Late October/early November is the ideal time to plan on how you are going to protect vulnerable plants from our severe winter. The first step has hopefully already been taken where you have chosen most of your plants that are hardy to your climate zone. Many gardeners, however, like to try a few “challenging” plants so these will need extra attention at this time of year.
Another important step in protecting plants from winter damage has also hopefully already taken place, and that is their placement in the garden. Tender plants — the most susceptible to winter damage — should have been planted in sheltered spots, perhaps near a fence or to the south of a row of trees. Also, ensuring that plants are located where they are exposed to favourable conditions such as enough sun, results in healthy, vigorous plants that will come through the winter in much better shape than those under stress because they are exposed to less-than-optimum surroundings.
Newly planted evergreens are at risk of injury; winter winds will increase the rate of transpiration when the plants cannot replace the lost moisture because the root zone is frozen. Strong winter sunshine can also have a desiccating effect on such plants. Erecting a burlap screen on the south and west sides will shade them from the sun, preventing desiccation and sunscald, while a sturdier barrier on the north and west sides will protect the plants from harsh winter winds. All plants benefit from going into the winter with adequate water around their roots because wet soil holds more heat than dry soil. Not only evergreen trees and shrubs, but other trees and shrubs as well as perennials should be watered thoroughly before freeze-up.
Wrapping the lower trunks of vulnerable deciduous trees with burlap offers protection from the sun and will prevent the bark from splitting. Winter sun can be damaging not only because it causes desiccation but also because the freeze-thaw cycle that sunny winter days produce is very damaging to plants. Some people plant very tender plants against the south wall of the house, thinking this is the most sheltered spot in the garden. It might be, but the constant freeze-thaw cycle that is created in such a sheltered spot can do great damage to plants during the winter. Another reason to wrap the trunks of trees and shrubs — but with a more substantial material than burlap — is to offer protection from damage done by rabbits.
The best way to protect plants from winterkill due to low temperatures is to keep the soil temperature constant and to keep it as high as possible by having good snow cover. Erect snow fences and use branches to catch snow so that deep drifts cover plants during the whole winter. This works well if there is significant early snowfall, but often, an outburst of very cold weather occurs before there is significant snow cover. In this case, mulches will serve the same purpose. A thick layer of mulch of an organic material such as dry leaves will provide insulation around plants to moderate the changes in the temperature of the soil. Whether dry leaves, straw, or another material, it must be kept dry to retain its insulating ability. Therefore such mulches are often covered with a plastic or Styrofoam cover. I have had good luck filling large plastic bags (originally held mattresses) with dry leaves and placing these bags on top of Oriental lilies.
Dry soil or peat moss can be mounded up over tender plants such as roses. A Styrofoam cone or some other material might be put on top of such a mulch to both keep it dry and to prevent winter winds from blowing it away. If the soil is mounded, however, it will shed a certain amount of water and this process is only done after threat of rain has past — any precipitation we get afterwards will be in the form of snow. Flax straw works well as mulch because its high oil content enables it to shed water and it remains dry during the winter. Wire cages can be used to hold mulch in place and the cages can be used to hold a covering for the mulch.
Tender plants such as a hydrangea can be cut back and then have a large cardboard box (bottom removed) placed over it. The box is next filled with dry leaves and the top closed and covered with plastic to make it waterproof. Perennial borders benefit from having a thick layer of organic mulch tucked in around the plants before freeze-up. I have a large basswood tree that supplies just such a mulch every year to a nearby flower border.
Never leave plants in containers as frost will enter the soil from all sides and the severe cold will kill them. Heel in evergreens and perennials into the garden that you have had in containers for the summer and put mulch around them. Offer extra protection to plants located in raised beds or planters for the same reason. If you must leave a plant in its pot, bury the pot up to its rim in the ground and then cover the whole thing with a thick layer of mulch. All of this effort at winterizing your garden will be worth it when you see your plants emerge next spring none the worse for wear no matter how severe the winter.