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Worm compost

Active worm composting bin with worms

Making worm compost (vermicomposting) is an easy and eco-friendly way of recycling your kitchen waste into good quality compost and liquid fertilizer. Although the idea of dealing with worms doesn't appeal to everyone, earthworms are efficient, hygienic composters.

Choosing a worm composting system

There is a wide range of wormeries available to buy, usually made from recycled plastic and supplied complete with bedding materials, instructions, and worms. They are generally either dustbin-like or modular stacking systems, varying in size from small bins designed to fit in a kitchen cupboard up to large wheelie bins. Domestic systems generally cost from about £40 up to £200 or more.

Which type is most suitable for you depends on the amount of waste you want to dispose of, how much time and effort you are prepared to put into taking care of it, and where you plan to keep the unit.

The easiest type to use are the stacking systems, which have several layers of trays for the worms to move up through, leaving the worm compost ready for use in the lower trays. They are ideal if you don't have large amounts of waste to compost, but must be kept under cover in a shed or garage as they are not rainproof. For larger amounts of waste a simple dustbin-like unit might be better. These come in various sizes.

Amazon stock a range of different types - *view them here.

It's also easy to construct your own worm compost bin - see our post on DIY worm composting bins.

Composting worms

Not all species of earthworm are suitable for worm composting systems. The long plump pink earthworm most commonly found in the garden is the lob worm, which is a deep burrower and will not survive in a wormery. The worms generally used for making worm compost are the European Nightcrawler or dendra, and the Red Tiger Worm or brandling/manure worm. Both live near the surface and feed on decaying organic matter.

Dendras are generally found living in damp leafy woodland, in compost heaps, and in rich soils. Brandlings are found in manure heaps and compost.

Worms don't have eyes but they are very sensitive to bright light and will try to hide as soon as exposed to light. They have no lungs but instead take in oxygen through their skin straight into their bloodstream. The skin must stay wet in order for the oxygen to pass through it, but they can drown if they are in too much water.

Worms can eat half their own weight in food each day. They have very small mouths and no teeth, so will only take very small particles of food which has already been softened by micro-organisms. This is mixed with grinding material such as sand, or fine grit, then contractions from the muscles in the gizzard grind the food into smaller pieces. After passing through the gut of the worm, the recycled organic wastes are excreted as 'worm casts' which look like fine soil. Secretions from the intestines make the nutrients in worm casts more concentrated and more easily available to plants than those in traditional garden compost.

Worms mature about three to six weeks after hatching and can then breed every three to four days. The cocoons are smaller than a grain of rice, yellow, and look like tiny lemons . Each contains between one and six worms and will take around 3 weeks to develop. The colour changes from pale yellow to mid brown as the worms grow inside. The baby worms are white. Cocoons are very resilient and, if the conditions are not right for hatching, can remain dormant for years ready to hatch when the conditions improve.

*You can buy worms online from Wormcity on Amazon Marketplace - see Composting Worms, or they also sell complete Wormery starter kits with worms, coir bedding, worm food and an instruction leaflet.

Siting the wormery

A wormery can be sited in a garage or shed, and some are even designed to fit into a kitchen cupboard. If the wormery is to be outside, it needs to be easily accessible from the house and in a position where it will be shaded in the summer and out of strong winds. (Stacking systems are not rainproof so should always be under cover.) In the winter it's best kept in a shed, utility room or garage, as worm activity ceases below 10 degrees centigrade. If the wormery has to be kept outside all year it will need to be well insulated by wrapping with old carpet or bubble wrap and tucking some straw inside.

Starting up the wormery

Put a damp sheet of newspaper into the bottom of the worm compost bin, and cover this with shredded newspaper which has been slightly dampened with water. Add a pile of worm bedding, well-rotted compost, or some leaf mould, and mix in a little garden soil. Place the worms on the bedding and leave the lid open for a few minutes until the worms have burrowed down out of sight. Put in a few handfuls of finely chopped kitchen waste, then replace the lid.

The worms now need some time to settle, and probably will not need any more food for about a week. In the early stages they should be left undisturbed to establish themselves in the bedding layer, then once they begin activity in the waste layer you can start adding more food.

It takes a while for the worm colony to establish, and so for the first week or so the worms are likely to try to leave, especially at night. Unfortunately it's impossible to prevent this, but some garden soil mixed in with the initial bedding does seem to help them settle. Keep the lid tightly on. A piece of damp cardboard on the lid and another below the bin act as a refuge for escaping worms so they are easily caught and returned to the bin. Or you can put the whole wormery in a bin liner and seal the top overnight so any worms which escape are trapped and can be returned to the bin the next morning.

Once the worms are settled they will generally stay in the bin.

Feeding the worms

It's best to feed the worms sparingly at first. The population will gradually increase and you can feed them more. Worms can eat up to half their own body weight every day so, as a rough guide, if you start off with 1 kilo of mature worms they will need up to 500g of food waste per day.

The rate of food consumption depends on the time of year, the temperature, and how long your wormery has been running, but you will soon become familiar with the workings of your own system. When it gets colder in the winter the worms will slow down and will not be able to digest as much food waste, so you will need to cut back on the amount you feed them - maybe use a bokashi system for some of your kitchen waste in the winter.

When working normally the wormery should smell earthy - bad smells generally arise when the bin is too wet or when excess food begins to rot and the compost becomes anaerobic. Never overfeed, and once a week or so carefully fork through the top of the compost and food to allow air to penetrate to keep the worms happy. Remember to drain off any liquid regularly - this can be diluted 1 to 10 with water and used as plant food. Check the tap regularly to be sure it's not blocked, as this is the most common cause of problems.

If you are using a stacking tray system, you will need to encourage the worms to keep moving upwards by adding some of the compost from the bottom layer each time you start a new tray at the top. This introduces bacteria needed for the composting process, and also gives the worms some bedding to move into. Allow the worms time to finish composting the food in the lower tray. A tray will probably take one or two months to fill, then about four or five months to finish composting, depending on the time of year.

The worms will eat most normal kitchen waste such as vegetable and fruit peelings, tea bags and coffee grounds, left-over cooked food, crushed egg shells and stale bread. They will also eat most other organic wastes such as pet bedding and the dust from the vacuum cleaner bag, shredded paper and torn-up egg boxes and cardboard. The more variety the better. Chop up any large pieces of food. You can also buy specially formulated *worm feed to add in every now and then to keep them fat and healthy.

The ideal balanced diet is composed of about half kitchen waste and the remainder a mix of paper, cardboard and leaves.

Worms are not keen on tough, woody waste, spicy foods, fat, raw onions, or very acid foods such as citrus fruits. Never add anything with traces of insecticides or pesticides, poisonous plants, salt, soaps or cosmetics, paint, solvents or non-biodegradables.

Pet bedding from rabbits, hamsters, guinea pigs, gerbils, mice etc - sawdust, straw, hay and paper bedding - can safely be added to the wormery.

Fermented food waste from a bokashi bin can be added to the wormery to finish composting. It is quite acidic so add it gradually mixed with other waste. (see our post on bokashi composting)

Dog and cat poo can occasionally be put into a wormery but if you need to compost this type of waste on a regular basis it will work much better in a separate wormery used solely for pet waste mixed with shredded paper. (See our post on disposing of dog waste) Dog and especially cat waste can contain dangerous pathogens, so never use the vermicompost where children play or where you grow food crops.

Chicken manure - before adding to a wormery, chicken manure needs to be well aged or pre-composted as it is very high in nitrogen and ammonia and will get very hot when composting which could kill your worms.

Horse manure - the worms love horse manure, but again it needs to be pre-composted to take the heat out before putting in a wormery.

Garden waste/grass mowings - a little is good, but too much is likely to heat up as it decomposes in the bin and will kill the worms.

Emptying the bin

Stacking trays 

This type of worm compost bin is designed to be harvested as you go along, taking the finished compost from the tray at the bottom each time and replacing the emptied tray at the top of the stack. Always allow the worms time to finish composting the food in the lower tray before removing it. They will gradually all migrate up into the next tray.

Dustbin systems

In order to separate out most of the worms, they should be fed plenty of their favourite food a few days before harvesting. This will bring a lot of them to the top of the bin. To harvest, open the lid and quickly remove the layer of uneaten food waste together with the worms. Put the whole lot in a bucket and cover over.

Tip out the remaining contents of the bin onto a plastic sheet. Make it into several cone-shaped piles, then leave these to stand for about half an hour. The rest of the worms will gather together in the centre of the piles. Carefully collect up the compost from all around the sides and the top until you are near to the worms, then remake the piles and repeat the process. Keep doing this until just the worms and a little compost remain. They can then be returned to the worm bin with the compost as starter bedding. The rest of the worm compost is ready for use. It can be stored before use in a bin to keep it dry.

Alternatively, fork out the worm compost from the bin into a garden sieve and carefully sift out the coarse material and the remaining worms. Collect these up to return to the bin. Leave some compost undisturbed in the bottom of the bin as this will act as starter bedding and contains most of the worm eggs to get the population re-established. The sieved compost is ready for use in the garden.

Once the bedding and the worms have been returned to the bin, replace the layer of uneaten food waste and the rest of the worms from the bucket. Leave the bin for a few days to settle, then simply start adding food back into the wormery as before.

Using the worm compost

Worm compost is very rich and can be used as a mulch on soil or mixed into the soil when putting in new plants or sowing seeds. For containers it's good for topping up the compost in pot-grown plants or mixing into potting compost.

The nutrient rich liquid produced by the wormery makes good plant food when diluted with 10 parts of water. The amount produced depends entirely on the type of foods being composted, and it may take months for any liquid to be produced.

For a good book on worm composting see *Composting with Worms: Why Waste Your Waste

*affiliate links

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