The forecast of temperatures hovering near the freezing mark often creates immediate concern for gardeners. A sudden drop in temperature, even slightly above \(32^{\circ}\text{F}\), signals a need to protect vulnerable plants from damage. Proactive cold protection is necessary for maintaining garden health, especially when dealing with non-hardy species. Understanding the relationship between air temperature and plant vulnerability is the first step toward successful gardening in fluctuating weather. Knowing when and how to apply temporary insulation ensures your garden survives an unexpected chill.
Is 35 Degrees a Threat?
The temperature of \(35^{\circ}\text{F}\) is technically above the \(32^{\circ}\text{F}\) freezing point, yet it still necessitates protective action for many garden plants. This discrepancy arises due to radiational cooling, which occurs on clear, still nights. Heat radiates quickly from the earth and plant surfaces into the atmosphere. The surface temperature of the plant tissue can drop several degrees below the measured air temperature, causing a damaging freeze even when the thermometer registers \(35^{\circ}\text{F}\).
This difference is why weather services issue a “frost advisory” when temperatures are forecasted to be between \(32^{\circ}\text{F}\) and \(36^{\circ}\text{F}\). Any moisture present on the leaves can freeze into frost crystals, which rupture the plant’s cell walls and cause widespread tissue damage. Without wind or cloud cover to insulate the ground, the chilling effect is amplified, meaning \(35^{\circ}\text{F}\) requires immediate preparation for susceptible species.
Identifying Vulnerable Plants
The need to cover plants is determined by the specific plant’s ability to tolerate cold, not just the air temperature. Plants are categorized based on minimum temperature tolerance, which helps prioritize protection. Highly tender plants, sensitive to any light frost, include tropical species, summer annuals like impatiens and begonias, and warm-season vegetables such as tomatoes and peppers. These species are easily killed when ice crystals form within their tissues and require immediate cover at the \(35^{\circ}\text{F}\) threshold.
Moderately tender plants, sometimes called half-hardy, can often withstand a light frost or temperatures dipping briefly down to \(28^{\circ}\text{F}\). This group includes cool-season crops like cabbage, kale, and pansies, along with newly transplanted seedlings that have not yet fully acclimated. Established hardy plants, such as most perennial shrubs and mature evergreens, generally require no protection at \(35^{\circ}\text{F}\) because they are genetically adapted to cold conditions.
Essential Covering Methods
The most effective covering materials are breathable and lightweight, such as old bed sheets, blankets, burlap, or commercial horticultural fleece. These fabrics allow moisture to escape while trapping heat radiating from the soil beneath. Non-breathable materials like plastic sheeting are less suitable because they hold moisture against the foliage, leading to severe freeze damage if the material contacts the plant tissue.
The mechanics of covering are as important as the material selection. The cover must be draped over the plant and secured so that it extends all the way to the ground. This creates a small dome that traps the residual warmth emanating from the soil, buffering against the cold air. Using stakes or a frame to keep the fabric from resting directly on the foliage prevents the transfer of cold from the material to the sensitive leaves, especially if the cover becomes damp with dew or frost.
Timing is a precise factor when deploying cold protection. Covers should be placed over plants before sunset to capture the warmth the soil has accumulated throughout the day. This trapped heat insulates the plant during the coldest hours of the night, which typically occur just before dawn. The coverings must be removed promptly the following morning, shortly after the temperature rises and the sun hits the plants. Leaving the cover on too long can cause rapid temperature buildup, damaging the foliage through overheating or scorching.
Protecting Container Plants and Hanging Baskets
Plants in containers and hanging baskets are uniquely vulnerable to cold compared to those planted directly in the ground. In-ground roots are insulated by the surrounding earth, maintaining a more stable temperature. Conversely, container roots are fully exposed to cold air on all sides, causing the soil to freeze much faster and deeper than garden beds.
The primary strategy for mobile plants is to move them to a sheltered location, such as a garage, shed, or a protected area near the house. Even an unheated structure provides a significant temperature buffer, protecting the roots from ambient air temperature and preventing the soil from freezing solid. If moving the containers indoors is not possible, the pots should be grouped closely together to share warmth and shield each other from wind exposure.
Wrapping the containers provides an additional layer of thermal protection for the roots. Materials like bubble wrap, burlap, or thick blankets can be secured around the sides of the pots to provide insulation. Because root damage is often the most damaging consequence of cold for containerized plants, focusing protective efforts on the pot itself is paramount.