Frost and freezing temperatures pose a significant threat to trees, especially young, newly planted, or tender species. Damage occurs when water within the plant’s cells freezes, forming ice crystals that rupture cell walls. This cell death leads to blackening of leaves, stem dieback, and structural compromise, making preventative action necessary.
Preparing the Tree and Root Zone
Moisture content in the soil is a primary factor in root zone protection because moist soil retains more heat than dry soil. Deep watering the root zone 24 to 48 hours before a predicted freeze is a highly effective pre-treatment. The water replaces air pockets and absorbs heat during the day, releasing it slowly overnight to provide a buffer of warmth for the roots.
Once the soil is hydrated, apply a thick layer of insulating mulch around the base of the tree. Materials like straw or wood chips, spread several inches deep, stabilize the ground temperature and minimize heat loss. This layer must be kept a few inches away from the trunk to prevent moisture buildup and potential rot near the root flare.
For thin-barked or young trees, temporary trunk wraps, such as burlap or commercial tree wrap, provide additional protection. This covering shields the cambium layer from rapid temperature fluctuations that can cause frost cracks. Protecting the cambium near the soil line is particularly important as this area is sensitive to cold damage.
Implementing Above-Ground Protection
Protecting the tree’s canopy and branches requires physical barriers that prevent heat loss and shield foliage from direct contact with freezing air. Suitable covering materials include specialized frost blankets, horticultural fleece, or burlap, which are lightweight and allow the tree to breathe. These materials should reach the ground to effectively trap the warmth radiating from the soil below.
It is important to avoid using plastic sheeting or heavy household blankets directly on the foliage. Plastic transfers cold where it touches the plant tissue, causing localized damage, and heavy materials can break brittle branches. Instead, set up temporary support structures, such as stakes or a frame, to hold the protective cover away from the leaves and branches. This air gap creates an insulating layer, enhancing the cover’s effectiveness.
For extreme cold events, supplemental heat can be introduced beneath the covering to raise the temperature around the canopy. Simple incandescent string lights can be woven through the inner branches, as they emit a small amount of radiant heat. These heat sources are most effective when used in combination with a cover that extends to the ground, concentrating the rising heat around the tree.
Timing the application and removal of the covers is important. Covers should be placed over the tree before sunset to trap the day’s retained heat. They must be removed the following morning after the temperature rises above freezing, preventing the tree from overheating and ensuring adequate light exposure.
Assessing and Treating Frost Damage
Following a freeze, the initial appearance of the tree can be alarming, often showing blackened, drooping, or wilted leaves. However, the true extent of the injury is not immediately apparent because damage to internal tissues may take time to fully manifest. The most important action immediately following a freeze is patience.
Immediate pruning is generally discouraged for woody plants, as seemingly dead branches may still contain living tissue that can sprout new growth later in the season. Removing damaged foliage too soon can also stimulate vulnerable new growth that subsequent late frosts may kill. Pruning should be delayed until late spring or early summer, when new growth appears and the boundaries between living and dead wood are clearly visible.
While delaying pruning, support the tree’s recovery by maintaining a consistent care routine. Ensure the tree has access to water, especially since a freeze can make soil moisture unavailable to the roots. Continued watering and avoiding fertilizer until the tree shows clear signs of recovery will help it focus energy on healing and generating new growth.