A late spring or early fall frost can quickly destroy an entire season’s fruit harvest. When temperatures suddenly drop below freezing, delicate flower buds, blossoms, or newly set fruit are highly susceptible to damage. For smaller or younger fruit trees, covering the canopy is a practical, low-tech intervention that creates a microclimate. This effectively shields the vulnerable parts of the tree from freezing air, which is often the difference between a successful harvest and a total crop loss.
Assessing Vulnerability and Timing
The decision to cover a fruit tree depends on the forecast and the tree’s current stage of development. Protective measures become necessary when frost occurs at 32°F (0°C) or below. However, the exact temperature that causes damage, known as the critical temperature, depends highly on the bud’s maturity.
A fruit tree’s vulnerability increases significantly as it progresses from a tight bud to an open flower. During the tight bud stage, many varieties can withstand temperatures in the mid-20s Fahrenheit. Once blossoms are fully open, the damage threshold rises sharply, often to 28°F (-2.2°C) or higher. Open flowers are the most susceptible because the reproductive structures, particularly the pistil, are easily killed by freezing temperatures. Monitoring local forecasts for expected overnight lows is the first step in protecting your harvest.
Selecting the Right Cover Materials
The choice of material directly impacts the amount of heat retained. Breathable fabrics like old cotton sheets, blankets, or burlap are the best options, as they allow for air exchange while insulating the tree. These materials trap the warmth radiating from the ground and the air surrounding the tree, effectively raising the temperature beneath the cover by several degrees.
Specialized commercial frost cloth, often sold as row covers or garden fleece, is also highly effective and lightweight. These materials are designed to be permeable to light and water while providing a thermal barrier. Use extreme caution with plastic sheeting, as it does not insulate well and can transfer cold, causing contact damage to foliage. If plastic must be used, drape it over a separate frame or stakes to ensure it never rests on the buds or branches.
Step-by-Step Covering Technique
Proper covering technique focuses on maximizing the heat trapped beneath the barrier, which is accomplished by sealing the cover at the base of the tree. Preparation should begin by ensuring the soil beneath the tree is moist. Wet soil absorbs more solar radiation during the day and releases more latent heat overnight. The covering process itself should be done in the late afternoon or early evening to trap the warmer air that has accumulated throughout the day.
For smaller trees, install sturdy stakes around the perimeter of the canopy to prevent the material from crushing delicate flowers or fruit. Drape your chosen material entirely over the tree and frame, ensuring the fabric extends all the way down to the ground. Secure the edges of the cloth to the soil using bricks, rocks, or heavy lengths of wood. This thermal seal prevents warm ground air from escaping and cold air from seeping in, maintaining a survivable temperature for the canopy.
Post-Frost Management
Removing the protective cover at the correct time is crucial. Leaving the material on for too long can expose the tree to risks, including overheating and sun scald once the sun is high. The covers should be removed promptly in the morning after the temperature rises above freezing and the immediate threat of frost has passed.
Delaying removal can also trap moisture, creating conditions favorable for fungal diseases, and prevent necessary pollination by insects. After removing the cover, examine the tree for potential damage, particularly the center of the flowers or buds. If the internal tissue, such as the pistil, appears brown or black, it indicates freeze damage. Do not prune immediately; wait until new growth begins to determine the full extent of the injury.