How to protect your worms from extreme summer heat

Heat and even heat waves are getting closer ! It is going to be hot !

We are told to stay in the shade or in a cool place, remain properly hydrated and avoid physical exertion…  In a word, to protect ourselves from the heat.

All these good advices also apply to the worms and particularly in case of heat waves.

But how can we handle the excessive heat in the worm composter during a heat wave ?

First, let’s see what’s happening in the worm composter when it’s hot. It will help you to easily understand all the advices to protect the worms and avoid catastrophe.

What’s happening in the worm composter by high heat ?

A heat wave is a period of excessive heat for at least three days and nights in a row. Therefore, the atmosphere does not cool down enough and it lets you feel the heat even more intensively. 

The temperature

For optimal performance, the worms and the whole composter ecosystem need a temperature between 15° and 25°C / 59°F and 77°F.

Below those temperatures, the ecosystem works at reduced level.

Above, it does not work properly and it can even be lethal for the worms above 35°C / 95°F.


The decomposition of waste

When the waste is not processed by the composter ecosystem, other organisms take over. Another process of decomposition of organic material gets started. The bacteria in action in this new process need a lot of oxygen but this creates an inhospitable environment for our wrigglers. This decomposition also changes the consistency of the fresh raw materials which becomes soft or liquid. Midges love it !
The bedding, at first rich in fresh raw materials, is going to change consistency very fast. It tends to mat and seals out oxygen.

The parameters : aeration, moisture, acidity

An enclosed composter keeps in the heat but it can be let out with good ventilation.

If your composter is not ventilated enough because of reduced airflow, packed bedding, many stacked trays, then, the heat builds up.

It’s like a heat wave inside the composter !

The moisture in the bedding works as a “heat-transfer fluid”. The higher the moisture is, the hotter the bedding gets.

The bacteria at work acidify the bedding and this process is increased with the lack of oxygen. Therefore, the bedding becomes far less hospitable for the “good” organisms of the vermicomposting.

Do not hesitate to visit this page for further information.

How can I see if the worms suffer from the heat ?

Above 25°C / 77°F, the worms and other organisms do not eat as much.

When it reaches 30°C / 86°F, you will notice that the worms are looking for fresh places. It may be on the side walls or under the lid. It may also be in the collector tray because it is a much more humid and cooler place.

At 35°C / 95°F, the worms try to flee from the extreme heat by getting out of the composter. Unfortunately, they fall on a dry floor. And that’s when you realize there were so many of them working in your composter. But when you notice it, it is often too late because the worms dry up very fast and die once they lie on the floor.

It is a sight you do not wish to see all the more so as you must add to it an unpleasant smell and maggots crawling around.

The only thing to do is to start from scratch again. The good point is that you won’t do the same mistake twice !

Let’s unveil the “heat wave plan” !

Keep an eye on your wrigglers !

Worms cannot withstand the extreme temperature of a composter placed outside during a heat wave.  Worms cannot survive at temperatures above 35°C / 95°F.

Put your composter in a fresh place in the shade like a cellar, a laundry room, a basement ….

Would you yourself stay outside all day when it is so hot ?

Reduce your amounts of waste

In hot weather, the organisms stop eating. It is a survival reflex.

The waste won’t be processed by the worms anymore but by some bacteria which will consume the oxygen. As soon as the fermentation starts, the waste get warmer and warmer.

So the good strategy is to stop adding any waste, or at least very little, in order to avoid the problems.

When it’s hot, what do you prefer ? A raclette or a light salad ?

Think about adding some thick cardboard to absorb excess moisture. It will also aerate the bedding. Sprinkling ground egg shells does a lot of good too.

Ventilate the vermicomposter

A well ventilated bedding – with some thick cardboard – allows good airflow.

Pieces of brown corrugated cardboard are what you need.

To allow the airflow, you can also keep the spigot open if there is one on your composter but don’t forget to leave a small bucket under the spigot.

If there is no spigot, empty the collector tray very often to reduce moisture inside.

Create some ventilation shafts to ensure proper aeration between the different trays. You only have to make a hole by pushing the waste material on the side. Good airflow allows the composter to cool down and to get rid of the excess of moisture.

Create a refuge area

Worms like to seek refuge in cool places. So it would be nice to get one ready for them.

But where ? A little hint : worms are used to wriggling down when it’s hot.

Now, it’s your turn to get this ‘Fresh Zone ready !


Empty the collector tray, put some cardboard or a wet cloth in half of the tray. The evaporation has a cooling effect and this ‘fresh place’ will even be cooler if it is well ventilated.

This ‘Fresh Zone‘ must be humid enough to welcome the worms but not too humid to allow a good evaporation effect.

The worms will soon find and appreciate this bath of freshness.

Make sure to keep this ‘fresh place’ very well ventilated and humid but not too much !

Bring in some freshness

If you keep adding some waste, think about leaving them in the fridge for a while. It will cool down the composter at the same time.

Be careful : do not put the waste in the deep freezer to cool down the composter afterwards.

Indeed, kitchen scraps contain a lot of water. When it turns into ice, the frozen water breaks the cell membranes. Then, the thawing process releases a lot of water very quickly and this can hamper the composting process.

The evaporation has a cooling effect. At school, we were told that water needs some energy – heat – to change from the fluid to the gaseous state. So when water evaporates, it cools down the composter. It absorbs the heat and turns itself into a low-temperature vapor.

If your composter is very well ventilated, you may think of putting a wet cloth or pieces of brown cardboard at the top of the compost. The ventilation will allow evaporation which will cool down the place a little bit. But forget about it if the whole system is not well ventilated because it would only add some more moisture and that’s not something you want.

Another strategy is to put an ice pack or a frozen water bottle outside on top of the composter or just on the waste inside. But be careful because it will also bring some moisture from condensation. So do not hesitate to add quite a lot of pieces of dry brown cardboard.

Is that enough to get the worms safely through the heat wave ?

Not too much moisture and good ventilation is what a vermicomposter needs to get through extreme heat.

Here are the tips to repeat over and over again when it’s very hot.

Keep an eye on your wrigglers. Avoid the sunlight !

Renew the material in the ‘Fresh Zone’ because it will soon turn into some compost.

Do not add too much food.

Of course, the best thing would be not to face those climate phenomenons anymore….

A solution ? It would help a great deal if everyone could compost their biowaste as it would reduce greenhouse gas emissions.

Do not hesitate to share your tips and good strategies with us.

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  1. Mihwa

    Thank you for the great tips. I am a new worm mom and the first summer is hitting here in Arizona where it will hit 114F this weekend. I will put some moist hay in the bottom of the bin tomorrow.

    • David

      Take care off them !

    • Mihwa

      When I checked the bin last Saturday when it was 112 F-113 F, my wormies were doing ok. They were chilling under the hay in the bin. Yesterday, the temp hit 114 F here in AZ. But I didn’t check the bin for them yesterday. Today, when I opened the bin, all the wormies were disappeared. I mean there is ZERO worms. I don’t even see a dead body. Why is that? My father-in-law says when it’s too hot they “melt”. Is it true?

    • David

      This temperature is indeed much too high for the worms.
      Where is the vermicomposter placed? The worms could have escaped into the soil if it was possible for them to do so (or die at the bottom of the vermicomposter).
      If the worms have “liquefied”, it must smell very bad in the vermicomposter. In this case, you need to empty it to clean it (you can use the vermicompost that is still good for plants).

  2. Patricia roe

    I have already learned how to protect worms in compost from heat. I live in Florida.

    • David

      Great! What tips would you add to protect worms from the heat?

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