A simple small polytunnel you can build yourself

Would you like earlier tomatoes, and aubergines and peppers that don't languish in the rain? A small polytunnel may be all you need. The model described here is 3 m long, 1.10 m wide and 1.80 m high. You don't walk inside it: you work from outside, rolling up the film along one long side. It shelters a single row of plants.

The principle (rebar driven into the ground, pipe hoops, a ridge pole and a film cover) comes from a much larger tunnel, designed by an American university as a cheap alternative to steel tunnels. Our reduced dimensions and reinforcements are a Semiterra adaptation. To be transparent: this model has not yet been tested in a real build. Treat it as a starting plan, to be adjusted to suit your site.

Before you start, check with your local authority: the rules vary from one country and one municipality to another, even for a garden shelter.

Why shelter these vegetables

  • Dry foliage: by keeping the rain off, the shelter limits diseases linked to wet leaves. On the other hand, those encouraged by humid air spread if the air circulates poorly, so ventilation is essential.
  • Less blight: it needs foliage to stay wet for several hours. Under cover, tomatoes are less affected, though not spared in some years.
  • A longer season: trials in Kentucky suggest you can plant and harvest about four to five weeks earlier than in open ground; other sources mention up to eight weeks gained in spring.

Materials

  • Pipe for the hoops: 25 mm black high-density polyethylene, in the 16 bar rating (the thickest wall, and therefore the stiffest), sold in coils for irrigation or drinking water. Each hoop has two straight sides of 1.25 m and a semicircle with a 55 cm radius, making about 4.25 m of pipe. Four hoops take 17 m, so a 25 m coil is enough. Its carbon black (2 to 2.5%) protects it from UV; one manufacturer quotes an operating range of −20 °C to +40 °C.
  • Eight lengths of rebar, 12 mm in diameter and 1.50 m long.
  • Four 3 m wooden battens: a ridge pole, two side battens and a roll-up batten.
  • UV-treated greenhouse film, about 150 µm; 200 µm is a common thickness. It lasts 3 to 5 years, compared with about 1 year for cheap green or reinforced tarpaulins.
  • Sandbags, strong string, screws, a sledgehammer, a saw, a drill-driver, gloves and safety glasses.

Avoid PVC pipe. Where it touches the film, it releases chlorine, which destroys the UV stabilisers in the polyethylene: a film designed to last five years failed in less than one. PVC also becomes brittle in the sun and loses impact resistance in the cold. If that is all you have, paint it with white acrylic paint or wrap it in PVC-free tape.

Bend in mild weather. Manufacturers disagree on the minimum bending radius: 15, 20 or 25 times the diameter at 20 °C, and 40 to 50 times at 0 °C. For a 25 mm pipe, that gives 37 to 62 cm at 20 °C. Our 55 cm semicircle falls within this range: shape the hoops on a mild day, never in cold weather. If the pipe kinks, don't force it.

Building it, step by step

  1. Mark out. Point one short end into the prevailing wind. Mark out the rectangle and check it is square by measuring both diagonals: they should be equal.
  2. Drive in the rebar. Four along each long side, one every metre, driven in 60 cm deep. That leaves 90 cm of bar inside each upright, which keeps the pipe straight. The original tunnel drives its bars in only 45 cm, but tunnel manufacturers ask for at least 60 cm for ground stakes. Its hoops are spaced about 90 cm apart, so our metre is slightly wider.
  3. Fit the hoops. Cut four lengths of pipe and slide each one over two facing bars.
  4. Fit the frame. Tie with string or screw the ridge pole beneath the top of the hoops, on the inside, then a batten halfway up each long side, also on the inside. A sturdy ridge pole stops the tunnel twisting, and the film will only touch smooth pipe.
  5. Lay the film in calm weather. A light breeze makes it awkward, a steady wind makes it impossible. Unroll it over the hoops and pull it taut without creases, so the rain runs off without forming pockets.
  6. Close the first long side. Let the film come down to the ground and weigh its edge down with sandbags.
  7. Prepare the opening side. Wrap the free edge several times around the roll-up batten and screw it in place: the film won't tear. To open, roll the whole film up on this batten, right to the top.
  8. Hold the film up. Tie two loops of string at the top of the two end hoops and slip the ends of the rolled batten into them. To close, take the batten out and unroll: its weight pulls the film taut. In windy weather, tie it to the base of the hoops.

The original tunnel held its film down with strings stretched from one rebar to the one opposite, which had to be re-tightened regularly: a possible alternative.

Ventilate without fail

On clear days, the air in a closed tunnel can go from below 0 °C to over 27 °C in a few hours, even in winter. Aim to keep the temperature below 25 to 27 °C.

  • In spring and autumn: closed at night, rolled up 30 to 60 cm around midday in calm weather.
  • In summer: open almost all the time, except on stormy days.
  • In wind: the film is either fully rolled up or fully down, never half-raised.

With the long side rolled up and both ends open, a through-draught is created that also lets pollinating insects in.

Watering

Under cover, no rain gets in: the plants need 25 to 38 mm of water a week, all of it supplied by you. Over the tunnel's 3.3 m², that works out at about 12 to 18 litres a day. Soak the soil but keep the foliage dry. Regular watering prevents fruit splitting and blossom end rot.

Drip irrigation is ideal: one emitter every 20 to 40 cm, with 30 cm suiting many soils. It can use half as much water as sprinklers. In-line emitters deliver from about 2 litres an hour; fit a pressure reducer and a filter, and wet the soil to a depth of 25 to 30 cm, letting it drain between waterings.

Add a mulch several centimetres deep, leaving a gap around the stems. With a water butt, the pressure gained is about 0.1 bar per metre of height; raised 30 to 90 cm, you remain well short of the 0.4 to 1 bar of a typical drip system. Choose emitters designed for low pressure and keep the line short.

Planting and growing on

  • Spacing: tomatoes 45 to 60 cm apart, peppers 38 to 45 cm, aubergines 60 cm.
  • Order: peppers and aubergines go in about ten days after the tomatoes.
  • Stakes: drive them into the ground. Never hang plants from the structure, which isn't designed to take their weight. Opinions differ: some do without a stake for peppers, others recommend one for each large aubergine.
  • Pruning: stop tomatoes two leaves above the top truss once they reach the top of their support, or after seven trusses. Pinch out peppers and aubergines at about 30 cm. One tunnel trial, however, achieved better yields without pruning the aubergines.
  • Pollination: open up, and tap or shake the open flowers.
  • Rotation: soil-borne diseases build up under a fixed shelter; against blight, a rotation of at least four years is advised. Dismantle the tunnel and move it from one bed to another.

Adapting the tunnel to your climate

Frost. At night, a tunnel is only a few degrees warmer than outside. A heavy fleece (35 g/m²) laid over the plants gives about 2 °C of protection, and can be doubled. Be patient: tomatoes set fruit poorly below 16 °C; peppers want at least 12 °C at night; aubergines, which are more tender, want nights above 10 °C, or at least 16 °C according to other sources. At altitude or in cold regions, plant later.

Wind. This structure is less sturdy than a steel tunnel, and its height exposes it more than a low profile would. Close everything up when strong winds are forecast: with these precautions, the original tunnels withstood winds of 72 km/h. A windbreak protects over ten times its height; bury or weigh down the edges well and keep the film free of creases.

Snow. Prop up the ridge pole with posts and brush the snow off once it exceeds about 13 cm, or remove the film out of season.

Heat. At the height of summer, open it right up: above 27 °C tomatoes set fruit poorly, and above 30 °C peppers produce less.

Find your dates

Every garden has its own rhythm. To find out when to sow and plant tomatoes, aubergines and peppers where you live, check the Semiterra calendar: it takes into account the climate and altitude of your area.