8 Supplies for Protecting High Tunnel Crops in Winter

As winter winds howl outside, a well-prepared high tunnel can remain a vibrant oasis of cold-hardy greens and root crops. However, keeping these plants alive through freezing nights requires more than just a single layer of plastic. Equipping your structure with the right insulation, heating, and monitoring tools is the key to harvesting fresh food all winter long.

The Challenges of Winter High Tunnel Growing

High tunnels are excellent season extenders, but they do not heat themselves. When winter sets in, the lack of thermal mass means inside temperatures closely mimic outside drops. Without intervention, a sudden cold snap can easily penetrate the thin plastic barrier and freeze your crops solid.

Freezing winds, heavy snow loads, and reduced daylight hours create a harsh environment for winter crops. While hardy vegetables like spinach and kale can tolerate some cold, repeated freeze-thaw cycles damage cell walls and ruin harvests. Protecting these crops requires a proactive approach to temperature management.

Managing humidity is another silent killer in the winter high tunnel. High moisture levels combined with stagnant cold air invite gray mold and other fungal diseases that can wipe out a crop overnight. Balancing heat retention with proper ventilation is the ultimate winter growing challenge.

Creating a Multi-Layered Insulation Strategy

Relying on a single layer of greenhouse plastic is a recipe for crop failure during a deep freeze. A successful winter strategy uses "tunnels within tunnels," layering insulation to trap ground heat close to the soil. This method creates multiple microclimates that buffer your crops against extreme temperature drops.

Every layer of protection you add acts as a thermal barrier. Soil absorbs solar heat during the day and slowly releases it at night; low tunnels and mulch trap this rising heat right at the root zone. This passive system can keep the air around your plants up to 10 degrees warmer than the outside air.

This multi-layered defense reduces the reliance on active heating, saving you money and fuel. It creates a stable environment where crops can coast through the coldest weeks of the year. By combining the right supplies, you can keep your soil workable and your crops productive all winter.

Row Cover – Agribon AG-19 Floating Row Cover

Row covers act as a blanket directly over your plants, trapping heat rising from the soil while still allowing light and water to pass through. This creates a microclimate that can keep plants several degrees warmer than the ambient tunnel temperature. It is the foundation of any low-tunnel system.

Agribon AG-19 is the industry standard for winter crop protection because it strikes the perfect balance with 85% light transmission and 4°F of frost protection. This lightweight, spun-bond polyester fabric is durable enough to withstand multiple seasons without tearing easily. It provides excellent wind protection while remaining light enough to rest directly on hardy crops.

• Weight: 0.9 oz/sq. yd.
• Sizes: Available in widths from 5 to 10 feet
• Best Use: Direct canopy cover or low tunnel layering

This fabric is ideal for growers protecting hardy greens like spinach, claytonia, and kale. It is not heavy enough for extreme deep freezes below 20°F without an additional layer, so northern growers should pair it with a heavier blanket.

Support Hoops – Greenhouse Megastore Steel Hoops

Support hoops keep heavy row covers off delicate plant leaves. When wet fabric touches foliage during a freeze, it can cause severe frost damage to the plant tissue. Hoops also create a neat, organized low tunnel system within the larger high tunnel, making harvesting much easier.

Greenhouse Megastore Steel Hoops are made of 9-gauge galvanized steel, offering excellent rigidity and rust resistance. Unlike flimsy DIY wire options, these hoops slide easily into heavy soil and will not bend under the weight of thick frost blankets. They maintain their shape season after season, providing reliable structural support.

• Length: 6 feet (creates a 4-foot wide low tunnel)
• Spacing: Space every 3 to 4 feet along the bed
• Compatibility: Works best with snap clamps or fabric pins

These hoops are perfect for growers utilizing heavy-weight frost blankets or working in windy tunnels. They are an unnecessary expense for low-profile crops grown without covers, but essential for structured low tunnels.

Greenhouse Film – Sunsaver 6-Mil Clear Film

The outer greenhouse film is your high tunnel’s first line of defense against the elements. It blocks freezing winds, sheds heavy snow, and traps solar radiation during the day. Without a high-quality film, a high tunnel cannot function as a viable winter growing space.

Sunsaver 6-Mil Clear Film is engineered specifically for heat retention. It features infrared (IR) additives that trap radiant heat inside the tunnel at night, slowing down the cooling process. It also contains anti-condensate properties that prevent water droplets from dripping onto crops and spreading disease.

• Lifespan: 4-year UV warranty
• Light Transmission: 88%
• Installation: Must be installed with the correct side facing inward

This film is a must-have for northern growers facing long, freezing winters. It is an unnecessary expense for mild southern climates where simple utility-grade plastic is sufficient.

Wireless Thermometer – SensorPush HT1 Smart Sensor

Temperature fluctuations in a winter high tunnel can be extreme and rapid. A reliable thermometer allows you to track these changes without constantly walking out to the tunnel in freezing weather. It takes the guesswork out of when to vent or apply extra covers.

The SensorPush HT1 Smart Sensor is incredibly accurate and connects seamlessly to your smartphone via Bluetooth. It records hourly temperature and humidity data, allowing you to spot dangerous dips and spikes. Its compact, weather-resistant build means it can sit directly in the crop canopy where it matters most.

• Range: 325 feet line-of-sight
• Battery: CR2477 (lasts over 1 year)
• Connectivity: Requires the SensorPush G1 Gateway for remote WiFi access

This sensor is perfect for busy growers who need instant, reliable climate data sent to their phones. It is not ideal for those who prefer manual, low-tech thermometer readings and have time for frequent physical checks.

Propane Heater – Mr. Heater MH18B Big Buddy

Even with great insulation, extreme cold snaps can push temperatures below the survival threshold of your crops. An emergency heat source provides a temporary safety net to keep the tunnel above freezing. It is your insurance policy against crop-killing freezes.

The Mr. Heater Big Buddy delivers up to 18,000 BTUs of clean, efficient propane heat. It features a low-oxygen safety shutoff and a tip-over switch, making it safe for enclosed high tunnel spaces. It can run on portable cylinders or connect directly to a larger propane tank for extended run times.

• Heating Area: Up to 450 square feet
• Fuel: Propane (requires ventilation to prevent moisture buildup)
• Clearance: Keep away from flammable plastics and dry straw

This heater is ideal for hobby farmers needing emergency heat during historic cold snaps. It is not cost-effective or practical as a primary, continuous winter heat source due to high fuel consumption.

Frost Blanket – DeWitt Deluxe Season Fabric

When the weather forecast calls for a severe freeze, standard row covers are not enough. A heavy-duty frost blanket acts as a thick shield to protect sensitive crops from extreme cold. It is the ultimate layer of defense for mid-winter survival.

DeWitt Deluxe Season Fabric is a heavy 1.5 oz. non-woven polypropylene that provides up to 8°F of freeze protection. It is highly durable and UV-treated, meaning it will not degrade or tear when exposed to harsh winter sunlight and winds. It retains heat far better than lighter fabrics.

• Light Transmission: 50%
• Weight: 1.5 oz/sq. yd.
• Management: Must be removed on sunny days to allow light in

This blanket is essential for growers in northern zones facing prolonged sub-freezing daytime temperatures. It is not suitable for growers who cannot commit to removing it during warm, sunny spells, as the low light transmission will stunt plant growth.

Snap Clamps – Grip Clips Greenhouse Frame Clamps

High winds can easily lift plastic film and row covers, exposing your crops to freezing air. Securing these materials tightly to your frame is critical for maintaining a sealed environment. Reliable clamps prevent wind damage and heat loss.

Grip Clips use a unique two-piece design that locks fabric and plastic to the frame without puncturing the material. Unlike cheap plastic clamps that crack in the cold, these are UV-stabilized and freeze-resistant. They provide a drum-tight grip that will not slip under tension.

• Sizing: Available for 1/2-inch, 3/4-inch, and 1-inch pipes
• Installation: Can be stiff to apply when cold; warm them in water first
• Reusability: Easily removed and repositioned without damage

These clamps are perfect for growers in high-wind areas who need to secure covers quickly. They are not necessary if you plan to bury the edges of your plastic in soil trenches.

Straw Mulch – EZ-Straw Weed Free Premium Straw

Insulating the soil is just as important as protecting the air around your plants. Straw mulch acts as a natural blanket for the root zone, preventing the ground from freezing solid. It helps retain soil moisture and protects beneficial soil biology.

EZ-Straw is processed to be 99% weed-free, saving you from a massive weeding headache in the spring. It is treated with a natural tackifier that helps the straw stick together, preventing it from blowing away in drafty tunnels. The finely chopped texture makes it easy to distribute around delicate plants.

• Coverage: One bale covers 500 square feet at 2 inches deep
• Application: Keep a small gap around plant stems to prevent rot
• Moisture: Avoid overwatering, as wet straw can invite mold and pests

This mulch is excellent for insulating root crops like carrots and garlic. It is not recommended for tunnels already struggling with high slug populations or excessive humidity.

Managing Ventilation and Humidity in Winter

It is easy to focus so much on keeping cold air out that you trap excessive moisture inside. High humidity is the primary cause of winter crop loss, as stagnant, damp air encourages fungal pathogens to take hold. Proper ventilation is critical to keeping your plants healthy.

On sunny winter days, even when outdoor temperatures are below freezing, the greenhouse effect can rapidly heat the tunnel. Venting is necessary to release this warm, moisture-laden air before it condenses on the cold plastic and drips back onto the crops. This daily air exchange keeps the environment dry and buoyant.

Open ridge vents or roll up side curtains slightly during the warmest part of the day, typically between 10:00 AM and 2:00 PM. Always close the tunnel early enough in the afternoon to trap some residual solar heat before sunset. This simple routine prevents the extreme humidity spikes that occur overnight.

Daily Monitoring Routines for Winter Success

Winter growing is not a set-it-and-forget-it endeavor. Success relies on a disciplined daily routine of checking weather forecasts, monitoring internal temperatures, and adjusting covers accordingly. Staying ahead of the weather is the best way to prevent crop loss.

Start the morning by checking for condensation and assessing whether row covers need to be pulled back to let light in. In the late afternoon, ensure all low tunnels are buttoned up tight and heater fuel levels are checked if a freeze is forecast. This ensures your crops are protected before the temperature drops.

Keep a simple logbook of daily high and low temperatures alongside crop conditions. This historical data becomes an invaluable guide for refining planting dates and insulation strategies in future seasons. Over time, you will develop an intuitive sense of how your specific high tunnel responds to winter weather.

Protecting a winter high tunnel requires a thoughtful combination of insulation, structural support, and daily management. By investing in quality supplies and establishing a solid routine, you can successfully shield your crops from the harshest winter elements. With the right preparation, your high tunnel will provide fresh, flavorful harvests all year round.