How to Compost Grass?

Tuesday, 4 February 2014  |  HOTBIN Composting

Composting Grass/Lawn Mowings in the HOTBIN

How to compost grass in the HOTBIN efficientlyYou can compost grass and lawn mowings in the HOTBIN. It is very quick to breakdown - typically creating mulch within 7 days when running at 60°C.

Grass is so quick, we recommend it to 'accelerate' and raise the temperature when setting up your HOTBIN. This post focuses on composting regular weekly grass cuttings. 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 mowings are a paradox. On one hand it is one of the easiest materials to compost (hence why the HOTBIN team recommends it to help increase temperature quickly); on the other hand, in traditional heaps, it is one of the most troublesome materials to compost. It often generates a lot of heat and produces a very distinctive whiff (ammonia/urine) for 2 days, before it 'collapses' into a cold 'wet slimy black' mass.

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 a 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 is 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. Below we outline a few options about different methods you might want to follow.

The traditional heap that generates a 'black slim' 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 slim' is created.

Thankfully 'black slim' 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 hands of wood chip 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.


Adding grass and lawn mowings into the HOTBIN1. 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). Add 1-part bulking agent to each 20-parts grass. Using a typical grass box of 40 litres of grass, you need to add 2 litres (a large mixing jug) 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 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! You will already probably pile the grass high somewhere to rot and have odour. We suggest you live with the odour from the dedicated grass HOTBIN and reap the benefits of fast compost without anaerobic slime.

Assuming finished compost removal every 90 days, then 80L of grass, bulk density 50%, water content 80%, degradable content 80% = 1.6Kg compost. At 1:20 ratio, 80L requires 4L bulking agent. At 50% density, 20% water, large pieces 5%degradable, small pieces 80% = 0.9Kg of bulking in compost. Each weekly cut will rapidly ‘disappear’ reducing from 80L down to 5L.

Mix each box of grass 20:1 with wood chip bulking agent, add all grass to bin each week(filling it up almost completely). After 90 days, take out first compost. This will be quite woody (50/50 ratio wood chip). Either use as mulching, or re-use as bulking agent again, gradually increasing humus content after each run. (It also reduces need for bulking agent). The above method assumes your grass is dry (i.e. cut on a warm day). If damp or moist, you will need more energy to drive of water. For wet grass, add 3-4 hands full of chopped up corrugated cardboard per box of grass.

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 is not 'mulching' correctly, or you are leaving grass too long before cutting. Although the debate still goes on whether this increases thatch, many find it does not. If you walk regularly on lawn - you may find bits get on your shoes and are walked back into the house!

4. Best of both world’s

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 on going use with food. After the spring cut, leave grass cuttings on lawn. Occasionally (e.g. when cutting hedges) add the grass box to 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.

We have had rave reviews on how fast and efficient the HOTBIN is with grass!