Compost is made from roughly equal volumes of soft green waste and dry brown material such as leaves and cardboard. Cooked food, meat and dairy products are left out.
A heap filled gradually takes six months to two years; the result is spread as a mulch or mixed into potting compost.
Composting turns garden and kitchen waste into a dark, crumbly material that improves any soil. It needs no special equipment and little work; the main requirements are a mix of ingredients and patience.
A heap needs two kinds of material in roughly equal volumes.
Too many greens, typically a thick layer of lawn mowings, produce a wet, airless, smelly sludge. Too many browns produce a dry heap that barely changes. Most problems are solved by adding the missing kind and mixing.
Many tea bags contain plastic and do not break down fully; loose leaves or bags labelled as plastic-free can be composted. Items labelled "compostable" are often designed for industrial plants and may persist in a garden heap.
Japanese knotweed must not be composted or put in garden waste collections; it is classed as controlled waste and has to be disposed of at a licensed site.
An open heap works, but a container keeps it tidy, warm and moist. Plastic bins with a lid suit small gardens. Wooden bays about a metre square hold enough to heat up properly and are easier to turn; two or three side by side allow one to be filled while another matures. Many councils run schemes supplying compost bins to residents.
The bin stands on bare soil, which lets worms in and liquid drain away, in a spot that is reasonably warm. Wire mesh under the base keeps rats from burrowing in.
A heap built in one go from a cubic metre or more of well-mixed material heats to 50–60°C within days as bacteria multiply. Turned every week or two, it can produce compost in a few months and kills many weed seeds and diseases.
Most household heaps are filled a little at a time and never heat much. This cold composting takes from six months to two years and relies more on worms and fungi. Turning the heap once or twice, or emptying and refilling the bin, speeds it up but is not essential.
Finished compost is dark brown, crumbly and smells of earth, with few recognisable ingredients. Twigs and other unrotted pieces are returned to the next heap. It is spread on beds as a mulch 3–5 cm deep in spring or autumn, forked into planting holes, or sieved and mixed with soil or leaf mould for pots.
Autumn leaves rot slowly by fungal action and are better kept apart. Packed damp into a wire cage or into bin bags pierced with a fork, they turn into leaf mould in one to two years. It is low in nutrients but an excellent soil conditioner and ingredient for seed compost.
A wormery is a stacked container in which composting worms turn kitchen scraps into a rich compost and a liquid feed. It suits households with a small garden or none, and can be kept in a shed or on a balcony, protected from frost and hot sun.
Cooked food and other waste unsuitable for a garden heap goes into council food waste collections, which are treated at high temperature. The arrangements are described under household recycling.
Bagged "compost" sold for pots is a different product from garden compost, made from materials such as composted bark, wood fibre, coir and green waste. Peat-free versions hold water differently from peat: the surface can look dry while the pot is wet beneath, so testing with a finger before watering avoids overwatering. Home-made compost is generally too rich and too variable for seeds, but mixed with soil or leaf mould it suits larger pots.