Composting Grass/Lawn Clipping in the HOTBIN
Yes - grass clippings can be composted and they break down very quickly. In fact, grass is one of the best materials for hot composting because it is rich in nitrogen and helps raise compost temperatures.
However, adding too much grass at once can create strong odours, wet clumps, or slimy compost. The key to success is balancing grass with dry materials and other waste to help maintain good airflow. This guide explains how to compost grass clippings successfully in a HOTBIN and how to avoid the most common problems.
Quick Summary
✔ Grass clippings compost very quickly
✔ They help raise compost temperature
✔ Always mix grass with dry materials
✔ Add grass little and often
✔ Break up compacted layers to maintain airflow
Follow these simple rules and grass clippings will become high-quality compost in your HOTBIN.
You can compost small to medium amounts of grass in the HOTBIN, just don't over do it. The recommend maximums are as follows:
| HOTBIN Model | Max grass volume |
| HOTBIN 100 | 20ltrs every fortnight (half a mower box) |
| HOTBIN 200 | 40ltrs every fortnight (a full mower box) |
| HOTBIN Mega 450/700 | 60 to 80 ltrs every fortnight (up to 2 full mower boxes) |
Why Grass Is Great for Composting
Grass clippings are considered a “green” compost material. Greens are rich in nitrogen, which feeds the microbes that break down organic matter and generate heat inside the compost bin.
Benefits of composting grass:
- Breaks down very quickly
- Helps raise compost temperatures
- Speeds up decomposition of other materials
- Produces nutrient-rich compost for your garden
Fresh grass contains high levels of nitrogen and moisture, which is why it decomposes rapidly in hot compost systems.
Grass Composting: Important Rule
Grass clippings should never be composted on their own. If a compost bin contains only grass, it will usually:
- Become wet and compacted
- Lose airflow
- Produce ammonia smells
- Turn into a black slimy mass
This happens because grass contains over 80% water and releases nitrogen quickly during decomposition.
To compost grass successfully, it must be balanced with dry “brown” materials as detailed below.
How to Compost Grass Clippings in a HOTBIN
For Small to Medium Lawns
If you collect one grass box every fortnight (about 40 litres), it can be composted in the HOTBIN 200.
Recommended method
- Add the grass clippings to the HOTBIN.
- Mix in woodchip and shredded paper (as per our guidelines below)
- Add any other kitchen waste you may have at the time
- Give the fresh materials a light stir to evenly incorporate
For example: 40 litres grass
- Add approx. 2-4 ltrs of woodchips or prunings
- Add approx. 8-10 ltrs of shredded paper/cardboard
Use these measurements a rough rule of thumb, adjust accordingly if the heap seems overly wet or dry.
What is the Odour When I Compost Grass?
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 may get 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 ~10 litres of paper - that's approx. 8-10 handfuls, which needs to be mixed into the grass layer.
Why is There Black Slime on the Grass I Compost?
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, they become 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 heap can turn 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 handfuls of woodchips for every box of grass clippings. (The wood chip helps aeration - the fact it is high carbon does not solve the ammonia odour issue).
Top Tip: Break Up Grass Layers
After adding grass clippings, check the compost after about one week. If you see compacted layers:
- Use the HOTBIN rake
- Gently break up the grass
- Allow air to circulate through the compost
Good airflow keeps compost hot and active.
Is Grass Worth Composting?
Absolutely.
Grass clippings are one of the fastest composting materials available and act as a natural compost accelerator.
When added correctly, they help:
- Boost compost temperatures
- Speed up decomposition
- Produce nutrient-rich compost for your garden
The secret is simple: Balance grass with dry materials and add it little and often.