Heaps of lawn mowings turn into a slimy, smelly mess because after the first burst of heat, the mowings collapse and all the air spaces disappear. Although the decaying process continues, the end result is not pleasant to handle or smell, so dig a trench and bury it, where it can continue to rot without offence.

Avoid this situation by mixing tougher, drier materials with mowings, so that air spaces are maintained and decomposition can progress quickly. Also ensure that air can circulate within the heap, but not so much that all the heat is lost. Keep a lid on the heap too, because rain can waterlog the contents and reduce the air spaces.

The heap can be turned to provide more air, but few gardeners do this, unless they use a bin that can be tumbled. Rebuilding the heap at intervals also incorporates air and mixes rotted with unrotted material.