Best Temperature for Worm Composting with Red Wigglers

Whether you start a worm bin in spring or middle of the winder you need to think about temperatures right from the beginning. The ideal temperature for worm composting allows red wigglers turn as much organic waste into living, nutrient rich worm castings as they can and reproduce well, too. 

Having the facts at your fingertips will help you to position the wormery in the right place, monitor the conditions, and not only avoid disasters but have a thriving worm team throughout the year.

Does Temperature Matter for Worm Composting?

Short answer is yes. It does matter a lot. Several biological processes in the worm bin are markedly influenced by temperature:

What is the Ideal Temperature  for Composting with Red Wigglers aka Tiger Worms?

Red wigglers are the most commonly used worms for composting. As they have no teeth they need to feed on soft, decaying organic matter. 

Most worm bins in a household receive green food waste that isn’t ready yet for worms to eat. It needs to start to decompose and soften enough for worms to eat. As we know from experience vegetables and fruit rot faster the warmer or even hotter it gets. So from a worm food provision point of view the higher the temperature the better.

In addition, worms have shown to produce most cocoons at 25°C (77°F) under lab conditions (see research here). At that temperature the worms also keep growing, gaining weight and reproducing for at least 3 years. This is based on the longest study ever done on red wigglers meaning there is a good chance they live longer – we just don’t know.

So if you could keep your worm bin at 25°C (77°F) to have food decay at a decent rate and have maximum cocoon production it would be perfect.

This is especially useful to know if you are just starting out and would like to increase the number of worms as quickly as possible. More on how to optimize conditions for rapid growth see here.

However, in reality, most households and gardens will have varying conditions. The temperature surrounding the bin as well as inside the wormery is likely to fluctuate during the day and with the seasons.

In fact, one of the main reasons the red wiggler is so popular as a composting worm is that it reproduces and keeps feeding across a fairly large range of temperatures between 15°C-25°C (59°F -77°F) according to worm expert Rhonda Sherman.

Truth is,t he same study mentioned above shows that worms do produce cocoons at temperatures as low as 10°C (50°F) although much, much less than at 25°C (77°F)

Furthermore, red wigglers on the whole cope well with fluctuating temperatures during the span of a day and in fact tend to have more cocoons when the temperature changes during the day. This makes sense if you think of its natural habitat being fairly close to the surface and therefore exposed to varying temperatures during day and night.

Ideally you would keep the inside temperature of your worm bin in the lower twenties degrees Celsius (seventies Fahrenheit). If you don’t already own one, purchase a  compost or worm farm thermometer. These are simple, easy to use instruments to monitor what’s going on inside your worm bin.

You’ll be surprised how often there is a large difference between the outside and inside temperature of the wormery.  

What Hot and Cold Temperatures Can Worms Tolerate?

Even though you now know the ideal temperature it’s also good to know your worms’ limits for hot and cold.

What if the temperature in the wormery goes over 27°C/ 80°F?

Here is the most important bit you need to know if you keep red wigglers: Prolonged exposure to higher  temperatures than 27°C will kill them.

How do you know whether temperatures have reached dangerous levels?

If the temperature has risen inside the bin as can happen if microbial activity hotcomposts even just a good handful of fresh food, you will see worms fleeing the scene in all directions: They might crawl up to the inside of the lid or squeeze close to the walls  and migrate downwards into lower, cooler trays if there are any.

A compost or worm bin thermometer can easily confirm your suspicions. If you don’t have a thermometer, yet, you can just hold the palm of your hand close to the surface and will probably sense any warm sports that way if some fresh food scraps have started to hot compost closer to the surface.

How to Ensure Safe Worm Farm Temperatures in Hot Weather

As explained above, red wigglers can die at temperatures over 27°C (80°F). At that point they will try to flee to cooler areas. Here are some ways to avoid those dangerous temperatures.

1. Move bin to a cool, shaded area

This is probably the easiest and safest way to ensure the bin doesn’t overheat. As most worm bins are a darker colour a shaded area prevents it from absorbing extra heat from sunlight.

Find a place that is not only cooler (no more than 25°C (77°F), ideally less in case there is food inside that can still heat up and increase the temperature from inside) but that doesn’t get any sun and doesn’t turn the bin into a heat absorbing death trap for worms.

If you have a cellar that could be the safest place for your worms to hunker down during a heatwave. The worms may slow down a little at temperatures below 20 C, but that’s a little price to pay compared to losing the whole worm population.

2. Improve Air Circulation

This can help prevent overheating in the first place and can also be a first aid measure when things have gotten too hot.

• Lift the lid so hot air can escape easily.
• Make sure the surface doesn’t dry out. Use a sprinkler or mister to moisturize the surfance without drenching the bin.
• Increase air flow over the bin. A natural breeze would be great. If desperate use a fan. A wet rag or towel over the bin helps drawing heat out of the bin.Increase air flow over the bin.
• Make sure there is enough moist carbon rich material like shredded cardboard in the bin. This provides air pocket, helps to keep worms skin moist and healthy (they breathe through their skin) and doesn’t heat up like fresh vegetable and fruit scraps.
• Every now and then, gently turn and loosen the composting material with your hands to aerate the bedding. 
• Separate the trays if you have a stackable worm bin. This create more surface area and heat can dissipate more quickly.

When Temperatures are already above 27°C (80°F)

• Put ice cubes in the top tray. You don’t want to add more water to the wormery, so put the ice cubes in a plastic bottle or inside newspaper for the newspaper to absorb the water from the melting ice. The worms will crawl closer to the cool areas.
• If you don’t have ice cubes at hand you could try placing a bottle of cold water inside the bin. You will have to monitor this more frequently as the water will fairly quickly absorb the heat from the bin.
• If you live in a climate where temperatures are sure to rise above 27°C/ 80°F and have no cool, shaded place for the wormery, consider creating worm towers over the summer. This is a good way to avoid having to monitor the situation all the time.

  Worms can crawl deeper into the cooler layers during the heat and move up at night when it’s cooler. Don’t worry about losing your worm population. They will stay where there is food and you will be able to retrieve a lot and move them back into the wormery bin for winter. If winters aren’t too harsh and hardly go below freezing you can just leave them in the worm tower in ground to hibernate. 

• Feed less. You want to avoid food heating up due to microbial activity and increasing the temperature inside the bin above safe levels.
  It’s best to add food in one area of the tray. That way worms can move away to cooler areas and come back when it has cooled down and settled to a comfortable range. At that point worms will thrive on the masses of microbes on the decomposing  food.
• If you have consistently hot summers, consider a different worm more suitable for your climate like the voracious African nightcrawler or Blue worm. 

What if The Temperature Of Your Bin Is Too Low?

Unlike hot temperatures there is a wider range of cold temperatures red wigglers can tolerate.

Below 10°C (50°F) the worms start hibernating. They will eat much less and stop reproducing and laying cocoons. 

Worms will die in freezing temperatures unless they can crawl somewhere where the temperature isn’t quite as low like the middle of a wormery.  Naturally, this is only possible if there are only short spells of frost.

CLick here for more information on outdoor worm composting in winter

In conclusion

Any temperature between freezing point and about 26°C / 79°F is tolerable for red wigglers. The ideal temperature is between 20 – 25°C / 68 – 77°F.

Overheating is the main concern for most worm bin owners because this could kill the worms.

The easiest is to find a place where the surrounding temperatures are unlikely to reach unsafe levels for worms. Also, measuring the temperature inside the bin with a compost or worm bin thermometer is an easy, cheap way to monitor the situation. That way you can rest assured that you provide the best conditions and the worm population keeps thriving throughout the year.

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