When you build a new polytunnel it’s essential to get the foundations right. Most especially you need to ensure that there is a 90-degree angle between the ends and the sides, and that the opposite sides of the hoops are level with each other. Otherwise, the tunnel won’t be strong enough to stand for very long.
Prepare the site
The first thing to do is decide on the position and orientation of the tunnel and then prepare the site. Sloping ground can provide a serious complication. While a polytunnel can be higher at one end than the other, putting one sideways-on along a slope is much more difficult. This is because each hoop must be exactly level from side-to-side. In our example the ground not only slopes across the tunnel but also slightly down towards one end, so it’s quite tricky.
If there is likely to be any livestock around the tunnel then the area must be securely fenced, or you’ll be sharing your harvest! An appropriate fence can also help with wild visitors such as rabbits and deer, and if wind is likely to be a problem then you might want to plant a hedge nearby as a windbreak.
Mark the first side
Initially, decide on the position of the first corner and push a bamboo cane into the ground at that point. Push another cane in to mark the opposite corner, forming what will become one end of the tunnel. Measure more than once to ensure you have the correct width, and tie a string between the canes.
Get the right angles, er, right
In order to figure out the exact angle for the tunnel sides, use the ‘3, 4, 5 triangle’ method. Place a marker on the string exactly 3 units (yards, metres, whatever) away from the first corner post. Then roughly position another cane along the line you think will become the tunnel side and place another marker on it exactly 4 units from the first corner. Then measure the distance between the two markers. If you can get someone to help, the next bit is much easier: move the cane marking the side until the distance between the two markers is exactly 5 units, and you have a right-angle triangle. You can place canes at 6ft intervals (6ft being the distance between the tunnel hoops) along this line, marking the positions of the ground tubes (also known as foundation poles) for that side.
Then run a string at roughly 90 degrees from the end corner, marking the exact tunnel width on it. Again, if you have help, it’s much faster to then adjust a string marking the other side until the side end marker and the width marker meet – and that’s the position of the final corner.
Put in the ground tubes…
For smaller tunnels, it’s just a matter of knocking the ground tubes into position at each marker and to the correct depth, making sure that they end up straight and level. For larger tunnels, you can use ‘anchor plates’ (see below) to provide more strength. Using concrete to set either type of foundation is not recommended because of probable planning permission complications.
…or the anchor plates
These are flat metal plates that are clamped to the bottom of each ground tube (these are sometimes called ‘foundation poles’). The two are then placed in a suitably-sized hole with the anchor plate at the bottom (in this case the holes were roughly 2ft square and 2ft deep). Set any backfill to one side when digging, and try to keep it separate from the topsoil. Then when you fill the hole in again, the subsoil will ensure a solid block of material that holds the anchor plate firmly in place.