Space hoops about 4 feet apart over a 3-4 foot bed, cut each hoop to the arc length of the shape you want, bury or weight the cover edges against wind, and plan for snow, which collapses a light tunnel. A semicircular hoop over a 1.2 metre bed needs an arc of π times 0.6 metres, which is 1.885 metres, plus the part driven into the soil. Cap the ends of every hoop at eye height and take the cover off or knock the snow down before it builds up.
A low tunnel is a row of arched hoops of wire, pipe, rod or cut wood stretched with film or fabric over a bed, used to protect crops from frost, wind and cold rain.
The Hoop Shape Fixes The Cover Width, The Height And The Strength
The carrier guide (Four-Season Growing & Season Extension) lists hoop materials in order of preference as 9-gauge or heavier wire, 1/2-inch PVC pipe, fresh green willow or hazel rods (lasting one or two seasons) and rebar stakes with PVC slipped over them; cover options as 6 mil greenhouse plastic (lasting 1-4 years), spunbond row-cover fabric and a double layer; and dimensions of hoops every 4 feet, 3-4 feet tall at the peak, covering a 3-4 foot bed, with edges secured by sandbags, boards or buried edges.
Worked Example: Length Of A Hoop
A semicircular arch of radius r has a height r, a span 2r and an arc length π × r. For a bed 1.2 m wide, r = 0.6 m, so the height is 0.6 m and the arc length is 3.1416 × 0.6 = 1.885 m. If 0.3 m of each end is driven into the soil, cut each hoop at 1.885 + 0.6 = about 2.5 m. The carrier’s 3-4 ft (0.9-1.2 m) height over a 3-4 ft bed is taller than a semicircle, so it needs a longer hoop; bend a string to the shape you want and measure it, then cut every hoop to that length. For the cover, add a skirt to hold the edges: a cover wide enough for the arc plus 0.3 m on each side is 1.885 + 0.6 = about 2.5 m.
Method
Step 1: Prepare the bed and mark the hoop line. Place hoops at the carrier’s 4 ft (1.2 m) spacing; closer in windy or snowy places.
Step 2: Set the hoops. Push wire or pipe hoops about 0.3 m into the soil on each side, or slip PVC over rebar stakes so the stake does the gripping.
Step 3: Stiffen with a ridge line. A cord or a lath tied along the top of the hoops keeps them from tipping in line with the row.
Step 4: Stretch the cover. Pull it taut along the row, so wind cannot flap it, and bury, board or weight the edges. Use sandbags or boards on the sides so you can lift the cover to vent.
Step 5: Close the ends with a flap you can tie open.
Step 6: Plan for snow. Light hoops of 1/2-inch pipe can collapse under wet snow. Brush snow off in the storm, or add a central support. If snow is heavy where you are, a low tunnel is not the structure for deep winter; use a stronger frame.
Common Mistakes
- Loose film — wind flaps it, abrades it and lifts it off.
- Hoops too far apart — the cover sags and pools rain.
- Burying a tight tunnel and forgetting vents — see Ventilation and Temperature Management.
- Using PVC in deep cold — it grows brittle; check before bending.
Safety
Exposed hoop ends are an eye hazard
The tips of wire hoops, rebar and cut rods stand at head height when you work at the bed. Cap every end with a bottle, a rubber boot or a wad of tape and rag. Do not walk into a hoop row at night or in snow, and do not crawl under a snow-laden tunnel that might collapse.
What’s Next
- Learn next: Ventilation and Temperature Management — what to do with the cover each day
- Combine with: Plastic Film & Alternatives — choosing the cover