Winter cold presents a significant threat to garden plants. The primary danger comes from the formation of ice crystals within plant cells, which can rupture the cell walls and lead to tissue death, often appearing as blackened or mushy foliage. Extremely cold temperatures can also freeze the soil, preventing roots from absorbing water, which leads to desiccation, particularly in evergreens susceptible to cold winds. Root systems, especially in containers, are vulnerable to freezing because they lack the insulation provided by the wider ground mass.
Preparing Plants for Winter Dormancy
Proactive steps taken weeks before the first hard frost are instrumental in helping plants naturally develop cold tolerance. Decreasing the amount of nitrogen fertilizer applied in late summer or early fall is crucial, since fertilization encourages tender new growth highly susceptible to cold injury. Instead, growth should be gradually slowed down by the shortening days and cooling temperatures. This process, called acclimation or hardening off, allows the plant to adjust its internal chemistry to better withstand freezing.
Controlled watering practices also aid in hardening off plants for winter survival. Reducing irrigation as temperatures drop decreases the chance of a late-season growth spurt. Plants should not be allowed to enter winter completely dry, however, as this increases the risk of desiccation. Strategic pruning should be limited in the fall, as new cuts can stimulate growth that will be damaged by cold weather. Leaving dead or dried foliage on perennials provides natural insulation for the crown of the plant.
Protecting Container and Potted Plants
Container plants are highly susceptible to cold damage because their root systems are more exposed to ambient air temperatures than in-ground plants. Tender species should be moved indoors to a sheltered location like a garage, basement, or unheated sunroom. For plants that still require light, they should be placed near a bright window or supplemented with a full-spectrum grow light.
For containers that must remain outside, grouping them closely together helps them share warmth and provides a buffer against cold winds. Place the most sensitive plants toward the center of the cluster or against the sheltered, south-facing wall of a building. Elevating containers off cold pavement or concrete with bricks or wooden blocks prevents frost from traveling directly into the root zone.
Insulating the pots protects the roots from fluctuating temperatures and prevents the container from cracking. Wrapping the sides of the containers with materials like burlap, bubble wrap, or old blankets adds a layer of thermal protection. A simple cold frame, which is an unheated structure with a translucent lid, can be used to overwinter hardy perennials and cool-season vegetables. The frame traps solar energy, raising the internal temperature by 5 to 10°F, but requires ventilation on sunny days to prevent overheating and fungal growth.
Shielding Outdoor and In-Ground Landscape
Established landscape plants, trees, and garden beds require physical barriers and soil modification to survive prolonged cold periods. Applying a thick layer of organic mulch, such as shredded bark, straw, or fallen leaves, insulates the soil and protects the root zone from freezing and the damaging freeze-thaw cycle. The mulch layer should be approximately 2 to 4 inches deep but must be kept a few inches away from the plant’s stem or trunk to prevent moisture buildup and rot.
Vulnerable above-ground plant parts can be protected using breathable frost blankets or row covers, which trap the heat radiating from the soil overnight. These specialized fabrics, often made of polypropylene, can provide several degrees of temperature protection and are available in different weights. For sensitive shrubs or trees, a temporary structure like a teepee made of stakes and draped with burlap or a frost blanket can prevent direct contact between the fabric and the foliage, reducing the risk of tissue damage.
Wrapping the trunks of young or thin-barked trees with commercial tree wrap or burlap helps prevent sun scald, which occurs when a warm winter sun heats the bark followed by a rapid freeze at night. Evergreens, which continue to lose moisture through their leaves in winter, can benefit from a windbreak constructed of burlap screens to reduce desiccation caused by cold, dry winds. The use of protective coverings is highly dependent on the plant’s natural hardiness and the expected low temperature.
Immediate Response to Unexpected Freezes
When an unexpected hard freeze is forecast, immediate action is necessary to prevent widespread damage. Thoroughly watering the soil a day or two before the temperature drop is a highly effective measure, as moist soil holds significantly more heat than dry soil, radiating warmth back into the air overnight. Water should be applied directly to the ground and foliage must be kept dry, since water on leaves can freeze and cause greater damage.
Covering tender plants with a sheet, blanket, or frost cloth just before sunset traps the residual heat released from the soil. The cover must extend all the way to the ground and be secured with rocks or soil to seal in the warmth. In a protected structure like a cold frame, a temporary heat source, such as an incandescent light bulb, can provide supplemental warmth. Remove or vent the coverings promptly in the morning once temperatures rise above freezing, as trapped sunlight can rapidly overheat the plant tissue.