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.

Let it snow

As we huddled inside during last week’s single-digit temperatures, I found one silver lining. The snow that fell in December was still on the ground, insulating my trees and shrubs from the worst of the cold winds. Thursday’s snow storm added more protection. Snow is good news for gardens when the weather is at its coldest.

Snow shields the garden from cold damage

    I’m particularly grateful for the blanket of snow this year, because during the summer and fall I went on a planting binge. In April I planted a replacement American persimmon (Diospyros virginiana). In June there was a blue juniper (Juniperus squamata), a yellow-flowering shrub rose (Rose ‘Kolorscape Lemon’), and a fragrant sumac (Rhus aromatica ‘Gro-Low’) for a new bed. 

Blue juniper settling in

Around the same time, I planted a spicebush (Lindera benzoin), an elderberry (Sambucus canadensis) and an eastern sweetshrub (Calycanthus floridus), native shrubs to fill in the spaces opened up when I cut down hemlocks. For the same spots, at a sale I scooped up a mountain laurel (Kalmia latifolia), a dwarf Alberta spruce (Picea glauca ‘Conica’), two inkberries (Ilex glabra), and an eastern white cedar (Thuja occidentalis) with the promising brand name ‘North Pole.’ 

Preparing 'North Pole' for winter with some extra watering

    In early October, I added a semi-dwarf Asian pear (Pyrus pyrifolia) and two honeyberries (Lonicera caerulea). By December, these new trees and shrubs were just recovering from transplant shock and, I hope, sending out some new roots. This winter poses the first severe test of their resilience.

This bare stick could grow into a 15-foot Asian pear, if it survives the winter

    Readers may recall that I planted my first persimmon in April 2016 before that summer’s severe drought. It didn’t survive. That makes me extra conscious that newly planted trees are vulnerable. Will this band of newcomers make it through to spring? With more than a foot of snow on the ground, their chances are better.

Second try at an American persimmon

    Temperate zone plants such as my new choices go dormant during the winter. Lengthening nights start the process in autumn, and falling temperatures trigger further readiness. Nutrients are stored in roots, leaves drop, and tissues revise their contents, replacing water with sugars and other chemicals that act as antifreeze so that ice won’t burst cell walls. 

     By this time of year, trees and shrubs have completed the processes that create endo-dormancy, a state of reduced metabolism that allows them to stay alive and save their resources for spring. When they reach a required chilling duration, they’re ready to switch to eco-dormancy. In this state of readiness, they’re able to respond to environmental conditions by opening buds and sending out leaves and flowers.

    During both of these stages, snow acts like a down jacket, its air spaces holding warmth in the ground. Under the snow, roots respire and even grow a little.

    Conifers and broadleaf evergreens like my new mountain laurel have a stern challenge in winter, because their foliage continues to lose water to drying winds.

Leaves held through the winter are vulnerable to cold winds

That’s why you’ll often see brown leaves on rhododendron branches in spring. Snow cover helps protect evergreens from desiccation. Snow may be a nuisance on paved surfaces, but in the garden, it’s a blessing.

Flowers open in April thanks to January snow

 

What Not To Wear, for shrubs

I can still remember my first down parka. Back then we were suffering through winter in uncomfortable wool coats. During the energy crisis of the seventies, I was walking to class early mornings in Cleveland. I was cold!

    A friend steered me to a new kind of coat, sold at a wilderness outfitter. I tried one on when I came home to Boston for the holidays. It was huge, dull blue-gray, and gloriously warm. The maker had dubbed it, “The Yeti.” 

A friendly Yeti

It wasn’t flattering, but that parka changed my winter experience.

     My neighborhood is dotted with swaddled shrubs, apparently wrapped up by soft-hearted owners who are trying to keep away the cold. But unlike me, plants don’t need the Yeti when temperatures are cold. 

Nice thought, but not necessary

     Plants don’t hold in warmth like us warm-blooded mammals. There are two putative reasons why wrapping shrubs could make sense. First, a windbreak can prevent loss of moisture. Desiccation is particularly a problem for plants that hold their leaves through the winter, such as the rhododendrons, Japanese andromedas, and mountain laurels that are so common in foundation plantings around town. 

Challenged by dry winter air

Second, support from wrapping the shrubs and tying them up with twine might prevent branches from being bent down by snow.

Heavy snow may bend shrubs temporarily

     My first objection to this practice is aesthetic. Why plant ornamental shrubs and then cover them in ugly wrappings for four or five months of the year? It seems to defeat the whole purpose of choosing broad-leafed evergreens. The reason we plant these shrubs with such monotonous consistency is precisely because they lend a bit of green and a sense of life to the winter landscape.

Why?

     On a more practical level, the wrappings don’t really help the shrubs much. The plants have evolved to deal with dry winter air, and branches that bend in the snow gradually resume their upright position in spring. 

Better looking, but still not necessary

Rather than wrapping, the better approach is to plant in the right place. A rhododendron likes a sheltered woodland spot with acid soil, not a windswept steppe of lawn where is has no cover from arctic winds. 

Rhododendrons prefer shade and shelter

     If the plants go into the winter with plenty of water, they’ll be able to hold on through the season despite the cold, drying air. 

     Last summer’s drought was very tough on our shrubs and trees. Despite recent rain and snow, we’re still way behind. Logan Airport recorded 33 inches of rainfall in the Boston area in 2016, compared to the annual average of 44 inches.
 
     The dry conditions took their toll. I notice dead branches on my own shrubs and some neighborhood evergreens that have finally turned completely brown, succumbing to long-term water deprivation.

 

Drought casualties

Perennials are tucked away underground at this season, storing everything they need for next year in their roots. Evergreen shrubs, on the other hand, have to keep their leaves and needles alive year round. Instead of winter coats, they should get priority for fall watering to help them stay hydrated through the winter.

When thawing is a bad thing

This year I tried a new approach for a difficult spot in my garden—an ornamental grass for a place where winter goes away temporarily when we dry our laundry.

      We had planted three boxwood shrubs (Buxus sempervirens) in a small north-facing bed next to the driveway. 
 

Happy  boxwood

They were expanding and thriving until we added two new dryer vents two summers ago. Having three vents instead of one definitely shortened the time it took to dry clothes. 

      Unfortunately, this gain in energy efficiency meant a death sentence for one of the boxwood shrubs, which stood right in the warm air outflow. By this spring, all the shrub’s leaves had yellowed or fallen off. I bowed to the inevitable and cut it to the ground. 

The unfortunate boxwood sprouting from the base this summer

As I understand it, what killed the boxwood’s leaves was desiccation.  

      As the nights get longer, trees and shrubs prepare for freezing temperatures by moving sugars they’ve made through photosynthesis into cells. Once there, the large molecules act as antifreeze, preventing the water in the cells from freezing, expanding, and bursting the rigid cell walls. (If you’ve ever over-filled a freezer container, you can imagine this process).

      Deciduous plants don’t have to protect their foliage from freezing. They cut their leaves loose and defend only their woody trunks and branches. But boxwood holds its evergreen leaves through the winter. Although the plant is dormant, it needs to maintain a low level of metabolism even in the cold to keep the foliage alive. 

      The problem is that during winter thaws—or when air from the dryer creates a warm patch around the shrub—metabolism speeds up. The leaves may open their stomata, pores that let in carbon dioxide for use in photosynthesis. This allows water vapor to escape. If it goes on for too long, the leaves dry out and die. 

      I pondered what to grow in place of the dried out boxwood. Another evergreen shrub would probably suffer the same fate. I needed a plant that could get by with mostly indirect sunlight in the bed’s northern exposure. I wanted something similar to the boxwood in height, width and density.  With my new goal of providing habitat for native insects, I preferred to replace the nonnative boxwood with a native plant. 

    I considered several bulky perennials, theorizing that since the above-ground part of the plant would die off, inconsistent air temperatures in winter wouldn’t be a problem. I decided on switchgrass (Panicum virgatum ‘Shenandoah’), a selection of an adaptable ornamental grass native to most of North America that has burgundy summer foliage. 

'Shenandoah' switchgrass in early summer

It came home from the nursery already quite tall (its expected full size is 4 by 4 feet) and soon sent up airy pink seedheads. It settled in with no fuss, apparently unfazed by intermittent blasts of hot damp air from the dryer during the summer.

      Next spring I’ll find out whether I was right in guessing that the switchgrass’s roots would survive the winter despite the challenging conditions next to the vents. 

Switchgrass, far left and ready to be flattened by snow, is positioned
 right where warm air comes out of the new vents