The big Autumn clear up – your next compost heap

In this article, we look at how to make a batch of compost in three to six months; i.e. make a batch over winter ready for spring use. We explain why some compost heaps take three to six months to break down to quality compost and others take 12 to 18 months.

The holidays are over, children back to school, most of the allotment harvest is in. Late September is the traditional time to start clearing up the garden and allotment. Very quickly a large pile of trimmings and old vegetation piles up. The pile will grow as the autumn leaf fall sets in. It’s the ideal time to build a new compost heap.
It would be great if the compost was ready to use in Spring and for some people this will be the case; for the majority it won’t be ready to use until next spring – 18 months away.

• We explain how to hot compost over winter
• We look at the reasons why hot composting is faster than cold composting

How to Hot Compost during the winter months

Tony’s top tips to improve your traditional heap

• Only use solid walls – no holes, no pallet boxes. Block off and build solid wood frames.
• Ensure you have a well fitted, solid lid. Weight or strap this down.
• Insulate your walls and lid. (Take note: any insulating material that absorbs water will rapidly lose all insulation property and conduct heat. Hence wet wood, foam, PU cell foam, ‘rook wool’ and carpets are hopeless – with the British weather, you’ll be lucky if these insulating materials stay dry for more than a week! Try 50mm Polystyrene sheets).
• Aerate – having blocked all the holes and gaps – you need to allow the heap to breathe. Either use a pallet in the base, or use lengths of irrigation or soaker pipe (i.e. pipes with holes in). This will allow air in at ground level and for it to be drawn up and out. 
• Keep excess rain out of the heap. Ensure the lid drains water away not into the heap.
• Kick start – if you are staring when the day temperature is below 10C (almost all days in Autumn!) then you need to kick start the bacteria. Take a plastic (PP or PE) 1-2 litre container (e.g. one used for oil or anti-freeze) and fill with boiling water and tightly lose lid. Put this in the centre of heap and cover with waste compost material as quickly as possible. The heat released will kick start the bacterial activity. (H&S Safety warning – boiling water scalds - do not use glass bottles – they can shatter when hot meets cold and do not use PET “coke” type bottles as they melt above 80C.
• Do not add large sods of soil or add cover layers of soils. Shake of all excess soil from roots. Small amounts (hand full) thinly spread (<0.5cm) are good, large amounts hinder.

If these tips sound a bit bonkers and you’re unsure if they’ll work, rest-assured they are tried and tested. The science and design has been used to create award winning patented hot composting systems by the author. 

Why is hot composting faster than cold (normal) composting?

Below we explain the science, it’s not that hard, have a read and wow your fellow gardeners.

In the UK the average 24 hour seasonal temperatures are roughly: Winter 0-5C, Spring 5-10C, Summer 15-20C, Autumn 10-15C. Over the year, 10C.

There is a law in science known as the Arrhenius equation. It states that for every 10C increase in temperature, the rate (speed, time) of chemical reaction doubles. (It is therefore sometimes known as the Q10 equation). Composting is a biological reaction and the law applies between 0-70C.

below we tabulate this into simple to follow approximation:

Notes: 

• The months have been approximated to make the maths very simple, 32 months/x32 rate.
• There is no single rate – each waste material has its own rate. Example Wood rate is slower than grass. The average rate varies depending on waste composition.
• Composting goes through phases. Only certain bacteria can work at 60C. The heap rarely stays at 60C for duration. It is entirely feasible to compost some materials is 30 days, however the average hot composting time is usually 90 days than 30.

What does this mean for gardeners and composters?

Shredding, turning, aerating, adding inoculants and activators may help. For example, shredding can double the surface area and half the time. However, only heat can create a X32 rate increase. Heat is the solution to fast composting. If you can keep your heap hot, you can compost over winter. 

However if, as in most UK gardens heaps, the temperature of your heap falls to below 5C, it will not decay by any appreciable degree over winter. You are in affect building a pile ready for it to start composting in spring.   

Many composters will state their heaps get hot, they can feel the heat and see steam rising – this is good news.

Many will notice a rise in temperature after heaps have been turned.  Many sites will state you need a big volume of waste to get a hot heap. Others will state you need lot of greens – yet you may have seen large piles of woodchip at council shredding sites hot and steamy. Let’s look at these and try to understand the importance of wind chill and insulation.

How hot is hot?

The water in your shower is normally 35-40C. In composting terms this is warm not hot. Stick your hand in the heap – it is usually warm rather than hot.

Touch a metal radiator going full blast – normally this will be 50-60C. Most hot water system have a safety cut out at 60C. Hot composting at 60C is really hot.

How much fuel do you have?

Consider a small camping gas canister and a large BBQ gas tank – one will heat longer than the other. The compost fuel is waste – a big pile is more fuel than a small one.  You have a bigger fire – i.e. more heat. 

How long do things stay hot?

It is quite usual to see a bin pile of grass get very hot. Then 24-48 hours later for it to collapse into a cold slimy mush. There are lots of reports of large (1m3, full pallet box of waste) getting hot – but a few days later they have cooled.  It is also possible for the same 1m3 bin stay hot for many weeks!

The difference is the speed at which heat is lost. It might be easy to compare this to a house with gas central heating. One house is insulated and one not. If they both have a cylinder of gas, the insulated house will stay hot for more days. Your heap is the same – the cool air and wind remove all the heat very quickly. You need to slow down the heat loss. 

Let us extend the analogy. If humans live in the house and it is completely sealed and no air flows - they will die of CO/CO2 poisoning. The house needs to breath – let in fresh air (oxygen) so the occupants can breathe (i.e. bacteria respire). The cold air comes in through the vent below the floor boards and eventually rises and leaves via the roof. Your heap can and will aerate using the same principle – insulated walls, with air in below and out the top via a small hole in the roof.  

Size matters

Big heaps are easier to heat and keep hot than small. Two things come into play. The surface area to volume favours large items retaining heat. The waste material acts as an insulator. 1m thickness of compost has similar insulation value as 50mm of polystyrene. In big heaps the outer layers hold in heat – the centre composts and the outer layers don’t. Turning moves the non-composted outer layers into the middle where it will heat up!  

The smaller your bin, the better the insulation and heat flow control needs to be.