Why does a compost heap get hot? + the secret ingredient

A compost heap will get hot as microorganisms break down organic matter. As microorganisms digest the organic matter the exothermic reaction heats up the pile. The compost pile itself acts like a blanket to keep the heat in and can quickly reach temperatures of 170 degrees Fahrenheit.

Hot composting is a great way to kill weed seeds and pest and diseases like rust that can damage your plants.

As your kitchen waste and other organic matter breaks down using oxygen this is called aerobic breakdown.

As all of the microbes break down the organic matter, using oxygen, this produces heat, and therefore your compost heap will get hot.

I have been making compost heaps for years of different sizes, different locations and different methods. If you have been reading about composting or have one yourself you may have asked the question, why does a compost get hot?

Temperature of a hot composting pile

Hot composting is a specific type of composting that can produce a quicker turnaround than ‘cold’ composting so many people want to try this method.

A hot compost can get to temperatures of 49-77 degrees Celsius or 120-170 degrees Fahrenheit after a few short days.  Using the hot composting method you can make compost in about 4 weeks, compared to at least 12 weeks or more for normal cold composting methods.

This perfect temperature range for your compost is warm enough so that it breaks down organic matter quickly but not so warm that the beneficial microorganisms are killed.

Large piles of old mulch can be turned into hot compost.

What you need to make hot compost

If you are keen to make a hot compost the ingredients, method and size are all important to make it work.  If you want to just start a small backyard compost, see my post here on ways to start composting kitchen waste.

Fall leaves are a great ingredient for hot compost

What you need to make hot compost is 1 part green 2 parts brown materials which are shredded down to as small size as possible.  This means maybe getting your blower vac attachment that sucks in the leaves.

These attachments generally come with an extra shredder mode which, when you suck up autumn or fall leaves, you can shred them down even finer.  This is the key to making a quick compost.  The smaller the materials are, the quicker they will break down.

What are green and brown compost materials?

In the table I have listed the main types of green and brown materials you may want to include in your compost.

Green materialsBrown materials
Kitchen scrapsFallen dried leaves
Fresh grass clippingHay or straw
Coffee groundsShredded newspaper
Sheep, cow or chicken manureSawdust or fine wood shavings

In my previous article I had discussed green grass clippings and how don’t recommend using them in your small backyard compost.  In a hot compost however they can work as you are working with a much larger volume of materials and will have more area to mix them in.  

Dry lawn clippings mixed with leaves make a great ingredient for hot compost.

Working out what a green and brown material is can be tricky.  Leaves that are still green are obviously green but as they sit around for longer and dry off them become brown.  Same goes for grass clippings, they start off green, but if left to dry they can be considered brown.

The trick really comes down to adjusting as you go.  If your compost becomes too hot you probably have too much green, mix in some brown and it should reduce the temperature.  If your compost is too wet this is also usually from too many wet green materials like kitchen scraps, add some brown and there you go.  If the compost is too dry and not heating up, there may be too much brown material, add green and it should fix this problem.

Add sugar cane mulch or hay to dry up a soggy compost.

Composts are technically a science but really an art and the more experience you have the better and better you will get.  You will start to understand your climate and the perfect mix for making hot compost work in your backyard. There is no such thing as a failed compost, it just needs adjusting and eventually it will work.

What other tools will you need to make hot compost?

  • Garden fork or shovel for tuning the compost
  • Compost thermometer to measure the temperature of your compost

Optional

  • Breathable tarp: something to contain or cover your compost
  • 1 cubic meter compost bin to contain your compost

How to make hot compost – 8 Steps to brilliant compost in 4 weeks

Step 1 – Find a space to fit your compost

The trick with hot composting that doesn’t apply so much to cold composting is that it needs to be a certain size to work well.  That size is around 1 cubic meter or about 4 feet deep, 4 feet high and 4 feet wide. This is the perfect size to heat up and keep the right about of moisture to break down quickly.

Step 2 – Shred your ingredients

Start by making sure that your compost is shredded down to as small as pieces as possible.  I recommend using autumn or fall leaves as they are a great material for making any compost, hot or cold. These are easily shredded even finer using a leaf blower.

If you are adding kitchen scraps for your green material, make sure these are all in small pieces. Adding whole fruit or veg will take too long to break down for this process, so chop or shred any kitchen scraps you might be adding.

Step 3 – Add the secret ingredient

For any compost the secret ingredient to make the process even quicker is to add a shovel full of soil or even better is to add a shovel full of compost from your previous compost pile.  If you don’t have this option another product available is a microbe mix.  

Adding this to your compost adds more beneficial microbes which will make the process quicker.  I have added various types of microbe mixes to my compost, even straight to my veggie garden to improve the soil quality.  This just ensures you have a good mix of microbes to facilitate the breakdown process.

Step 4 – Add water

You will find with hot composting that you need to add extra water most days.  Depending on the weather, especially if you live in a warm climate, extra water will be needed to keep the pile moist.

The microorganisms are like us and need moisture and food to survive. You are aiming for a damp sponge level of moisture.

Step 5 – Mix your ingredients together and add them to your compost area

Get your shredded green and brown materials and mix them together with your secret ingredient and water.  It is important that you have at least 1 cubic meter of materials for this process to work.

As the compost breaks down it will reduce in size but it is important to start with this much because it is the magic size that will help it break down quickly.

After you make your compost you may want to cover your compost pile with a permeable tarp.  That way it will keep any material from blowing away, it will keep any mice, rats or possums out of the compost whilst still allowing moisture in and out.

Step 6 – Monitor the temperature

Each day you will need to monitor the temperature of the mix.  It will gradually rise in temperature over the first 5 days and get up to 120-170 degree Fahrenheit.  What will then happen is it will then start to cool.

Once it gets below 110 degrees Fahrenheit you get your fork and give it a mix which will introduce more oxygen and the pile will heat back up again.  

Step 7 – Keep turning

You will need to turn your compost every 4-5 days and after about 4 weeks you will see your compost heap look like the dark compost you are used to seeing.  The temperature will be low now, around 85 degrees Fahrenheit (or less than 29 degrees Celsius).

Step 8 – Let it rest

The next step is just to let your compost rest for around 2 weeks.  This just completes the process, allows the temperature to come down and it is ready for your garden.

What causes a compost to heat up?

The basic reason compost heaps heat up is because of the activity of the microorganisms that break down the organic material in your compost.  As they chew up and break down the green and brown material, they use up oxygen and release heat. This heat gets trapped in your compost pile and gradually warms up.

If you are making a hot compost this will take a few days to get up to heat and level out.  If the compost gets too hot, eg. above 77 degrees Celsius or 170 degrees Fahrenheit it will be too hot for many microbes and might kill them off.

The simple solution is to add some more brown materials and mix them in.  It is really unlikely that your compost will reach this temperature but spreading it out over a larger area to allow it to cool, then piling it back up again may  be necessary in the short term to release some heat.

What are the benefits of hot composting?

The best thing about hot compost is that it breaks down quicker than a traditional cold compost.  A well made hot compost will break down in just 4 weeks.  A cold compost will take at least 8-12 weeks but often much longer. Conventional composts can sit for 6-12 months before they are completely broken down (especially if you don’t use the tricks I taught you in my previous article here).

Hot compost will also kill most weed seeds or pathogens that may be nasty for your garden.  Technically you can put weeds that have gone to seed in the mix and as long as it heats up they should be no longer grow.

There are some exceptions however and many weed seeds can survive.  I generally avoid putting weeds that have gone to seed into my compost just to be safe.  The best way is pulling and adding weeds before they have gone to seed.

Just as a side note, the best way to deal with weeds that have gone to seed is to make a weed tea. I will write an article on how to make this soon.

How long does a compost heap stay hot?

A compost heap will gradually heat up over 4-5 days and then start to cool down.  It really depends on your climate but once it gets below 110 degree Fahrenheit then turn it over and make sure it is moist enough. This will introduce more oxygen and then it will start to heat up again.

After 4 weeks it should drop to a lower temperature where you can leave it for 2 more weeks then use in your garden.

Why I don’t use the hot composting method in my backyard

This might be a weird topic to finish on after I just spoke about how great hot composting is, but the reason i don’t use it is I just don’t have enough space or ingredients to make the compost.  If you have the space and can access enough green and brown material than this is a great method for you.

If you only have a small backyard with a small amount of fall leaves and kitchen waste you can still compost.  Just read my other article on composting and start there.

If you have the space then go for it.  Hot composting is a great way to make compost fast, you trap carbon and will return valuable nutrients and structure back to your soils.

Whether you choose to make a hot compost or use a traditional ‘cold’ composting method, either way this is great for the environment and your garden so get composting!

Check out my previous article if you haven’t already on ways to start composting kitchen waste.

Why does a compost heap get hot – FAQ

Can a compost pile get too hot?

Compost piles can get too hot if there is enough organic matter in the pile and the heat cannot escape. Test your compost pile with a compost thermometer and to release extra heat turn the compost and remove some of the material.

A compost pile that is above 170 degrees Fahrenheit is too hot and should have material removed. This temperature will kill microorganisms and stop the breakdown process.

What can prevent a compost pile from getting hot?

Too much moisture, not enough nitrogen, not enough volume and if the compost pile gets too dry it can prevent it from getting hot.

Make sure the compost pile is at least 4 feet high by 4 feet wide and has at least 1/3 green materials and 2/3 brown materials. Water the pile regularly to keep it moist but not wet and turn the pile every 1-2 weeks to get it heating up.

How do I cool down my compost pile?

Cool down your compost pile by mixing the materials and removing up to ¼ of the mix. This will allow more air flow and allow the heat to escape.

Removing the excess materials will cool down the compost pile down to a temperature of around 120-160 degrees Fahrenheit. This is the perfect temperature for microorganisms to break down the organic matter without being too cool and killing them off.

How hot does a compost heap get?

A hot composting heap can get up to 170 degrees Fahrenheit but a good range is anywhere from 120-160 degrees Fahrenheit. The more organic matter, moisture and volume that is in the pile the hotter it will get.

When microbes break down the compost heat is released which is trapped by the compost pile causing it to heat up. If your pile gets above 170 degrees, mix the pile and remove ¼ of the mix.

How often should you turn compost?

Turn compost every 1-2 weeks if you have time. As a minimum, turn hot composting piles every 4 weeks as this will increase the rate of break down as you introduce more oxygen for the microorganisms breaking down the organic matter. Check the temperature with a compost thermometer and add remove extra material if the compost is above 170 degrees Fahrenheit.