Val Bourne
Firstly you can partially rot your clippings down before you add them. Spread them out on a plastic sheet after mowing and leave them until they brown: this usually takes a day. Turn them and leave them for a second day. Then add the brown clippings to the heap, or use them as a brown mulch under shrubs, raspberries or trees.
The second method involves adding plain cardboard boxes. These can be ripped up very roughly and mixed among the clippings to create air pockets.
The third method is to add a natural accelerator. If you have a pet rabbit or hamster, or keep poultry, their droppings are ideal. But comfrey leaves also work well and if you plant an upright Russian comfrey (Symphytum uplandicum) close to the heap it can be cut back every few weeks and the leaves added.
Finally cover your heap to keep the moisture in.
Which type of bin shall I use for compost?
Think of a compost heap as a multilayered sandwich with lots of ingredients and remember it's got have air and moisture to decompose. The plastic compost bins with lids, though useful in small gardens, tend to exclude air and for this reason I would build or buy a square wooden bin with slatted sides and a removable front. Ideally you should have a bay of three - one to add to, one that's rotting down and one that you're digging.
Shredding helps
Add chopped up material when possible because chopped material makes fine, friable compost. A mechanical shredder is useful simply because it creates more air gaps among smaller pieces. You can also shred woody material.
When shall I start?
If you haven't started making compost, September is an excellent time to make a start because you'll be cutting down some of your plants. Look for leafy stems and avoid adding hard, woody material to the heap unless you can shred it first.
What not to add
Never add any plant material that looks diseased. Black spot affected rose leaves, blackened hellebore foliage, virused plant material, mildew affected leaves etc are best left off the heap.
And although hamster, chicken and rabbit droppings make excellent accelerators, dog and cat faeces shouldn't be used. They are too acidic.
A Cover Really Helps
If you are using an open wooden bin you need a cover to keep in moisture. You can use large cardboard boxes or a piece of old carpet (rubber side down) as a lid.
The Difference Between a Hot Heap and a Cold Heap
Most gardeners add material throughout the growing season and their heaps decompose slowly. These are known as 'cold' heaps. They are usually dug out several months later.
These heaps still warm up fairly quickly and you can often smell them decaying. Touch the heap with the flat of your hand to gauge when it cools. When this happens you need to turn the heap, putting the less rotted material on the inside. This will reignite the process - but compost from 'cold' heaps still takes months rather than weeks.
Hot heaps work much faster because they reach a higher temperature and involve different, swifter acting microbes. To make one gather a lot of green material together and build the heap all at once and then cover it. I have used a willow and hazel frame to contain hot heaps. Once covered it will soon reach a very high temperature. When you sense it's cooling, turn it and add a natural accelerator like comfrey or small animal droppings. Within six weeks you can have some wonderful compost.
What can I use my compost for?
From a Traditional 'Cold' Heap
Friable compost is the ideal material to add to the hole when planting new plants. It also makes an excellent conditioner when dug into the soil over winter. Cold heaps do not kill seeds and therefore the compost is best used below the ground and not spread on the soil surface - where seeds will germinate. For this reason you cannot sow seeds in 'cold' heap compost.
From a Hot Heap
This can be spread on the surface and it can be used for seed sowing too because the high temperatures kill unwanted seeds. You will need to sieve this compost before sowing seeds however.