Many plants shut down and go dormant for the winter to protect themselves from cold weather. As the days become shorter and the temperature drops in the fall, plants shift their energy from flowering and producing new growth to gradually hardening off their tissues.
The most important influence on the timing of dormancy is shorter daylight hours. For example, even during an Indian summer in autumn, plants continue to go dormant because of the change in light. If temperatures drop too suddenly before a plant has had time to shut down, the plant's tissues can be injured and it won't produce new growth in the spring.
The biggest culprit in winter damage is not the cold, it's the heat. Mid-winter warm spells create a cycle of freezing and thawing. This cycle causes frost heaves, which can expose fragile roots.
Winter burn and sunscald, two other common problems, are also caused by warm, winter sun. Winter burn is common among broadleaf evergreens such as rhododendrons and azaleas. It occurs when sun and wind remove moisture from the leaves faster than these shallow-rooted plants can replace it. Sunscald, similar to sunburn, thaws and then burns bark tissue of young trees, fruit trees, beeches and sycamores.
Other problems that plague the gardener in winter are salt damage from paths and roads, disfiguration and damage to hedges and fragile plants and dieback. The best way to ensure that your plants successfully overwinter is to plant them in the right place, based on growing and drainage requirements, and to make sure that they are healthy and well watered as they enter the winter season.
Tips for Winterizing Your Garden
- Be careful about salt damage from paths and sidewalks. Salt burns plants and kills root systems. Instead of sodium chloride (salt), try de-icing products that contain calcium chloride or magnesium and potassium chloride.
- To prevent damage caused by snow falling from eaves, direct ice and snow away from shrubs with water diverters. Wires on the roof also help to gradually melt snow.
- Mark your driveway with reflectors before the snow starts to pile up. Make sure that you don't plow snow onto valuable trees or shrubs. The force of the plowing often damages branches and you will be piling up harmful salt residues.
- Gently remove heavy snow from hedges and trees. Use a broom or broom handle and slowly push upward. If the snow has iced over, wait until the sun warms it up; don't try to break off ice crystals. Avoid using a shovel; it can damage branches.
- Remember that snow is a great insulator. As long as it is not crushing a plant or placing too much weight on vulnerable branches, let it naturally pile up around your plants.
- The best way to winterize hydrangeas and other shrubs that are susceptible to winter dieback is to place 4 to 6 stakes around the plant and wrap it with burlap. You don't need to cover the top of the plant; the windbreak created by surrounding the shrub with burlap is generally sufficient protection. Burlap can be attached to the stakes with industrial-sized staples, zip ties, string or nails.
- If you are worried that your arborvitae or other evergreens will be buffeted by snow (particularly columnar or upright forms), stake the trees and secure the trunk with nylon straps or tree savers that you can buy at a garden center. For arborvitae and yew, you can also take twine, attach it at the bottom of the trunk and wind it around the tree in a spiral to prevent the branches from being forced down and broken by the weight of the snow.
- If you have newly planted trees whose roots have not had a chance to get established or are weak, remember to stake them in the fall and remove the stakes first thing in the spring. This is particularly important for newly planted evergreens which can act like a sail in the winter wind.
- To protect from frost heaves and to retain moisture, surround newly planted trees with a 2 to 3 inch layer of mulch. Don't use more than 2 to 3 inches; mulch volcanoes can kill plants by causing cankers and disease on the lower trunk. Never mulch right up to the base of a tree or shrub. Likewise, keep mulch off the crown (the root/stem juncture) of a perennial.
- Winterize containers by wrapping them in bubble wrap. If the contents of the container are not hardy, place the container in an unheated garage so that the plants can go dormant during the winter.
- Make sure that plants and containers are well watered before winter sets in.
- Don't prune or fertilize your plants after mid- to late summer; it will only encourage new growth that risks being killed by frost.
- To prevent sunscald, don't prune in early or mid-winter. The canopy is important for shading out the intense rays of the winter sun. Wait until February to prune young or susceptible trees. You can also wrap the trunks of young trees in highly exposed areas on the south or west side of your property with tree wrap purchased at a garden center. However, it's generally best to let the tree tough it out and get used to its environment. Tree wraps can sometimes be an invitation for pest and disease problems.
- The best way to prevent winter burn on evergreens is to water them properly in summer and fall. Healthy plants have the best opportunity to successfully survive the winter. Siting is also important: broadleaf evergreens (such as rhododendrons) planted near a white house can often be damaged by the sun's reflected rays. Water plants on warm days in January, February and March. Adding a 2 inch layer of mulch will reduce water loss. In extreme cases, create a windbreak with stakes and burlap. You can also try anti-desiccants. Apply in late November and again in early February. Spray when it is 40°F or warmer. However, we rarely use anti-desiccants at the Garden and make efforts to properly site our trees and shrubs so that they can fend for themselves.