Adventures with a Compost Tumbler

(Images courtesy of Harrod Horticultural)

They come in different sizes, but they can be expensive.  They claim to produce usable garden compost in two weeks.  Do they work and are they worth the money?
The pile of green waste was growing, and it was taking a year or more to produce the crumbly, soil-like stuff that we gardeners crave. And by that time it was a few day’s work, with innumerable barrow-loads, to dig it out to use as mulch or soil conditioner. I wanted a far quicker process and less hard labour. So I invested in one of the large tumblers with a capacity of 112 bushels.  If you’ve seen the adverts in the garden magazines, you’ll know that the drum looks pretty big.  It is!  But the first surprise was that the whole thing arrived as a flat-pack. The drum is made of three flexible metal sheets that bolt together and slot into the circular end pieces.  Self-assembly is fairly straightforward, but it would have been nice to have some text to accompany (or elucidate) the diagrams.  But there is a very good booklet on the theory and practice of composting, using the tumbler.  It is worth following this closely initially.
The composition of what you load into it is crucial. Grass cuttings for example are an ideal accelerator, but they heat up very quickly and turn to a revolting squelchy mess nearly as quickly.  So a balance is needed.  Specifically about 30% of the bulk should be dry, brown material such as shredded prunings from shrubs or hedges, or torn up newspaper, egg boxes or cardboard (uncoated, like the inside of a loo roll : not the shiny packaging of so many super-market products). This provides carbon to balance the process. Too much green and the whole thing becomes very wet; too much brown and composting is inhibited because the mix is too dry.  Experience quickly enables you to judge the correct balance.  But be prepared for trial and error.
The theory is that the internal temperature should rise steadily over the first 7-10 days.  Having reached a peak it then cools gradually as the bacterial action, having done its job, slows down. That action depends on continuous aeration, so you have to remember to turn the drum daily to ensure the material does not become compacted. The crank handle has a low gearing so that turning the drum is surprisingly easy, despite the weight of material it contains. The loading/emptying hatch detaches very simply and enables you to monitor what is going on inside.
Key criteria for rapid composting so far are; a good mix of basic materials, and regular aeration.  A third criterion is filling the drum completely at the start (and this may affect your choice of the size of tumbler.) If you only partially fill and then add to it over subsequent days or even weeks, the process will be much slower.  Equally if you leave the drum only partially full, there is insufficient bulk to create the required temperature.  Whilst the internal temperature is generated spontaneously, it does seem to me to be affected by ambient temperature, since composting appears to be somewhat slower during the winter.
So; can you get good usable garden compost in 2 weeks as claimed?  No, I don’t believe you can, even with a good supply of soft green matter (eg grass cuttings) as a starter.  But if you get the mix right you can certainly have usable material in 3-4 weeks – a vast improvement on cold composting in the traditional compost heap. It is also the case that reaching the right internal temperature will ensure weed seeds are killed, so you don’t end up weeding the mulch!  A process that is 10 or 12 times faster has obvious benefits, but if the tumbler produces the stuff faster than you can use it (not a problem for me so far) you can still stack it for later use and the volume will have been hugely reduced as a result of the composting.
Although I have not yet been able to produce results as quickly as the makers claim, it is nevertheless impressively quick and I have been pleased with the quality of the output.  I do not regret the expense, but whether it is worth the money is very much a personal judgement.  I am glad to have fewer and smaller piles around the garden.  And it beats putting potentially valuable material in plastic sacks and taking them to the local dump.

Roy Stickland 

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