Due to the slow decaying nature of their high carbon content, dry leaves break down far more slowly than most other compost ingredients. This can be overcome either by placing the collected leaves wet in plastic bags (taking care to avoid collecting from areas that may be subject to high levels of pollution, e.g., roadsides), or in specially constructed wire bins, to encourage fungal action. To accelerate this fungal breakdown, it is useful to keep the leaves wet and avoid the drying effects of wind. The traditional wire enclosure may slow down the process by allowing the contents to dry out unless it is lined with cardboard or similar material.
Time and process
Leaves alone can take between one and two years to break down into rich humic matter with a smell reminiscent of ancient woodland. While not high in nutrient content, leaf mold is an excellent humic soil conditioner. To speed up the decomposition process, fallen leaves can be shredded, for instance by using a rotary lawn mower. Adding fresh grass clippings to autumn leaves will also speed the process. For best results watch the pile to keep moisture content high enough, observe temperatures, and turn the pile occasionally to improve the cycle.