There are occasions when you should not mulch your lawn. For example, your lawn may have been allowed to grow much longer than usual on occasion, perhaps because work or family commitments have prevented you from getting the grass cut when it should have been. In addition, wet weather sometimes persists in the summer months, necessitating cutting the grass whilst it is wet. Under these circumstances, it is unwise to use the resulting clippings as a lawn mulch, they will be much better used as compost as wet clippings will be unevenly distributed and will form clumps which may harm the grass underneath.
In addition, it is not wise to mulch the lawn every time you mow as this can lead to two problems. The first is an accumulation of clippings on the surface of your lawn which can blow onto paths and borders and, in wet conditions, get carried into the house on shoes which is a major nuisance! The second is an overdevelopment of the normal lawn thatch. Thatch is an accumulation of grass roots and stems above ground and is normal in all lawns but can cause problems if it gets too established. If thatch gets to about ½” thick, stop mulching and take steps to de-thatch your lawn. Otherwise your lawn will suffer from drought (as water will be unable to effectively penetrate to the roots) and from starvation (as sunlight will be restricted from reaching the blades of grass and photosynthesis, the process by which all plants manufacture food from sunlight will be hindered).
If you have a petrol lawnmower of the push type, it may be possible to fit a mulching attachment. It is possible to buy a ride on mower that offers a combination system where some of the grass is collected and some mulched. When mulching your lawn, leave the grass about half an inch longer than normal and always water your lawn after mulching, not before. The reason for this is that dry clippings will disperse more readily and then, if you water, they will settle and not blow onto areas where you do not want them!
The great benefit of mulching lawn clippings is that you will be able to reduce, or perhaps even eliminate the need for additional fertiliser as about 4% of the clippings comprises Nitrogen, 2% Potassium and 1% Phosphorous the decomposition of the clippings puts these elements back into your lawn.
About 80% of the grass clippings is water and it is obviously a great benefit to put this back onto the lawn in times of drought or extreme heat. Mulching will also help to prevent moisture loss