How to Build a Low Tunnel (Hoop House)

A low tunnel is the workhorse of season extension: a row of arched hoops covered with clear plastic or garden fleece, forming a long, knee-high mini-greenhouse over a single bed. It costs a fraction of a walk-in greenhouse, goes up in an afternoon, and can shift your effective climate by a full USDA zone or more — letting you start crops weeks early in spring, push harvests deep into autumn, and overwinter hardy greens under cover. The build is genuinely simple, but a few details separate a tunnel that thrives from one that overheats, blows away, or crushes under snow. This guide walks through materials, hoop spacing, a step-by-step build, anchoring against wind, and the ventilation routine that keeps your crops from cooking on sunny days.

Low Tunnel vs. Cold Frame vs. Row Cover

All three are passive, unheated season extenders, but they suit different needs. A cold frame is a fixed, rigid box — durable and great for small batches and hardening off, but limited in size. A floating row cover lies directly on the crop with no hoops, giving a few degrees of frost protection and pest exclusion at minimal cost. A low tunnel sits between them: hoops lift the cover off the foliage, creating an air space that traps more heat and gives plants room to grow, while still being cheap, modular, and easy to move. If you want to cover a whole bed or a long row and gain real warmth — not just a couple of degrees — the low tunnel is the tool. Choose plastic when you want maximum heat and a humid microclimate; choose fleece when you want breathable frost and wind protection without the daily venting chore.

Materials You’ll Need

Hoops. The most common and affordable option is 1/2-inch PVC pipe, cut to length and bent into an arch. For a 3–4 foot wide bed, 10-foot lengths of PVC give a comfortable arch about 2–3 feet tall at the center. Metal options — 1/2-inch EMT electrical conduit bent with a hoop bender, or 9-gauge galvanized wire for shorter spans — are sturdier and won’t degrade in UV like cheap PVC can over years. Avoid undersized, flimsy wire on wide beds; it sags under snow.

Anchors. Short lengths of rebar (12–18 inches, 3/8 or 1/2 inch) driven halfway into the ground make perfect sockets — slide a PVC hoop over each exposed stub and it stands rigid. Metal conduit hoops can often be pushed straight into the soil. Cover. For warmth, use 4–6 mil clear greenhouse polyethylene (UV-stabilized, so it lasts several seasons). For breathable frost protection, use spunbonded polypropylene row cover/fleece in a medium weight (around 0.9–1.25 oz/sq yd for frost; heavier for harder freezes). Fasteners. Snap clamps or spring clips made for the hoop diameter hold the cover to each hoop; you’ll also want sandbags, boards, bricks, or soil to weigh down the edges. A ridge line of twine over the top, tied to end anchors, helps the cover shed wind and snow.

Step-by-Step Build

1. Prepare and orient the bed. Build over a bed roughly 3–4 feet wide. Run the tunnel north-to-south where possible for even light, and pick a sheltered, well-drained spot. Sow or transplant your crop first — it’s far easier to plant before the hoops go up.

2. Set the anchors. Drive rebar stakes into the ground along both long edges of the bed, directly opposite each other, spaced 3–4 feet apart down the row. Leave 6–9 inches of each stake above ground. Closer spacing (3 feet) makes a stronger tunnel for snowy or windy sites; 4 feet is fine in mild conditions.

3. Install the hoops. Push one end of a PVC length over a rebar stub, bend it into an arch, and slide the other end onto the opposing stub. Repeat down the row. The hoops should stand 18–36 inches tall at the center — tall enough to clear your crop at maturity. Aim for consistent height so the cover sits evenly.

4. Add a ridge line (optional but recommended). Run a length of strong twine or a thin PVC/conduit spine along the apex of the hoops, lashed or clipped at each one, and anchor it firmly at both ends. This ties the hoops together so they don’t splay, and it helps the cover shed snow and resist wind uplift.

5. Drape and secure the cover. On a calm day, unroll the plastic or fleece over the hoops, leaving plenty of extra material at both ends and along both sides. Clip the cover to each hoop with snap clamps so it can’t shift. Then anchor the perimeter (see the next section). Leave the cover slightly loose rather than drum-tight — a little slack lets it flex in wind without tearing.

Anchoring Against Wind and Snow

Wind is the number-one killer of low tunnels — an unsecured cover acts like a sail and is gone by morning. Anchor the long edges by one of these methods: bury the cover edge in a shallow trench and backfill with soil; lay the surplus material flat and weigh it down with a continuous line of sandbags, lengths of lumber, bricks, or rocks; or clip the cover to a ground-level pipe or wire. The ends take the most strain, so gather and twist the excess material and tie it off to a stake or weigh it heavily with a sandbag, leaving enough to reopen for venting. Clamps on every hoop stop the cover from sliding lengthwise.

For snow, slope and support matter. Steeper, well-tensioned arches shed snow better than flat, saggy ones, and a ridge line plus closer hoop spacing prevents collapse under wet snowfall. After heavy snow, brush it off promptly — even a sturdy tunnel can buckle under accumulated weight. In very exposed gardens, double-anchor the windward side and consider a windbreak of fencing or shrubs.

Ventilation: Don’t Cook Your Crops

With a clear-plastic tunnel, ventilation is not optional — it is the difference between a thriving bed and a row of wilted, scorched plants. Sealed plastic traps solar heat aggressively: an outside temperature of 40°F can push the interior past 90°F within an hour on a sunny day, and heat that high cooks roots, bolts greens, and stresses everything inside. Vent the tunnel whenever it’s mild and sunny. The easiest method is to open both ends so air flows through; on warm days, also lift one long side and clip it up to the hoops to dump heat. Then close everything by mid-afternoon to trap warmth for the cold night ahead. In spring and fall, expect to vent most sunny days; in deep winter, vent only on the mildest, brightest days.

Fleece (spunbonded row cover) is far more forgiving because it’s breathable — air and rain pass through, so it rarely overheats and needs little to no daily venting. The trade-off is that fleece holds less heat than plastic, offering frost protection of a few degrees rather than a warm growing microclimate. Many gardeners run plastic in the coldest months for maximum warmth and switch to fleece in the shoulder seasons for hands-off frost and pest protection. Whatever you use, watch the inside temperature for the first week and learn how your tunnel behaves on your site.

What to Grow and When

In spring, a low tunnel warms the soil so you can direct-sow radishes, salad greens, carrots, peas, and beets two to four weeks ahead of the open garden, and protect early transplants from late frosts. In autumn, it keeps crops growing and harvestable well past the first frosts. Through winter, it overwinters genuinely hardy crops — spinach, mache, kale, chard, scallions, and overwintering onions and garlic — for harvest on mild days. Water sparingly under plastic (the enclosed space stays humid and fungal rot loves wet, stagnant air), always in the morning, and ventilate to keep leaves dry. Pull the cover off entirely once spring warms up, then store it clean and dry for next season. Built and tended this way, a single low tunnel pays for itself many times over in extra harvests.