Protecting plants in freezing weather

Rhododendron leaves curling in cold
Rhododendron leaves curling in cold

When winter temps drop below 0, Boston-area gardeners may need to exercise special care to protect plants. As average temperatures have warmed in recent years, the USDA has shifted its plant hardiness zones, so that much of eastern Massachusetts now falls within zone 6b, where it was 6a in the 1990 edition. In layperson’s terms, this means that many of the “hardy” zone 6 plants sold by local nurseries may only tolerate an average winter low temperature of -5 to 0 degrees. In addition to marginally hardy plants, newly-planted material may have tender growth that is more open to damage.

Frequently planted zone 6 cultivars include:

  • Ilex crenata ‘Sky Pencil’ (Japanese Holly)
  • Acer palmatum ‘Red Select’, ‘Sereiyu’, Orangeola’ (Japanese Maple – other cultivars may be hardy to zone 5)
  • Cryptomeria japonica
  • Oxydendrum arboreum (Sourwood)
  • Liquidambar styraciflua (Sweetgum)
  • Cornus florida (American Dogwood)
  • Buddleia davidii (Butterfly Bush)
  • Magnolia virginiana (Southern Magnolia)
  • Hydrangea macrophylla (Hortensias – varieties that bloom on old wood may lose flowers next spring)
  • Buxus spp. (Boxwoods – depends on cultivar, most not hardy)

If you have any of the above, particularly planted in an exposed or north-facing spot, take steps to protect them to prevent casualties. Damage may include bud loss, stem and twig die-back, or even complete death. Snow will provide some protection to the root zone, but you may also consider applying 4-6″ of mulch over the root zone. More importantly, wrap above-ground parts with floating row covers, burlap, or light blankets (not comforters or plastic!), secured against wind using bricks or large stones. Old-fashioned incandescent lights can also add heat around branches.

Remember that plants in containers are two full zones more susceptible to cold, so unless you have planted things for zone 4 and below, insulate pots or bring them indoors. Wrap pots with bubble wrap, blankets, or haybales, group plants along heat-reflective patios or walls, and cover exposed soil with evergreen boughs.

After temps warm, hold off on pruning branches that appear dead. Pruning too long before plants “wake up” in spring may invite further damage. Your landscape professional can help asses the extend of damage in late winter/early spring, to avoid removing more of the plant than necessary.