Last winter was remarkably tough on many landscape plants. Rhododendrons were especially hard hit, as well as numerous other evergreen and deciduous trees and shrubs. While it is impossible to entirely prevent plant injury from severe winter weather conditions, there are a number of practices gardeners can employ to help keep their trees and shrubs healthy.
Proper care during the growing season and right up through the fall is a crucial part of keeping evergreens alive through the winter. Providing adequate water is essential to keep plants from suffering from stress, as healthy plants are much better prepared to survive the winter. For optimum growth, most woody plants require one inch of rainfall or supplemental irrigation every week. In the fall, when the air temperature drops below that of the soil, shoot growth ceases and roots continue to develop until the soil dips below 40℉. In order to encourage maximum root development, and by extension improve winter hardiness, water thoroughly and consistently, applying enough water to moisten the soil to a depth of eight to ten inches at least once a week, continuing until freezing temperatures arrive.
Wrapping with Burlap and Making Windbreaks
Broadleaf evergreens, such as Rhododendrons, are particularly susceptible to drying out in the winter months. Even in cold weather, leaves and needles lose water in a process called transpiration. Water loss is greatest during periods of strong winds and mild sunny weather. In super cold temperatures, the ground freezes and cuts off the water supply to the plant’s roots. When water is transpired faster than it is taken up, the leaves begin to desiccate and turn brown.
Desiccation can be mitigated by erecting windbreaks made from burlap or canvas attached to frames around the plants. These barriers should be placed on the side of the prevailing winds. Some plants such as arborvitae have growth habits which lend themselves to a complete wrapping of burlap. If wrapping plants, never use black plastic as it causes extreme temperature fluctuations. Wrapping with burlap and building windbreaks isn’t always enough to prevent winter injury, but it can help. If nothing else, plants wrapped with burlap are less likely to be browsed by deer.
Gardeners frequently ask whether anti-dessicant sprays should be used to protect evergreens from winter damage. There is evidence that anti-dessicants can be helpful when applied correctly but they can be ineffective or even damaging when used inappropriately. For best results, make sure to read and follow all instructions on the product label. Most anti-desiccants are best applied when temperatures are around 40-50 degrees. Within this temperature range, the spray should have good coverage on the foliage. Because plants lose water through both the upper and lower surfaces of their leaves, all parts of the plant should be sprayed. Make sure not to apply too early. Spraying anti-dessicants before plants are dormant increases potential for damage, because the spray can trap excess water in leaves, which can freeze and cause cells to rupture. Wait to apply until evergreens are fully dormant in the late fall.
Mulching to Protect Roots
Perhaps surprisingly, snowy winters are often best for tree and shrub survival. Snow cover insulates the soil and helps prevent it from reaching a killing temperature, and it limits freezing and thawing. Thus, woody plants are more likely to suffer cold damage in winters where there is very little snow to protect their root systems. Since there is no guarantee of adequate snow cover, mulching trees and shrubs (especially those that have been newly planted) becomes very important. Aim to apply at least two inches of woodchips or straw over the root zone, taking care not to pile mulch against trunks. Extra mulch can be removed in the spring once the ground begins to thaw. When available, cut evergreen boughs can also provide good insulation.
Preventing Deer Damage
Throughout much of New Hampshire, white-tailed deer have become a major garden and landscape pest. When food is scarce in winter months, deer will heavily browse on some evergreen plants, including arborvitae (Thuja occidentalis) and yew (Taxus sp.). Installing fencing around plants susceptible to deer is the most effective method of protection, but it can be quite expensive and impractical in some landscapes. Many repellants are also available on the market with mixed degrees of effectiveness. In general, repellents that trigger a fear response are most effective. These products generally contain putrescent egg solids, predator urine or slaughterhouse wastes, and they may not be appropriate for use near heavily-trafficked walkways or buildings. Alternatively, you can place area repellents near affected plants, such as bar soap or garlic “sticks.” These repellents are usually clipped or hung from the branches of trees and shrubs that deer enjoy. If deer are only an occasional issue in your garden, you may find that they provide enough protection. Even in the best of circumstances, repellents will never completely eliminate deer damage, but they can help reduce it. For best results, make sure to apply repellents according to the product label.
Note that all deer-proofing methods work best when they are employed early in the season. Start applying repellents in the mid-to-late fall to discourage deer from making regular visits to your landscape.
Plant Selection and Planting Location
Even when you do everything you can to protect plants, winter damage is still a possibility. Some plants are simply better adapted to survive than others. Many winter injury issues can be solved by choosing appropriate plants, and hardiness is the first thing to consider. Trees and shrubs should be hardy enough to survive in the zone where they are planted without too much extra care. In most of New Hampshire, this means selecting plants which are hardy in Zones 3-5.
Another thing to keep in mind is planting trees and shrubs in the proper place in the landscape. Winter winds and sun can be extremely damaging to evergreens so they should be planted in protected spots out of the prevailing winds. Broadleaf evergreens in particular should be planted on the north, northeast, or eastern sides of buildings, or behind barriers where they are protected from the elements.
Broadleaf trees that have thin bark, like maples and cherries, are susceptible to frost cracking. This type of injury occurs on the southwest side of trees on sunny days in the winter when the sun warms the bark enough for the sap to flow. When the temperature drops quickly, the bark contracts and splits vertically. Sunscald can also occur when the temperature drops suddenly. Cells that have become active on the sunny side of plants are killed, resulting in dead, sunken areas. If planting a tree with thin bark, try to place it in a location where it will receive some protection from winter sun instead of in the open landscape. Even consider placing smaller shrubs near the base that will shade the south side of the tree and reduce the likelihood of frost cracking.
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