Composting grass clippings is a great way to get nitrogen rich compost 'greens'. Add to that the fact that about half the states in the US have banned grass clippings from landfills and you have a compelling reason to compost grass clippings yourself.
However, grass clippings, because they are very fine and soft, do not have much structure. Unless used carefully they tend to compress into a water and air impenetrable mat. This can change your lovely aromatic aerobic compost into a stinky, slimy, anaerobic mess.
The key to avoiding problems is composting grass clippings in very thin layers. Three ways to make good use of your grass clippings are
Grasscycling is as easy as it gets. Instead of mowing, bagging, moving, piling and then turning your grass clippings you simply let the grass fall onto and decompose on your lawn.
Essentially grass cycling is darn close to doing nothing. Call it very thin sheet composting. It mimics nature's decomposition - a process which usually happens in thin layers and rarely in the big hot compost piles we concoct.
Doing almost nothing helps your lawn and its soil. Grass clippings have an NPK of 4 - 0.5 - 2, plus small amounts of other plant nutrients. In fact by bagging your grass clippings you are removing about half the Nitrogen fertilizer you paid for and put on your lawn. Grasscycling feeds the soil microbes and can provide up to 30% of your lawn's fertilizer needs.
Leaving your grass clippings on your lawn does not contribute to lawn thatch build up. In fact it may even decrease thatch build up. The microbes chowing down on the grass clippings are the same critters who eat thatch. Dethatching the lawn may become a chore of the past with consistent grasscycling.
The key to successful Grasscycling is Mow High, Mow Often, Leave the Clippings,
There are times when you'll want to collect your grass clippings. For example:
Remember that grass clippings need special handling because they are too fine to maintain air passages and are very moist. They tend to compress and mat. Once that happens air and water can no longer get into your pile and you'll soon have a stinky, anaerobic mess.
Mulching is a good way to control weeds, cut down on watering needs and in the case of grass clippings slowly feed your soil and plants.
Essentially the same rules apply to mulching with clippings as composting with clippings. Keep the layers thin - at most an inch of clippings at a time. You need to watch this - make sure water and air are still getting through to your plants.
There are at least two long lived herbicides that do not fully break down in the composting process - clopyralid and picloram. These are sold under a number of trade names. Avoid composting or using these clippings as mulch as your problems will last several years.
Do Not mulch or incorporate into the soil grass clippings that have been treated with any herbicides. This includes weed and feed combinations. There is a good chance the herbicide will be active and damage the plants you are mulching.