Funnily enough I received this e-mail from Rolawn today on this very subject. It's a few years now since I scarified my lawn but it is well worth the effort.
Stan
"Scarification is a very important operation. In spring (and autumn for that matter) scarification is best combined with other operations, like top dressing, seeding and applying fertiliser. However, where scarification is combined with other planned work, we must stress that it should be completed first. We have had discussions with lawn owners who have fertilised and top-dressed and then remembered that they should also scarify. If this happens, it will be better to leave any intensive scarification until the autumn. That said a little light scarification could be beneficial during good growing conditions in the summer, once the effect of the other spring operations have borne fruit.
Assuming that the work has been planned well and the lawn is scarified prior to any other spring operations (excluding mowing, which should continue on a regular basis both before and after scarification) effective scarification will improve the quality of the lawn. It will also improve the effectiveness of the other operations that follow. In this regard, if you have the energy, you do not have to wait to do any other planned work once the scarification is complete. Seeding and top dressing can follow straight away!
Scarification is carried out to remove organic matter from around the base of the grass plants and tidy up any straggly lateral growth. It removes material, probably in the form of thatch or moss that will otherwise prevent good dense grass growth. If you do not scarify, debris will build up and lead to other problems. Thatchy and mossy lawns will not be very wear or drought tolerant. Once the space has been created around the base of the grass plants, the trick is to encourage the grass to fill the space. There are two important points to make here. Firstly, do not scarify too early in the spring. It is important that the grass is growing fairly well and that any space created is filled by desirable lawn grasses. If growth is slow, there is a higher risk that the gaps will be filled with unwanted weeds or weed-grasses. Secondly, encourage grass growth after scarification. This is where other operations like fertilisation will be beneficial.
Finally, be careful not to over do it. Hand scarification is hard work. If you have a large lawn, it will be better to use a machine which are available for hire".
