A little bit of Eden
I will add more pictures in due course
Project Week in the woods
Many of the woodland based craft training suppliers such as Tim Gatfield’s Cherry Wood, Mike Abbots Living Wood and Gudren Leitz at Clisset wood have some form of project/development week in the Spring and/or Autumn where for a contribution to the food bill you can take a break from your usual routine and volunteer to undertake some new practical tasks such as hedge laying, coppicing or building a new toilet block.
And so it was that during the sunniest week in April I was challenged to build a 16ft diameter geodesic greenhouse in Cherry Wood.
With many courses running through the year and many mouths to feed the Cherry Wood project needs some 10kilos of vegetables every week. Tim is passionate about food and food miles, mainly using what is in season and supplied by local organic farmers. Alongside this Tim also wants to be as self sufficient as possible by growing in the forest garden, raised beds and the new greenhouse.
After being challenged (a few weeks before) I knuckled down to some serious research about the science of geodomes, the benefits of the amount of growing space, the all year round growing conditions and how bountiful the crops would be.
The principals are based on the work of an American architect of the 1950,s called R.Buckminster Fuller who specified the architectural science of building with this method in the hope of developing cheap housing. The design was exploited by the military and became most popular as observatory and radar domes.
We are most familiar with the infamous Millennium dome in London (now the 02 arena used for concerts) and more inspiringly The Eden Project in Cornwall which clearly demonstrates the ability to create a carefully controlled climate within the dome along with the huge advantage that the structure is self supporting with no columns interrupting the floor space.
Geodesic icosahedrons; (as they are more formally called) are designed in phases from 2v (phase) the least complicated through to 5v for larger more complicated domes. We will be building a 2v which will require 65 struts or poles to complete the construction. A great resource is www.geodome.co.uk which has an on line calculator to help you with the maths. It is not important for you to use Imperial or metric measurements as long as you are accurate and consistent.￼
Our available space was approximately 18-20 ft and we needed to allow some working room around the dome. This given, we had a diameter of 194 units which led us to requiring 35 struts of 60 units and 30 struts 11% shorter at 53 units. We also needed 10x 4 way hubs for around the base 6 x 5 way hubs for the centres of the pentagons and 10 x 6 way hubs for the centres of the hexagons.
Part of the geometry and strength of the structure depends on it integrity. If the structure is placed on the ground the options for doors will always be very low or you will need to break into the structure to create a passable height door. The alternative is to raise the structure onto a dwarf wall which should gain you the valuable extra height. If the dome is to be move,d as in a tent for accommodation, the idea of a wall may not be feasible.
Our site is in a clearing of the woods which will get sunshine for most of the day. The ground however is solid clay which would be impossible to grow anything on presenting us with the only option of raised beds to grow in and therefore give us the dwarf wall to sit the dome on and therefore the height for the door and extra headroom around the edges.
The dwarf wall is built from 8×2 softwood felled just days earlier and converted on site by a woodmiser mobile saw mill. These boards are going to take a ton of soil pressure so are screwed to 3×3 posts driven deeply into the clay.
The chestnut poles are extracted from the woods on the morning of day 1 and a group of willing hands set to stripping of the bark using draw knives and shave horses. It is necessary to clean the bark off to prevent rot and insect infestation in the humid atmosphere of the greenhouse. They are pretty chunky at 2-3 inches and need to be drilled and fitted with a hub plate at one end fixed with nuts and bolts.
￼For the hubs we were using galvanised plate drilled to take bolts to join them and connect them to the struts. Some designs, using lighter timber such as coppiced Ash poles, use blue water pipe heat shrunk on to the spars and flattened at one end drilled and bolted. (See pic)
This design is described as a stretched skin dome with the skin being horticultural polythene. Whilst acknowledging the less than green credential of plastic sheet we must balance this against the alternatives and accept that at only 15% of the build its effects are relatively low.
The material is highly specialised for optimizing growing with special filters and coatings to reduce condensation, minimize aphid growth and allow the correct light for maximum growth.
We used 180 Micron Sterilite HDF from www.plasticsbypost.net
After gathering all the ingredients together and a core team of 3-4 we lay out the 10 equal length sides of the base and drove in posts to mark the boundary. It takes half a day or so to get the walls raised and we are then ready for real dome building.
As you can see we started to raise the first phase and it was pretty good at being self supporting. Maybe the poles were a little on the heavy side and the hub plates a little thin so as we got 2/3rd of the way around we had to put some temporary supports in to stop a collapse. Once the last section was in however it was self supporting if a little wonky.
Day 3 saw us building a scaffold tower in the centre of the dome to facilitate the building of the second phase and the crown. This needed a few more hands to pull￼ it together and there were a scary few minutes when nothing seemed to fit with one doubting Thomas suggesting we had built it wrong. After a few checks as to the accuracy of the detail and a little brute force we managed to twist and beat the last few joints into submission and all was standing proud.
Day 4 was spent building the internal wall to the raised beds in a similar way to the outer wall. This however is built in a convoluted pattern to give, small walkways, maximum growing space and reachability to ever corner without having to compact the soil in any way.
The walls of the beds are covered in membrane to prevent soil leakage and to keep as much moisture as possible off the wood.
With this completed it was time for the soil to be brought in by barrow and spread around the beds and a round pool in the centre of the dome. This pool is part of the climate management of the dome creating moisture and helping to keep temperatures down and more constant. This is not very fancy just a garden trug ￼popped in and filled from a hose as required.
The paths inside was covered with scalpings and then a thick layer of charcoal. This makes a comfortable path and acts as a heat store to give off vital extra warmth during cool nights.
With all of these activities complete we were able to shift the dome to an exact fit on the wall plate and screw it down in its final position, create a door frame and make a window for vital ventilation and air flow through from the door to the roof.
The final day saw many hands carefully unfurling the skin over the dome. After successful draping it was then a case of shaping darts in the material by folding, ￼rolling and tapeing down, thus creating a snug fit to the frame. The bottom edge was then rolled around 2×1 batons, stretched as tightly as possible and nailed to the base.
With the door and window fitted we were all but complete but for planting, which in the cool of the evening was done at the end of day 5.
A few weeks later the produce is growing strong and the lemon tree is establishing itself. It is estimated that a dome greenhouse of this size is sufficient and capable of providing fresh produce for a family of 6 throughout the year – a pretty good return on your investment and a little piece of Eden to enjoy on a rainy day in November, so why not think about creating your own?