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