The covering for this dome arrived the day after I had finished the frame – great timing. I ordered 3 metres of 7.3 metre wide Sunmaster Tunnel Covers from Carters Packaging in Cornwall where I live. I cut oversize equilateral triangle sections and nailed them on with small, flat-head, roofing nails onto the framework, folding over the sheeting to produce a stronger edge. This is where the fresh-cut timber came into its own – as it was still sappy it took quite a lot of pounding without splitting at all. I’m not sure the same would be true of purchased 2×1, especially if it were old stock.
Three (oversize) equilateral triangles (eg ∆∆∆ – giving 5 triangles ) fitted the width of the poly sheet and allowed for anomalies created by the wild wood and the fold-over onto the struts. I later tidied-up these edges with a staple gun.
The grow dome worked very well in the spring, nearly all the seeds sprouted but there were a couple of problems. Firstly there was not enough ventilation and while the earth was damp inside the greenhouse it got extremely humid when the sun came out. Some of the seedlings didn’t like this at all. Leaving the ‘door’ open during the day helped but the greenhouse would benefit greatly from some kind of ventilation at the top which would also supply a hole where trapped insects could get out easily. Unfortunately, leaving the door open meant that one rainy day the polythyene filled with so much water the weight of it snapped the wood of the door frame – well, easily mended!
Secondly – the wind. I had fully intended to anchor the dome but didn’t get round to it – until it was too late. We get some pretty ferocious winds on the edge of Bodmin Moor in Cornwall and on one particularly windy day I heard a large ‘flapping noise’ coming from that part of the garden where I had put the icosahedron.
I went round the house to see that the dome had moved 25 feet, scattering all the tables full of seedlings across the garden like some kind of airplane wreck. It had landed the right way up and from the small rips in the polythene, made by table and chair legs, it appeared to have completed a somersault. It took myself and Sarah most of the afternoon to pot on all the seedlings and suitably humiliated I tied the dome down to some stakes hammered as far into the ground as I could get them.
The dome, apart from a couple of small rips, was completely unharmed and structurally sound. As I write this now it is full of tomatoes and the spring seeds are all out in the garden. For a while it housed my inflatable paddling pool which worked well as a solar heat sink, but I emptied it because there were some mosquito larvae in the water.
All in all this garden growing dome cost me £30 for the polytunnel covering – but I did have a stock of roofing nails and wire and rope in the garage.
My next project is to build one of these with every large triangle subdivided by three (eg nine trianges in each) – a ‘three-frequency, five eighths, alternate icosahedron’ – which is theoretically more of a dome shape.