8 Supplies for Building Cold Frames and Low Tunnels

Late autumn frosts can wipe out healthy crops overnight, but simple season extension structures keep harvest yields steady straight through winter. Building dependable cold frames and low tunnels requires materials that withstand heavy snow, howling winds, and intense moisture without collapsing or degrading. Investing in purpose-built components ensures your winter greens and early spring starts stay protected through the harsher months.

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What to Consider Before Building Cold Frames

Microclimates and sun exposure dictate structural placement. Positioning frames along a south-facing wall or slope maximizes critical winter solar gain while offering natural wind protection. Keep structures close to high-traffic pathways so daily monitoring during freezing weather remains convenient.

Thermal capacity varies wildly based on structure design. Simple unheated cold frames rely solely on soil thermal mass and direct sunlight, making tight construction essential. High-humidity environments created under tight plastic require accessible venting pathways to bleed off excess heat on clear afternoon days.

Materials must balance budget constraints with multi-season durability. Temporary polyethylene structures provide low-cost cover for quick crop turnover, whereas rigid polycarbonate installations offer superior insulation for multi-year raised bed operations. Matching structure design to local wind loads and expected snowpack prevents costly structural failures mid-winter.

Polyethylene Film – Farm Plast 6 Mil Clear Film

Standard hardware store drop cloths break down under sunshine within weeks, leaving shredded plastic across beds. A true season-extension setup demands heavy-duty greenhouse plastic engineered specifically to trap radiant heat and endure severe weather. Farm Plast 6 Mil Clear Film provides the primary weather barrier that sheds rain, blocks freezing winds, and maintains high interior temperatures.

This four-year rated polyethylene sheet features built-in UV inhibitors that prevent premature yellowing and brittleness. Its six-mil thickness balances exceptional tear resistance with optimal light transmission, allowing over 90 percent of photosynthetically active radiation through to crops.

• Thickness: 6 mil heavy-duty construction
• UV Protection: Integrated light stabilizers rated for 4+ years
• Light Transmission: 90%+ clear light passage
• Best Uses: Low tunnel covers, cold frame tops, caterpillar tunnels

Proper installation requires stretching the film drum-tight over support hoops to prevent water pooling during heavy rainstorms. Never substitute general construction sheeting, as non-UV plastic breaks into micro-fragments that contaminate farm soil. This film is perfect for growers seeking dependable, multi-season tunnel covers, though permanent glass or polycarbonate remains better for heavy structural cold frames.

Tunnel Hoop Conduit – Allied Tube Metallic Tubing

Flexible PVC pipe often snaps under winter snow accumulation or sags under heat tension during early autumn. Rigid metallic conduit forms an unyielding skeletal frame that keeps heavy covers off delicate crop foliage. Allied Tube Electrical Metallic Tubing (EMT) provides superior structural integrity for low tunnel arches without adding excessive weight.

Manufactured from high-strength galvanized steel, this tubing features a smooth internal and external coating that prevents rust stains and fabric abrasion. The triple-layer inline galvanizing process ensures the metal withstands constant contact with damp soil and high-humidity interior environments.

• Material: Galvanized steel EMT conduit
• Standard Sizes: 1/2-inch and 3/4-inch trade sizes (10-foot lengths)
• Corrosion Resistance: Galvanized interior and exterior walls
• Best Uses: Bent low tunnel hoops, ridge purlins, structural ribbing

Bending 10-foot lengths of 1/2-inch or 3/4-inch EMT requires a specialized hoop bender to achieve smooth, uniform curves without kinking the metal. Once bent, these steel arches withstand severe wind gusts and unexpected snow loads that crush plastic hoops. They are ideal for serious market gardens and backyard growers building semi-permanent field tunnels.

Frost Cloth – Agribon AG-19 Fabric Row Cover

Heavy poly film retains heat well, but direct contact with freezing plastic burns tender plant leaves. Floating row covers act as a thermal blanket, creating a protective buffer zone directly over foliage. Agribon AG-19 Fabric Row Cover offers mid-weight freeze protection while allowing essential moisture and sunlight to reach crops.

Fabricated from spunbond polypropylene, AG-19 delivers 28°F to 30°F frost protection without trapping excessive ambient heat during warm spells. It provides 85 percent light transmission while serving as an effective physical barrier against insect pests and hungry winter wildlife.

• Weight: 0.55 oz/sq. yd. medium-weight non-woven fabric
• Thermal Protection: Buffers down to 28°F (-2°C)
• Permeability: Pass-through design for air and water passage
• Best Uses: Direct plant wrapping, interior low tunnel lining, pest exclusion

Drape AG-19 directly over crops inside poly tunnels to create a double-layer insulation system for sub-zero winter nights. Handle the fabric carefully around sharp wire edges or rough wood to prevent fraying and tears. It is indispensable for shoulder-season greens and hardy brassicas, though heavy snow will push non-supported fabric directly into plants.

Film Clamps – Circo Innovations Snap Clamps

Tying down tunnel covers with twine or weighing them down solely with rocks creates stress points that tear expensive plastic. Dedicated pipe clamps anchor sheeting securely along structural ribs without piercing holes into the material. Circo Innovations Snap Clamps deliver high clamping pressure across metal or PVC tubing to hold covers taut.

These heavy-duty clamps are manufactured from durable ABS plastic, providing significant grip force and strong resistance to cold-weather cracking. Their bevel-edged design allows for quick snap-on attachment and smooth removal when opening tunnels for harvest or ventilation.

• Material: High-impact, UV-stabilized ABS resin
• Compatibility: Fits schedule 40 PVC and metallic EMT tubing sizes
• Design: Beveled edges to prevent film snagging
• Best Uses: Securing poly film and frost cloth to tunnel hoops

Match clamp size precisely to the outer diameter of your hoop conduit to ensure a non-slip grip during high wind events. Space clamps every two to three feet along end hoops and side purlins for maximum hold. These clamps are ideal for setups requiring frequent side adjustments, though permanent channel-and-wiggle-wire systems remain better for high-tunnel end walls.

Polycarbonate Panel – Lexan Thermoclear Sheet

Traditional glass cold frame lids are extremely heavy, prone to shattering under hail, and offer poor thermal insulation. Multi-wall polycarbonate panels provide structural rigidity alongside dead-air insulation space to retain maximum heat during freezing nights. Lexan Thermoclear Sheet stands out as the premium choice for hard-sided cold frames and durable raised bed covers.

Featuring a double-wall twin-flute construction, this panel traps insulating air channels to yield an exceptional high thermal R-value. It delivers high impact resistance—virtually unbreakable compared to standard glass—and incorporates proprietary UV protection on the exterior face to prevent yellowing over time.

• Structure: Twin-wall flute insulation design
• Durability: High impact resistance against hail and drops
• UV Treatment: Integrated single-side UV protective barrier
• Best Uses: Cold frame lids, rigid end-walls, permanent raised bed covers

Always orient the UV-protected side outward toward the sun during assembly to prevent rapid panel degradation. Seal the open fluted edges with breathable aluminum tape to stop moisture condensation, mold growth, and insect entry within the air channels. Lexan is ideal for homesteaders building long-lasting rigid frames, though its higher cost makes it impractical for spanning long outdoor field rows.

Vent Opener – Bayliss Autovent XL Controller

Trapped solar energy inside an unvented cold frame can cook winter greens in less than an hour on a bright, sunny afternoon. Manual venting requires constant physical attendance, which is difficult for part-time farmers working day jobs. The Bayliss Autovent XL Controller provides non-electric, automatic temperature regulation to safeguard crops from heat stress.

Powered by a solar-sensitive wax-filled hydraulic cylinder, this unit expands when internal temperatures rise, mechanically pushing cold frame lids open. As temperatures drop in late afternoon, the wax contracts, allowing the heavy arm to close the lid tightly against night frosts.

• Power Source: Non-electric mineral wax thermal cylinder
• Lifting Capacity: Operates vents weighing up to 12 pounds
• Temperature Range: Adjustable opening range between 55°F and 75°F
• Best Uses: Cold frame top lids, small greenhouse roof vents

Mount the controller securely to solid wood or aluminum framing, ensuring the vent hinge moves freely without binding. Disconnect or unhook the cylinder before deep winter sets in if heavy snow loads might force the vent mechanism past its mechanical limits. It is a mandatory investment for working growers, though unnecessary for open-ended field tunnels that rely on manual roll-up sides.

Ground Rebar Stakes – Sandbaggy Steel Rebar

Ground hoops inserted directly into loose garden soil quickly loosen and pull free during strong autumn storm fronts. Inserting heavy steel anchor pins deep into the ground provides an immovable foundation for structural arches to slide over. Sandbaggy Steel Rebar Stakes anchor low tunnel frameworks securely into high-wind field sites.

Cut from heavy-gauge structural steel, these ribbed rebar pins feature textured outer ridges that bite into dirt and gravel for exceptional ground holding power. Their clean-cut top ends allow standard 1/2-inch EMT conduit hoops to slip smoothly over the exposed stake tops.

• Material: Heavy-duty Grade 60 steel rebar
• Dimensions: Available in 18-inch and 24-inch lengths (1/2-inch diameter)
• Surface: Deformed ribbed texture for soil retention
• Best Uses: Low tunnel hoop ground anchors, timber frame stakes

Drive stakes into the soil at a slight 15-degree angle inward using a heavy hand mallet, leaving roughly six to eight inches exposed above grade. Wear heavy leather work gloves during handling to prevent scrapes from rough steel edges. These stakes are essential for loose field soils and exposed sites, though unnecessary if mounting hoops directly to wooden raised bed frames with metal conduit straps.

Conduit Bender – Quick Hoops Low Tunnel Bender

Attempting to bend steel conduit over a knee or around a tree produces irregular, kinked arches that fail under snow pressure. Achieving uniform, structurally sound arches requires a fixed-radius bending jig designed specifically for agricultural tubing. The Quick Hoops Low Tunnel Bender transforms straight metal conduit into perfect, high-clearance tunnel ribs.

Engineered to mount directly to a sturdy workbench, picnic table, or wooden post, this heavy steel jig features a leveraged curved tracking guide. It allows a single operator to shape standard 10-foot lengths of EMT conduit into uniform 4-foot wide arches without crimping or weakening the metal.

• Mounting: Bolt-down base for wooden surfaces
• Compatibility: Formed specifically for 1/2-inch EMT metal conduit
• Finished Dimensions: Creates 4-foot wide, high-clearance gothic or smooth arches
• Best Uses: Custom low tunnel hoop fabrication

Lever movement should be steady and smooth down the length of the pipe to create identical hoop profiles that line up evenly down the bed. Keep a spare piece of pipe handy to extend your leverage arm if bending heavier 3/4-inch conduit. This specialty tool pays for itself rapidly if building multiple row tunnels, but is an unnecessary expense if covering only one small bed.

How to Manage Temperature and Humidity Inside

Closed microclimates act as powerful solar collectors, spiking interior temperatures rapidly even when outside ambient air stays near freezing. Consistently monitoring high-low interior thermometers prevents leaf scorch and heat-induced bolting in overwintering spinach and lettuce. Venting must become a daily habit: open structure ends or lift frame lids whenever interior temperatures cross 65°F.

High humidity accumulation is the secondary threat inside enclosed winter structures. Transpiration from plant leaves combined with soil evaporation creates stagnant, saturated air that breeds downy mildew, botrytis, and root rot. Venting for even fifteen minutes during mid-day drives out moist air and replaces it with fresh, drier ambient air.

Layering materials inside structures provides flexible thermal control without trapping excess moisture. Laying frost cloth directly over crops inside a clear plastic low tunnel creates a buffer zone that adds several degrees of protection during extreme temperature dips. Remove inner fabric covers on bright days to let radiant heat warm the underlying soil bed.

Securing Your Structures Against Strong Winds

Wind uplift poses the greatest structural threat to winter season extension setups. High winds act like a wing across broad plastic surfaces, lifting unanchored hoops straight out of the ground and tearing covers away. Eliminating slack in the outer skin prevents wind from grabbing looseness and whipping plastic to pieces.

Tensioning purlins down the ridge line of low tunnels locks individual hoops into a single rigid structure. Run continuous lengths of poly rope, webbing, or a center EMT purge line tied firmly across every arch rib. Anchoring the end-hoops with diagonal guy-lines anchored to heavy screw-in earth anchors keeps the overall frame stretched end-to-end.

Secure the bottom edges of film covers using heavy continuous weight or deep soil burial. Placing sandbags or smooth heavy stones every two feet along side-skirts creates an effective seal against ground-level wind penetration. In high-exposure areas, trenching the windward edge of the plastic directly into soil remains the most reliable anchoring strategy available.

Essential Maintenance Tips for Off-Season Storage

Season extension equipment lasts significantly longer when properly cleaned, dried, and stored during warm summer months. Remove dirt, splash-back, and organic residues from plastic film and frost cloth using clean water and a soft bristle brush before packing materials away. Storing wet fabrics creates mildew that degrades synthetic fibers and ruins transparency.

Polyethylene sheets should be folded loosely rather than creased tightly to prevent stress cracks along fold lines over summer storage. Store plastic and woven fabrics inside sealed plastic totes elevated off barn floors to keep nesting rodents from chewing through folded rolls. Store metal conduit hoops stacked vertically or nested neatly on wall racks away from direct ground moisture.

Inspect hardware components like snap clamps, autovent openers, and rebar stakes before putting them away for the season. Wash dirt from snap clamp interiors to preserve their elasticity and check autovent wax cylinders for hydraulic oil leaks. Replacing worn clamps and damaged stakes over the off-season ensures your winter gear is instantly ready when autumn cold fronts return.

Building robust cold frames and low tunnels transforms unpredictable shoulder seasons into reliable, year-round harvest opportunities. Equipping your farm with commercial-grade film, formed steel hoops, and dependable hardware protects your soil investment against harsh winter weather. Set up your structures early in autumn, manage daily ventilation carefully, and enjoy fresh garden harvests straight through the deepest freeze.