Freezing temperatures present a significant hazard to garden plants. When the air temperature drops below freezing, water in the spaces between plant cells turns into ice crystals. Since ice has a lower water potential than liquid water, this ice formation draws water out of the plant cells. This process dehydrates the cells, similar to a physiological drought, leading to damage to the cell membranes and tissue collapse. Recognizing susceptible plants and applying the correct protection method is necessary to mitigate this harm.
Prioritizing Vulnerable Plants
The need to cover plants is directly related to their natural hardiness, making species from warmer climates the highest priority. Tropical varieties, such as hibiscus, citrus trees, and crotons, are extremely sensitive to cold and can sustain damage below 40°F. These plants lack the physiological mechanisms to tolerate ice formation and must be sheltered immediately when a freeze is predicted.
Warm-season annuals also require immediate attention since their life cycle occurs during mild conditions. Plants like impatiens, petunias, and basil possess fleshy stems and leaves highly susceptible to freezing. Protecting them prevents immediate loss and allows for a longer growing season.
Any plant grown in a container is significantly more vulnerable than its in-ground counterpart. The soil mass in a pot is fully exposed to cold air, causing the root ball temperature to drop quickly. This lack of thermal mass increases the risk of root damage, even for marginally hardy species.
Newly planted shrubs and trees should also be covered because they lack the established root systems of mature specimens. Young plants have not developed the reserves or deep roots that buffer against temperature fluctuations. Protecting these additions ensures their survival until they become fully established.
Identifying Plants That Do Not Need Covering
Not every plant in the garden requires protection. Established perennial plants native to the region are generally safe because they are adapted to local winter conditions. These plants, such as coneflowers, hostas, and peonies, enter a natural state of dormancy where their roots can easily withstand standard freezing temperatures.
Mature, woody shrubs and trees rated for the local hardiness zone rarely require covering. Once established, their wood and bark provide sufficient insulation for the inner tissues and buds. Attempting to fully cover a large tree is often impractical and unnecessary.
Many cool-season annuals and vegetables, including pansies, ornamental cabbage, and kale, are bred for cold tolerance. While they might look wilted after a hard freeze, their roots are resilient enough to recover quickly when temperatures rise. Focusing protection efforts on truly tender, high-risk plants saves effort and resources.
Essential Techniques for Covering Plants
Effective covering techniques rely on trapping the radiant heat released from the ground while preventing direct contact between the material and the foliage. Breathable materials are the preferred choice, as they allow for air circulation and moisture escape while providing insulation.
- Old bed sheets
- Burlap
- Blankets
- Specialized frost cloth
Avoid using plastic sheeting directly on the foliage, as it can transfer cold to the plant tissue and increase the risk of damage. If plastic is used, it must be supported by a frame, stakes, or wire hoops to create a protective air barrier between the plastic and the plant. The goal is to create a small, insulated microclimate.
The covering must extend all the way to the ground and be secured around the perimeter with bricks, rocks, or soil. This anchoring is necessary because the heat radiating from the soil is the primary source of warmth being trapped beneath the cover. A cover that only wraps the top of a plant allows the warm air to escape and offers minimal protection.
For larger plants, construct a simple structure by driving stakes into the ground around the plant to support the covering. This prevents the material’s weight from crushing branches and ensures the insulating air space remains intact. Layering multiple, lightweight coverings can also significantly increase insulation during prolonged or severe freeze events.
Pre-Freeze Preparation and Post-Freeze Care
Preparation for an impending freeze begins about 24 hours before the temperature is expected to drop. Thoroughly watering the plants is a beneficial step, as moist soil holds significantly more heat than dry soil. This saturated earth acts as a natural heat sink, radiating warmth back up toward the covered plant and enhancing the covering’s effectiveness.
Container plants should be moved to a protected location, such as a garage, shed, or covered porch, to shield the exposed root ball from freezing air. If containers are too large to move, group them closely together and wrap the pots with burlap or blankets. This provides additional insulation for the roots and helps the pots retain residual heat.
After the freezing event, the timing for removing the covers is important to prevent heat buildup and sun scald. Covers should generally be removed once the air temperature rises above freezing and the sun is fully out. Removing them too late can cause the plant to overheat quickly under the material.
Post-freeze care requires patience, as the full extent of the damage may not be immediately visible. Resist the urge to immediately prune off brown or wilted foliage. Damaged plant parts can continue to provide insulation for the living tissue beneath them, so wait until new growth appears in spring for an accurate assessment.