JARS v64n1 - What to do About Heavy Snow-inflicted Damage to Gardens!

What to do about Heavy Snow-inflicted Damage to Gardens!
Steve Whysall

Modified from an article in the Vancouver Sun, January 7, 2009

Okay, the snow has finally gone. Now, what to do about all the damage done to your garden. In Vancouver, BC, during the winter of 2008/2009, trees were broken and shrubs were crushed by the unusually heavy snowfalls and prolonged sub-freezing weather. In Van Duzen Garden, one of the city's main show gardens, cedars that form the maze were warped and branches were snapped off. At the University of B.C. Botanical Garden, many of the broadleaf evergreens were crushed and some other plant treasures left broken. A strawberry tree (Arbutus unedo) and some of the Oregon grape (Mahonia aquifolium) plants were flattened. The yew hedge around the popular Physic Garden was buckled and knocked out of shape, branches were snapped off large Douglas firs, and an elegant Japanese black pine was left with a large gash when a branch was torn off.

The heavy snow also inflicted significant damage to evergreen magnolias while the deciduous ones were left undamaged. "They all have fairly brittle branches, but the evergreen Magnolia grandiflora seems to have been hit the hardest," Doug Justice, curator of the UBC collection, noted.

Attempts were made during the storm to minimize damage. Gardeners went around knocking snow off trees and shrubs with 5.6 m (15 foot) bamboo poles. Justice says he had to come to the rescue of a few plants in his own home garden such as a beautiful Azara serrata tree from South America that was "almost flat on the ground." He knocked snow off it and helped it back into place.

In home gardens throughout Metro Vancouver, shrubs were squashed and trees lost branches, but the biggest problem seems to have been damage done to hedges, which were twisted and warped by the weight of wet snow. Most prone were hedges that were too tall to allow homeowners to knock snow off the top. Most of these, like the buckled yew hedge at UBC and the warped cedar hedges at Van Dusen, will bounce back into shape by spring. Others will need to be pulled back into position using guide ropes and stakes. In the worst cases, hedges will need to be pruned and possibly replaced.

Columnar trees and juvenile conifers have also been forced over and left bent or crooked. These, too, will need to be helped to straighten out by using guide ropes to pull them back into position. Trees that have lost limbs will also need to be taken care of. If a branch came off and left a ragged edge, the wound will need to be cut cleanly.

It is not recommended to use wound paint or "tar" to seal a tear or gash. Tests have shown that wound paint actually hinders the healing process and promotes fungal disease. It's best to leave it alone and let the tree heal itself. The collar - the slightly mounded part where a branch connects with the tree - has cells in it that promote healing. If a new cut is necessary to clean up the tear, it should be made just outside the collar.

Crushed shrubs are probably the biggest problem home gardeners are going to have to deal with over the next few weeks. Many of these can be pulled back into position, but some will need to be propped up.

Vines and climbing roses that have been ripped away from walls and trellises will also need to be put back into place. Some pruning may be needed where stems have been broken or crushed. Nandina and Spirea are two popular shrubs that often get squished by heavy snow. Both of these can be pruned in early spring or late winter and will regenerate.

Many evergreen shrubs such as Rhododendron will also be showing signs of cold damage on leaves, which will appear discoloured with noticeable brown patches. The damaged leaves can be removed in late winter and the shrub will, in most cases, produce new growth in spring. One of the biggest problems, however, in coastal gardens over winter is the damage caused by recurring freeze-thaw cycles. Moisture in the ground freezes, then thaws, then freezes again, causing damage to root systems. This is one of the main reasons plants die. Shrubs and perennials in poorly drained ground are most prone. Unfortunately, the damage is not always evident until spring or early summer when the plant fails to thrive.

Also, if the freezing is early in the winter before plants have sufficiently hardened off, the sap in exposed trunks may freeze and split the bark, which can kill this portion of a plant.

One of the good aspects of the heavy snowfall is that snow acts as an insulator to protect plants such as perennials that are dormant in the ground. The temperature under the carpet of snow stays around freezing whereas plants above ground are often exposed to subfreezing temperatures. Sometimes, the part of a plant above the snow will die, while the trunk beneath the snow will survive and be able to throw up new shoots in the spring. This is why many plants in gardens on the Prairies manage to survive the extremely cold winters there.

The temptation after snow has left plants damaged is to want to prune early and as much as possible. However, it is not a good idea to leave plants with open wounds that are exposed to moisture in the middle of winter. This can lead to fungal diseases. It is best to let winter do its worst and then assess the damage in the garden nearer to spring. Annual pruning and clean up in general is best left until a mild spell in late winter.