Cool tips for composting in a heatwave

The Great Green Team

16 June 2022

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The temperature is 28 degrees Celsius as I write this article, but as we know the British weather gods like a laugh so it might well be bucketing down by the time you get to read it.

Not to be put off, we’ll take the risk and keep that provocative little word ‘heatwave’ in the title.

A heatwave is defined as a period of excessive heat for at least three days and nights. Whether what we’re experiencing is a heatwave or what other countries might call summer, it’s still worth paying special attention to what’s going on in your compost. (After all what else would you talk to friends about?)

The summer months are when the composting process is at its quickest. Bacterial activity is faster, using up more water and more evaporation takes place. Heat is an important element in composting but if temperatures get too high the aerobic microbes digesting the waste die off and the process stalls.  We need to ensure the microbes are getting the oxygen and moisture they need to survive and thrive.

A compost thermometer is useful to keep an eye on temperature. If the compost temperature gets above 70 degrees Celsius there are steps we can take to cool it down to prevent anaerobic microbes starting to dominate. A sign that compost has turned anaerobic (without air) is if there is a bad smell.   Healthy compost smells neutral and earthy. 

Steps to take: 

  • If using an insulating jacket on a Green Johanna this should be removed.
  • Open the vents in the lid on your Green Johanna by twisting to the maximum position (or on other composters if they have this feature).
  • Check water levels – compost should always be moist like a wrung-out sponge. Moisture levels should be about 50 per cent. Check this either by using a moisture monitor or by doing the squeeze test – take large handfuls of compost and squeeze; one or two drops of liquid should be visible. Less is too dry, more is too wet. 
  • Increase moisture levels in dry compost by adding materials which contain a lot of water, such as fruit and veg peelings and grass mowings. Fresh grass is about 85% water.
  • Add grass in small amounts and mix in well as you aerate the compost materials so the clippings are dispersed. Beware of adding large amounts of grass mowings at once as they can clump together and become a slimy mess. Add them in thin layers mixed with carbon-rich materials that are good for providing air pockets, such as wood chips, shredded twigs or torn corrugated cardboard.
  • Give the compost a few turns with a garden fork to allow heat to escape.  You will also be able to see how dry the compost is inside.
  • If your compost is very dry and you need to add water, it’s best to use rainwater from a water butt if you can rather than tap water since chemicals in the water system that are safe for humans can kill some of the microbes you’re trying to nurture in your compost.  Don’t soak the bin as the water will not be distributed evenly. Add water in different dry places as you turn the compost by using a small watering can with a fine rose head.
  • You can ensure oxygen reaches deep into the compost by making a chimney – push a stick down into the compost from above and remove it so you have opened up a pathway of air.   
  • Adding dried leaves or hay will slow down decomposition in the compost, helping it to cool down.
  • Adding bokashi bran or the fermented pre-compost contents from a bokashi bin to a composter increases the temperature inside the bin – sometimes by as much as 20 degrees, so you don’t want to add these to compost that is already close to 70C.

Keeping wormeries cool

  • Worms work best in a constant temperature that isn’t too hot or too cold, ideally between 15-25 degrees Celsius. If the temperature in the bedding is getting close to 30C you should take action to cool it down.
  • A cool area such as a cellar or basement is a good spot for a worm bin during a hot summer. 
  • Worms stop eating in hot weather so stop adding waste or at least add very little.
  • Add some corrugated cardboard to aerate the bedding, adding airflow to allow the wormery to cool down.
  • If adding waste you could leave it in the fridge for a while, which will also cool the bin down.
  • Put an ice pack or frozen water bottle outside on top of the wormery for a short time.
  • Adding water is important in hot weather. You can flush your Maze Worm Farm with half a small bucket of water (5L) once a week to keep conditions moist. When doing this, replace the liquid collection tray with a container that will hold the sudden influx of water.
  • Pre-soak any dry materials such as newspaper before adding to the worm farm.

And finally, enjoy the sunshine while you can…

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