Eisenia fetida – the heroes of your compost bin

Composting is the most sustainable way to recycle your food and garden waste, and nourish your garden. And when you add the right mix of organic material to your compost bin – you also create an environment for worms to thrive.

Composting worms play a key role in breaking down organic waste to create rich, fertile compost. And having some insight into their habits can help you optimise your composting efforts.

What are composting worms?

Composting worms (Eisenia fetida) - also known as brandling worms, red wriggler worms, or tiger worms - are different to the earthworms residing in your garden soil.

These surface-feeding (epigeic) worms are smaller and more active than soil-burrowing worms, with a reddish appearance, obvious stripes and well-defined segments. They’re often found naturally in leaf litter, manure piles or under plant pots - but they also love the nutrient-rich, moist environment of a well-balanced compost bin.

How compost bin conditions affect worm behaviour

Composting worms are highly active in the right environment; however, they will migrate to a more suitable location if the conditions are not ideal.

It’s common to find worms congregating on the underside of your compost bin lid - but don’t panic if you find them hanging out in this way. They know what’s best to optimise the thriving ecosystem of your compost bin, but you can also look at how the following might be affecting them.

1. Compost bin temperature

Worms are cold -blooded creatures, so their surroundings directly affect their body temperature. When you add materials to your compost bin, the thermophilic bacteria generate heat. This can make it too hot for worms, so they will try to escape to a cooler area of the bin. Once this thermophilic phase of the composting process is over, they will return to help further break down the organic matter. In winter, when the ambient temperature is colder, they usually go the other way, huddling together in the warmest area in the middle of the compost heap. The optimum temperature for composting worms is 12-24°C, but they would struggle to survive at +35°C or much below 5°C. It’s not only temperature that can affect worms, but a change in air pressure can also cause worms to migrate. Although worms don’t have ears, they’re sensitive to vibrations, so heavy rain on the compost bin lid can cause them to try to escape these vibrations.

2. Moisture/oxygen levels in your heap

Worms are sensitive to moisture levels, so if the materials in the bin become too wet, it can create an anaerobic environment (lack of oxygen). Worms need a steady supply of oxygen to survive, so will move to a dryer area of the bin if it becomes too wet or too compacted. If the heap is too dry, however, it means there’s not enough of the nitrogen-rich food that worms love, so they’re likely to move on.

3. Abundance of worm food

Worms are always looking for food and will naturally move towards areas where decomposing organic material is abundant. If you’re regularly feeding your compost bin with the right balance of food and garden waste, composting worms will be able to seek it out. If the food supply is in short supply, they may climb up towards the lid in search of more to eat.

4. Overcrowding and reproductive safaris

Worms are excellent at regulating their populations according to the amount of space and food available to them, so they may decide to migrate to maintain this balance or try to escape the bin if it becomes overcrowded. They may also head out to look for a suitable mate especially in the spring or early autumn when their reproduction rates are at their highest.

5. Compost heap pH and nutrient levels

Worms may migrate to another area of your compost bin if it becomes too acidic. While they love most vegetable peels and fruit scraps, they don’t tolerate citrus, garlic or onions well and too much of this type of material in one go might make the composting environment unpalatable. They may also up-sticks to seek out calcium or another mineral they might be lacking in.