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How to compost grass lawn mowing

To get fast superb results when composting grass lawn mowings in the HOTBIN: 

Add 40 parts grass with 20 parts shredded paper and 1 part wood chip (bulking agent)

Typically this is a medium sized lawn mower collection box (40 litres) with a full carrier bag of shredded paper (20 litres) and 4 hands full (one 2-litre measuring jug) of bulking agent.

If you want to view a photographic sequence of grass compost stages, you can jump to our post on steps / stages of grass composting. 

Below we explain why this recipe works, why just adding large amounts of grass can be problematic and offer 6 different options for handling large volumes of grass cuttings 

The problem often seen when composting grass is you end up with a black slimy layer that stops the compost heap working.

In a HOTBIN you should get brown mulch in 7 days

Grass is one of the quickest materials to compost. In the HOTBIN you can typically convert grass to mulch within 7 days. Grass is so quick to heat up to 60-70C, the HOTBIN team recommend it to help accelerate and increase temperature quickly.

BUT! You can end up with a black anaerobic slime

In traditional compost heaps, it is one of the most troublesome materials to compost. Grass often heats up for 2 days and produces a very distinctive whiff (ammonia/urine). After 2-days it then ‘collapses’ into a cold, wet, slimy black mass that smells horrible (anaerobic mush).

Funnily enough we don’t have a picture of this as if you do it correctly in a HOTBIN it shouldn’t happen!

Composting grass successfully requires a little bit of extra composting knowledge but the real secret is matching the amount of grass you generate with the time and effort you have available.

Why does grass turn into a slimy putrid mess?

The ‘black slime’ is due to anaerobic conditions, i.e. excess water and too little airflow. Grass (lawn mowing) has a high water content (>80%) and no lignin (i.e. no woody stalk). As grass starts to decompose, the plants cells break down and become soft; water is released. The grass collapses and forms a thick impervious layer and airflow decreases. This in turn means the water is trapped, the process slows and a vicious circle is created where water is not removed, all oxygen stops flowing and aerobic bacteria cease to release heat. The heap cools and anaerobic bacteria take over releasing obnoxious odour and resulting in a ‘black slime’.

The golden rules for successfully composting grass are:

  1. Remove excess water
  2. Keep the grass aerated
  3. Balance the mix to avoid ammonia odour

So how do we get rid of excess water, keep the waste aerated and avoid both ammonia and or anaerobic odour?

  • To remove excess water
    You need lots of heat, i.e. you need to be ‘HOT composting’
  • To keep the grass aerated (i.e. get air/oxygen into the grass layer)
    You need buoyant airflow which requires a temperature gradient and a structure with spaces and gaps so the air can flow up.

Unless you have the means to force airflow (e.g. a pump / blower), or you can constantly turn/tumble (yes we mean constantly), then you are reliant on “buoyant airflow via the chimney principle” – and this requires the grass to maintain a structure  with small air spaces (ie not a thick wet slimy mass!).

To get a structure that stops grass collapsing into a slimy heap, you need to add what we refer to as a bulking agent (typically this is wood chip). The bits of wood chip act like ‘stacking blocks’ and the air flows around them.  Simple but essential!

  • To avoid ammonia odour
    To prevent excess ammonia, you need to be adding a fast/easy to digest carbon material like shredded office paper or chopped up corrugated cardboard.

The odour is caused because grass has an excess of nitrogen which the bacteria are unable to use as fast as it is released. So it forms ammonia gas and evaporates away. You are most likely to notice this when composting and/or turning large quantities of 1-2 day old grass lawn mowing. After 3 days things slow down and the nitrogen is no longer in excess. Turning grass heaps does not prevent the odour – it enables the trapped gas to escape ‘all in one go’. (If you have done this job, you may well come back inside the house and realise your clothes smell of ammonia!).

The HOTBIN does have an odour filter in the lid that does remove ammonia odour. But, when you add a whole box of grass in one go without anything else, the filter gets temporarily overload for 2-3 days. To prevent the odour during the initial 2-3 days you need to balance the carbon/nitrogen ratio.

You achieve this by adding a dry high carbon waste. The key here is to add ‘easy to digest carbon’ such as corrugated cardboard or paper shredding. Woody items like sawdust, shavings, wood chips are high carbon – but they are not easy to digest, so will not balance the C/N during the critical 2-days of intense activity. Here is the challenge – you need a lot of dry carbon! A 40L grass box (a typical mower box), needs 20L of paper – that’s a whole carrier bag full. It also needs to be mixed with the grass. Not everyone wants to do this, especially after cutting the grass. Below we outline a few options about different methods you might want to follow.

Large amounts of grass waste need extra steps to compost quickly, without ammonia and without turning putrid. Is the extra effort worth it? We think so! Each year fertilising grass lawns consumes considerable inorganic fertiliser – adding the nutrients back via compost is environmentally better.

Below are six options/choices for composting grass. Often you can ‘mix ‘n’ match’ routines at different seasons and times of the year to cater for the varying grass volumes.

1) Small to medium lawns – add grass into your HOTBIN each week
The HOTBINn will easily compost grass from a small-medium lawn (approx 40 litres/week or 1 large grass box per week, filling about a quarter of the bin each time).

This will generate some odour that you may well notice for 2 days. If this bothers you, there are a couple of methods to solve this:

  • add shredded paper or corrugated cardboard in ratio 2 parts grass to 1 part paper
  • Only add half a box, then return 3 days later add the other half


Grass volume / Weight To Avoid wet slime To Avoid Ammonia
40 litre (approx 20 Kg) Add 2 litre (a measuring jug) of bulking agent. Mix in well Add 20 litre (a full carrier bag) of shredded office paper or chopped up corrugated cardboard).

Results in approx 1.6 Kgs of compost in 30-90 days

2) Large lawn – use a dedicated HOTBIN for grass

If you have a large lawn and generate 3, 4 or more boxes each week, then you will need to consider a dedicated HOTBIN. It will cope with 2-4 boxes (about 60-80L) per week.

The same rules apply – but adding and mixing in large amounts of paper is intensive and requires a high degree of commitment – perhaps not what you want straight after cutting the lawn! Large lawns allow the HOTBIN to be located away from your seating area – so we suggest you save your effort of adding shredded paper to eliminate ammonia odour – just leave the HOTBIN down the garden and reap the benefit of fast compost without anaerobic slime.

Grass volume / Weight To Avoid wet slime To Avoid Ammonia
80 litre (approx 40 Kg) Add 4 litre (a measuring jug) of bulking agent Leave remotely and accept ammonia for 1-2 daysAdd 40 litre (a full carrier bag) of shredded office paper or chopped up corrugated cardboard).

Results in approx 1.6 Kgs of compost in 30-90 days

3) Leave the cuttings to compost on the lawn
Many gardening sites now actively promote leaving grass cuttings on the lawn. Normally you use an adapted/special mower blade that chops the grass into very small pieces (2-5 mm) and thoroughly spreads them. The method is to weekly trim of top third of grass and spread this evenly so it composts quickly, adding nutrients back to soil, but not creating thatch. If you have the grass ‘trail line’ down side of mower, then this will rot into mulch that blocks light and growth and does create thatch. Please refer to manufacturer for correct mower blades/settings. If you walk regularly on your lawn – you may find bits get on your shoes and are walked back into the house!

4) Best of both worlds’
Add the first few cuts of the year which tend to be large (say 3-4 boxes) into your empty HOTBIN. The bin is full for a week or so, and then rapidly becomes half-empty allowing ongoing use with food. After the spring cut, leave grass cuttings on lawn. Occasionally (e.g. when cutting hedges) add the grass box back on the mower and collect grass to complement garden ‘browns’.

5) Transfer grass to Local Authority
This is unlikely to interest HOTBIN users, but it is possible to have grass collected at the kerbside and taken to the council recycling centre. We are strong believers in home composting and believe in the environmental benefits of saving fuel and transport.

6) Allocate a large, remote area of garden to build smelly grass mounds
The mounds will tend to be smelly and go anaerobic, but it is fast to empty and dump lots of grass. We had rave reviews on how fast and efficient the HOTBIN is with grass – so maybe you do not need this option anymore!

So in essence it is easy to compost grass in your HOTBIN but depending on the amount you need to consider which methodology is best for you.

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  1. I’d use one if someone bought one for me but I’m not paying silly money for a composter!

  2. Well it is a unique design and achieves hot composting effectively to compost all food waste as well as garden waste including grass. The HOTBIN is made from an expensive high grade material called expanded polypropylene – it is built to last.

  3. I have a question for you. I have Rabbits and Chickens. Are their dopnripgs better for the garden if put directly on the garden? or are they better if Composted first? Thank you for your help!

  4. think this might enhance your product it is just new onto the market and is flexible with most types of wheely bin’s

  5. Thanks – it looks really great – but unfortunately our HOTBIN does not have the wheelie bin rail to hook the BinPin around.
    i have been reading with interest “composting in the hotbin”
    regarding buying a bin is there anyway you could purchase one and pay in instalments

  6. I prefer comsopt tumbler for fast comsopting it only take less than a week for the organic garbage to turn to humus. Plus the humus goes to my vermicompost bin as beddings for my worms making an even more better natural fertilizers. Just try it.

    Don’t do what I did and add too much grass in one go it clogged up my hotbin and I spent ages digging it out. I’m new to this so I don’t think I got my bulking agent etc ratio correct. Hopefully it’s all sorted now

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