For several mornings, my garden has been coated with hoar frost, a warning that soon the earth will be frozen solid. This is the time I begin to winterize all my roses, giving special attention to ‘Clair Matin’, a climber supported by the open framework of a ginkgo’s branches.
The ‘Clair Matin’ canes are entirely exposed to winter temperatures and even lower wind-chill conditions. Unlike other climbers trellised on a warm brick wall, ‘Clair Matin’ gets no help beyond the hilling up of soil I provide at the base of the shrub. The time of greatest vulnerability occurs in December, when roses begin to enter dormancy in a period of acclimation. The gradual decline of cold temperatures in December guides rose wood into hardening and increases its ability to withstand the coldest mid-winter temperatures. If a sudden cold snap occurs in December, accompanied by 24 to 48 hours of plummeting temperatures, rose wood is liable to suffer dieback. If December temperatures will just stick to a slow and gradual decline, ‘Clair Matin’ will again be a beautiful plant next summer.
I’m always interested in ways to help roses get through an unpredictable winter. Although recent winters have been remarkably mild, it’s always possible that we’ll return to many weeks of sub-freezing temperatures. So, I prepare roses for the worst while hoping for the best. Even though roses in my garden are all frost hardy, I still provide protection for them. My aim is to preserve as much live wood as possible, knowing it will result in a larger plant with more flowers next summer. Here are the two simple steps I follow:
1. Limit pruning to removing only the long canes that can whip around and be damaged in winter wind. Long canes that can be anchored, staked or tied in place should be allowed to remain. If canes are cut short in autumn, any potential frost dieback in winter will leave an even smaller shrub next spring. If canes are left long in autumn and then pruned in early spring, the result will be a larger plant with more healthy wood for the new season.
2. Hill each rose shrub with soil (from another part of the garden) or growing mix from summer containers, along with leaves, by making a mound over the crown and root zone. The mound can be 12 to 18 inches (30 to 45 cm) high, or even higher if you live in hardiness Zones 3 or 4. The soil and leaves will help to insulate the vulnerable bud union and the lower portions of canes. When buds on exposed canes begin to redden and swell early next spring, pull back the soil and leaves, and spread them around the plants as a mulch.
What’s hoar frost?
The thin layer of ice crystals that coats leaves and grass blades is called hoar frost. It forms when heat escapes from the earth overnight, causing the surface temperatures of plants and objects (like empty pots on the driveway and garden gloves left on the front steps) to be lower than the surrounding air. Hoar frost is beautiful to see, but it’s also a warning that frost is slowly creeping into garden soil.