How to Compost Grass?

Tuesday, 4 February 2014  |  HOTBIN Composting

Composting Grass/Lawn Mowings in the HOTBIN

How to compost grass in the HOTBIN efficiently

You can compost grass and lawn mowings in the HOTBIN - it is very quick to breakdown. Grass decomposes very quickly producing a heat boost in the process which is great for raising the temperature when setting up your HOTBIN.

This post focuses on composting regular weekly grass cuttings so if you want to use grass to get to 60°C, follow the link to 'How to get to 60°C. Composting grass successfully requires a little bit of knowledge but the real secret is matching your method to the amount of grass you generate and the time and effort you have available. Grass/lawn mowing’s are a paradox. On one hand it is one of the easiest materials to compost; on the other hand, in traditional heaps, it is one of the most troublesome materials to compost producing a lot heat, a distinctive odour (ammonia/urine) before collapsing into a wet slimy black mess.

Grass and Odour

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 - in this instance, turning enables the gas to escape 'all in one go' so it is stinky after turning. (if you have done this job, you may well come back inside the house and realise your clothes wreak of ammonia!).

The HOTBIN does have an odour filter in the lid that does remove ammonia odour. But, when you add a whole box 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 has to be '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.

Grass and Black Slime

A traditional heap that generates a 'black slime' is due to excess water and too little airflow. Grass/lawn mowings have a high water content (>80%) and are low in lignin (i.e. no woody stalk). As it decomposes, the cells breakdown, becomes soft and water is produced. The grass collapses and forms thick layers reducing airflow. This in turn means the water is trapped, the process slows and a viscous circle is created which water is not removed, the heaps turns anaerobic and the 'black slime' is created. Thankfully 'black slime' is a rarity in the HOTBIN. The excess water is removed as steam when hot composting and ensuing good airflow in the HOTBIN is a breeze - just add 3-5 handfulls of woodchips (bulking agent) for every box of grass clippings. (The wood chip helps aeration - the fact it is high carbon does not solve the ammonia odour issue).


So we can compost small and large amounts of grass in the HOTBIN. We know large amounts need extra effort to avoid odour. Is it worth it? We think so you get great compost and lots of it. However below are we outline six options/choices that you might like to consider. Often you can 'mix n match' routines at different times of the year to cater for the grass you generate in spring versus summer. Below we outline a few options about different methods you might want to follow.

Adding grass and lawn mowings into the HOTBIN
1. Small to medium lawns

Add grass into your HotBin each week The HOTBIN 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). We recommend adding 4 parts wood chip (bulking agent) to 20 parts grass cuttings (1:5). For example, if you are adding a typical grass box of 40 litres of grass, you will need to add 8 litres of bulking agent. It is important you mix bulking agent into the grass. Adding it as a single layer before or on top of the grass will not solve the aeration issue. Adding the bulking agent into the grass box before cutting seems to mix it well! This will generate some odour that you may well notice for 2 days. If this bothers you, there are two main ways to solve:

• Add shredded paper or cardboard in ratio 2 parts grass to 1 part paper

• Only add half a box, then return 3 days later add the other half.

Remember the first few cuts in spring tend to generate 2-3 times the volume of grass as later cuts, so you may well have to adjust with the season.

2. Large lawn

If you have a large lawn 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!

3. Tips

Once grass has been added in, check after a week and look inside the HOTBIN. If there are any matted grass layers break these up with the rake to maintain airflow but you don’t need to mix with material underneath. When adding grass, try to add a mixture of waste types (easy and hard to digest) to ensure the constant production of heat.