Lichens appear as brown or grey flat structures that grow horizontally in the turf.  They can be problematic on lawns because, as well as affecting the appearance, they block light from reaching the grass preventing growth and can make the surface slippery.

Lichens occur in damp often more shady areas of the garden, where drainage is poor and the soil compacted.

Treating them with chemicals is difficult as nothing works that well. The most sensible approach is to address the cause of the problem through non chemical controls.  If the surface layer is poorly drained aerate to a depth of 7.5-15cm (3-6in) by spiking with a garden fork every 10-15cm (4-6in). Where the lawn is only compacted or waterlogged in patches, aerate during wet conditions every four to five weeks. On heavy soils a hollow-tine aerator will give better results

After aerating, top dress with sharp sand, working it into the holes to improve drainage.  Avoid walking on the lawn when it is waterlogged as this will only aggravate compaction.

Provide good all-season care, especially in spring and summer and most importantly in autumn.

In the worst cases, if your lawn is waterlogged for much of the year, consider installing artificial drainage.