Building a Garden Composter ...

This year, as is fairly typical, we've bought in excess of 700 litres of multi-purpose compost for the garden, usually when there were 3-for-2 or other special offers on large bags at the DIY chains or garden centres.  Even on offer, the costs add up over the year for such a large quantity. 

If 700 litres seems a lot, then to put it into perspective I've just started off next year's garlic crop, with each seed clove in its own 2-litre pot until the spring, and there's 25 of them so there's 50 litres for a start ...

So we use the compost for starting off seeds, filling tubs & planters for flowers, shrubs & trees (generally mixed with topsoil and or / grit and pearlite) and for the tomatoes, cucumbers, sweet peppers and aubergines etc which we grow in large pots in the greenhouses.  When the greenhouse crops are finished in the autumn, we empty the compost from these pots onto the beds and borders as a mulch and to improve the soil structure.  The bed at the front of the garden sits on particularly heavy clay, and so every year we try to add more organic material.

But every year, we also need to dispose of large quantities of green and brown garden waste in the form of grass cuttings, hedge clippings, prunings, deadheaded flowers, fallen leaves and other dead plant material from all the vegetables and annuals.   We already have a small, upright plastic compost bin which can contain a couple of hundred litres of material, but it's certainly not sufficient for a season's worth of waste.

So, we decided to build a new two-bay composter in front of the large shed.

After looking over the location and producing an outline drawing, the first job was to lift some of the paving stones in this area into which the composter could be placed.   These are 450 x 450 mm flags, and we removed eight of them and cut another four almost in half to create a sunken composter base area of around 1,800 x 1,100 mm.

paving slabs lifted ....

Removing the slabs will put the compost mix in direct contact with the earth, so the worms and other beasties can get into it.

We also used the removed slabs to extend the paved area in front of the shed by 450 mm, stripping away the lawn turf and then laying the slabs.  By doing this, we created enough extra space on the paved area to the north of the new structure for large shrubs in containers to partially hide the composter from view.  On the photos above and below, you can see one of our landrover wheel-and-tyre planters with a Loropetalum Chinensis 'Fire Dance' shrub - planter and shrub together are already around 800 mm high, and the shrub should fill out further over the next couple of years and make a half-decent screen. 

It was then off to the local timber yard, where I bought 150 wide x 22 mm thick wooden gravel boards plus some 50 x 50 mm framing timbers.  The cutting machinery in the yard was out of commission, so I had to mark and cut the 3.6 m and 4.8 m standard lengths into small enough pieces to get them in the car, using my cordless reciprocating saw.

As usual when I buy timber from this place, it had been stacked outside in all weathers, and what with the fresh pressure-treatment preservative still in the wood, plus the heavy rain we've had this autumn, the timber was sopping wet.  

I know from experience that if I assemble the composter box with the wood in such a wet condition, when it eventually dries out in the spring then 4 - 6 mm gaps will open up between the boards as they shrink.  Usually for the construction of sheds and other buildings, you don't want small gaps opening up, but for a compost bin they're fine and will let more air get in to the mixture. 

Onto the construction, using the cut-off saw I trimmed all the framing, side and rear boards to the exact lengths required from the drawing, and pre-assembled the side and central divider panels by attaching the ends of the boards to the framing members.  I carried these pre-assemblies down the garden to finish the build adjacent to the installation location, because it's quite a substantial box and would be heavy and awkward to lift over a longer distance.

during a construction tea-break ...

It's a very simple construction and didn't take long to put together and then move into position.  

basic box assembled, and in its place ...

The rear, sides and divider panels are all permanently fixed with screws, but the closing panels on the front of each bay are made by simply sliding seven individual boards into position from above, within guide runners formed by two framing members.  

This means we can open up the entire front of each bay to periodically turn the heaps over to mix and further aerate them, and to easily shovel the final composted mix into the wheelbarrow for use around the garden.

the runners for the front boards ...

To finish off, I measured the gaps across the insides of the guide runners and cut all the front panel boards to suit.

all done ...

And so that's it.   You could make something very similar, and much cheaper, from any scrap wood or pallets etc you have lying around, but we didn't have enough offcuts for such a large box and we want it to last a long time, so we used new pressure-treated timber throughout.

For now, we'll leave the top uncovered - if necessary, it will be a simple job to add a basic roof made from thinner fenceboards, or even just a tarpaulin.

Each of the new composter bays is >900 litres in volume.  Hopefully one bay will be enough to handle all the waste generated in the garden in a year, so we can fill one up over the season and then leave it for another full year to break down while we use the other bay for fresh material.   

We're almost due a full clean-up of all the fallen leaves lying around the garden - we usually wait until the trees are completely bare, otherwise we just end up doing the job several times - so they'll form the first layer of brown waste material in one of the bays.   There's also the autumn-fruiting raspberry canes to be cut down to ground level, another source of brown material.   And then we've green waste from the vegetables etc to start off a first green layer on top. 

From what I've read on composting, the difficulty is getting enough brown material (high in carbon) in the mix to stop it just becoming a slimy mess which takes a very long time to break down.  There's never a shortage of green materials in the growing season (high in nitrogen), what with regular grass cutting etc.   

To supplement the brown materials, we can also use cardboard packaging materials and paper - we eventually shred every piece of snail mail we receive anyway, so this is another source.