Over time, lawns suffer a build up dead or decaying organic matter - such as lawn clippings - which is know as thatch. If this is allowed to keep accumulating without any sort of removal, this layer of thatch will start to cause problems and adversely affect the health of your lawn.
Lawn Scarification (or dethatching) involves removing a portion of this build up from within a lawn. Depending on the size of lawn to be worked, this can either be achieved using a spring bok rake or a motorized lawn scarifier - some cylinder lawn mowers have optional scarifying cassettes which are interchangeable with the cutting blades. Both of these methods penetrate into this layer below the grass and remove the organic matter that contributes to the thatch layerline at the base of the sward and help to prevent lateral growth, with the added benefit of removing the seed heads of undesirable grasses such as annual meadow grass.
Deep scarification cuts through the grass roots resulting in tillering. Tillering is the grass thickening by increasing its rootzone by sending out new shoots. Regular root pruning such as this promotes a thicker lawn, inhibiting weed and moss growth.
Along with other lawn care practices such as aerating and feeding, scarifying will lead to a healthier lawn. It is usually performed in spring following moss treatment in order to remove the moss or in the autumn as part of your annual lawn care, before top dressing or overseeding. Overseeding can take place afterwards to help fill in any thin or bare areas.