The worm bin

A worm bin transforms kitchen waste and soft garden waste into the best organic fertiliser with little effort. This fertiliser can be used for countless balcony and container plants. And even a small worm bin can produce an ideal plant booster for an entire vegetable garden.

Vermicompost can be produced on a small scale in a garden, on a balcony or on a patio from the household's kitchen waste. Vermicompost refers to the transformation of organic waste by compost worms. More precisely, at moderate temperatures (5-30oC), compost worms in tandem with various other soil organisms and under the influence of oxygen transform organic waste into stable clay humus complexes, in other words valuable worm humus. Compost worms are a special kind of earthworm. Vermicomposting is an odourless process, and does not produce any heat. Compost worms eat organic waste and then excrete worm humus. This is ready for use immediately without the need for further maturing, and has three to five times the fertilising effect of regular compost.  
Vermicompost is particularly well tolerated by plants, therefore it can even be used in higher concentrations than regular compost. Unlike with conventional composting, the heap does not need to be turned over. The volume of waste is reduced by up to 90% and, last but not least, the worms reproduce so quickly if properly looked after and fed that you can share starter populations for other worm bins. Compost worms digest around 50-100% of their body weight per day. This means that a worm weighing 0.2 grams can eat 0.15 grams a day on average – provided the food is "predigested" by fungi and bacteria (because the worms cannot eat hard stalks or dry foliage).

Fertiliser made from organic waste
Worm bins are suitable for many uses: in cities they are the best way of transforming kitchen waste and green cuttings from a balcony straight back into organic fertiliser. You save having to dispose of the organic waste in the organic waste bin and you also no longer need to buy organic fertiliser for your balcony or indoor plants. Worm bins have no smell – provided they are properly looked after. They don't get out of control, don't need a lot of attention and study after study confirms that the worm humus produced is among the best fertilisers available.

The worm bin in winter
Unlike regular compost, you have to make sure that the worms don’t freeze to death in winter. In compost heaps, the worms simply withdraw into deeper soil layers. They can't do this in the bin, and would simply freeze to death. Smaller vermicompost bins must therefore be brought inside in winter. In regions with mild winters, it is also enough to wrap them well (in fleece or using compost mats also sold in garden centres as over-wintering protection for tub plants). Although the worms in the bin will not then be active over the cold season, they will enter a dormant phase and in this way easily survive the winter.

How big should a worm bin be?
The size of the surface is what determines how productive the bin will be. Because the worms only work at the surface, the greater the surface, the more material can be processed. The bins should therefore be on the shallower side.

What does a compost worm need to live?
Compost worms are low maintenance compared to other house pets. However, they are still living, breathing things. For a good life they need air as well as adequate humidity, warmth and darkness. And, of course, they also need food. However, since you want them to thrive rather than just survive so that they eat as much as possible and produce as much fertiliser as possible, you must offer them optimal conditions and also regularly monitor them.

Temperatures between 15-30°C suit compost worms best. This is the range where the worms eat the most and give the best performance. It is relatively large, which means that an earthworm bin can stay outside over a long period during the year and deliver a good yield. Worms steadily become less active from 15°C to approx. 5°C, cease all activity below this temperature and die at -2-3°C. Worm eggs (cocoons) can survive much lower temperatures. At the height of summer, however, heat can also become a problem. Worms quickly become stressed at temperatures above 30°C. This means that if your earthworm bin is on a hot, south-facing balcony, you should either put it in the shade or water it regularly on hot summer days.


Temperature                                    Activity
Below -2 to -5oC                                Worms freeze to death
5 to 15oC                                          Dormant phase
15 to 30oC                                        Best performance and highest rate of reproduction
30 to 40oC                                        Poor performance
above 40oC                                       Worms die from heat

Table: Optimum temperatures for compost worms

Andrea Heistinger
VIKING gardening expert



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