Leaving them on a lawn that has healthy grass becomes a problem only if they are too thick (that is, the grass was too high when you mowed it) or the lawn is wet:

  • When they are too thick, they mat down.
  • When the lawn is wet, grass clippings stick together (just as it is easier to make snowballs out of wet snow than fluffy snow).

In each case, they block the healthy flow of air to your lawn.

If you mow the lawn before it gets overly tall, the mass of the grass clippings will not be sufficient to warrant raking (or bagging, if you use a bag attachment on your lawnmower). They do not contribute to lawn thatch build-up.

But what if you do not want to have to worry about getting the timing just right or bother with emptying out a mower bag? A good way to get around having to bag or rake grass clippings is to mow with a mulching mower. Mulching grass clippings chops them up finely enough that they cause no harm to the lawn. But regardless of the type of mower that you own, you should not be mowing when the grass is wet (if for no other reason than because it is dangerous to mow wet grass).

The advice above specified that leaving clippings on the lawn is all right, under the right conditions, provided that the grass in question is healthy. But if you have patches of diseased grass on your lawn, you should bag the clippings and dispose of them. Otherwise, you risk letting the disease spread to other areas of the lawn.