Last year most of my brassicas got devastated by caterpillars despite have a net over the crop. The issue last year was the size of the netting – I had gone for 20mm butterfly netting but they still got in. As this is my first year on the allotment, I have taken advise from the locals who have worked those plots for years and it appears that there ia threat from Pigeons who love to eat new crops.
I had already decided on a semi-circular tube cage made form MDPE pipe and fine mesh netting. Last week I put a photograph of the pipe and netting on Instagram and was asked by one follower if I could show how I built the brassica cage.
So here we are………
Firstly, the dimensions of the bed and the materials required.
The bed measures 5m long, 2m wide and I wanted at least 1m height in the centre. The pipe was to be 25mm MDPE which I could buy in a 25m roll. The pipe support (driven into the ground) were the remains of an old tomato greenhouse. and the lateral supports were 2.4m and 1.2m bamboo canes. The net is 4m x 10m with 5mm x 7mm holes.
The first task is to calculate the length of the tube hoops. For those that remember geometry from school, this will be fairly straight forward.
What I am creating is a semi-circle with a diameter of 2m (the width of the bed). Using the formula for the circumference of a circle Pi x d, where Pi = 3.14 and d = diameter 2m, the circumference of the circle is 6.28m. However, I only need a semi-circle, so I can divide 6.28m by 2 and the length that I require is 3.14m for each hoop segment.
However, due to the amount of pipe I had, I decided to cut each length to 3.5m. This would give me a slightly higher cage but well within the dimensions of netting that I had purchased. Before cutting all of the hoops, I cut one and assembled in to make sure that it did what I wanted it to do.
The hoops are secured to the ground using lengths of metal tubing which I salvaged from an old tomato grow house. These lengths are driven into the ground leaving approximately 20mm above the ground level. One of the lengths is placed each side of the bed and the hoop pushed down over each tube.
Once the dimensions were confirmed I could then cut the rest of the hoops until I had a total of 6. The second hoop was installed at the opposite end to the first then the remaining support posts were equally spaced along the bed. and the rest of the hoops installed.
So we now have 6 hoops along the length of the bed but in order to prevent the netting sagging too much and to add additional rigidity, bamboo canes were tie wrapped along the length. One set along the top and a set along each side. Due the length of the bed, each set consisted of 2 x 2.4m and 1 x 1.2m canes, all secured with tie wraps.
The netting will be installed once the first of the brassicas go into the ground in a couple of weeks time. Overall, with the cost of the MDPE, the bamboo and the netting, I probably spent no more than £35 and I will still have some netting left for other uses.
If you are going to build one of these, you may not need as much MDPE piping if you have a smaller bed to cover. You may be able to get away with 3 or 4 hoops. If this is the case, try checking out local builders to see if they have any offcuts they cannot use. All you will need to know is the minimum length of each hoops.
If you cage is a small one, then you may be able to get away with a single set of bamboo canes on the top and use PVC cord ties down the sides to prevent net sag.