Putting the cover on the frame turns what has been merely a construction zone into a polytunnel. Immediately, the interior warms up and gives you an idea of what kind of growing space you’ve just added to your garden. If it’s your first tunnel, you’ll be amazed.
The final stage of frame assembly is the door frames. These comprise two uprights and a lintel at each end, and an additional ‘spacing’ beam between each upright and the adjacent corner on larger tunnels. Uprights are held in place using ‘P’ clamps attached to the end hoops.
This is supplied by the manufacturer and is a sticky-backed plastic foam tape that should be applied to the frame anywhere it may come into contact with the cover. A single strip is enough for the intermediate hoops and the ridge. Keep back enough extra to add to the bits you forgot – things like the screws on the outside edge of the diagonal braces. If you have enough, you can add a double-wide strip on the hoops at each end, one facing towards the tunnel side and the other the tunnel end.
The instruction manual will tell you that the cover can easily be pulled into place by two people, and while this may well be true for smaller tunnels, for large ones like this, the more the merrier – so invite a few friends and turn it into a party. The cover is heavy, and as the tunnel will be close to three meters high at the ridge a lot of lifting is involved. A stepladder (or two) will help. Lift the cover as evenly as possible, because although it’s extremely strong a small area can become stretched by pulling on it too hard. And, obviously enough, don’t attempt it on a windy day.
Once the cover is over the entire frame you can more easily pull the whole thing into the final position, with even amounts of excess cover on all sides.
A common tunnel construction method is trenching, which relies on laying the edges of the cover into a 30cm (1′) deep trench all the way round. The earth is firmly pressed into place on top of it to provide the tension, and the weight of the earth holds everything in place. However, digging a trench all around the tunnel is very hard work and may not be practical – for example if a pathway already exists along one side. An alternative method is to use ‘base rails’ made of timber or aluminium.
Base rails run all the way around the tunnel (except for the doorways) and are attached, close to ground level, to all the hoops. If the rails are timber, the cover is fastened to them with battens. The clamps holding the timber rails to the hoops are then loosened one at a time, and the rails are pushed firmly down – usually by having someone inside the tunnel stand on them – before they are re-tightened.
Aluminium base rails work in the same way, except that the cover is held in a channel by two plastic inserts (as shown below), thus avoiding the need to hammer any nails through it.
If you’re using either type of base rail, the ‘tail’ of the cover will only need to be covered by a few inches of earth all the way around, to stop the wind getting under it. A flat spade quickly makes a nice, straight slot to push it into. To make this easier, try to remember not to add big rocks to the top few inches of the fill for the foundation pole holes on the outside of the tunnel.