairn in Snow, Caspar David Friedrich, 1807

     This compilation of government research also predicts less snow and a shorter snow season in our region. That won't be the worst effect of climate change, but it'll be one that gardeners will regret.

    I’ll replace the lavender plants that died with a variety such as 'Munstead' that reportedly survives winters north of us in USDA hardiness zone 4—we’re in zone 6b. 

Lavender 'Munstead'

But that won’t mean they’re safe without snow. If I want them to survive, I’ll have to remember to cover them with leaves or straw and then wrap them in burlap for the winter months. That’s more work than I had in mind, and it looks bad. I’m not a fan of burlap-swaddled lumps in the winter landscape.

I don't find this decorative

    The vinca in the front yard is on its own. There’s no way I can cover it up without creating an eyesore or sending mulch material blowing all over my neighbors’ lawns. Fortunately by May, this winter’s dead brown leaves had fallen off and been replaced by new foliage. I hope that will keep happening.

The vinca bounced back

    Garden plants’ tolerance for cold and heat are rated on two useful scales. The USDA Plant Hardiness Zone Map focuses on the lowest temperatures in an area. The American Horticultural Society’s Plant Heat Zone Map ranks areas according to the number of “heat days” over 86 degrees. For example, right now ‘Munstead’ lavender can grow in USDA hardiness zones 4 to 9 and AHS heat zones 5 to 8. 

     Who knows how a warming regional climate will change these facts of plant life? Maybe New Englanders will also need to learn to rate plants’ tolerance for winters without snow.