Posted: 21/03/2017 at 14:40
Yep. Moss and thatch. You need to raise the cutting level on you mower as grass which is cut too short simply hasn't enough lead area to feed its roots and compete with weeds and moss.
Unless poor drainage is a serious issue (flooding n winter?) you can improve the lawn thus -
Take a good, long pronged gardening fork and use it to make vertical holes at frequent intervals working all along and across the grass. Stand on it so the tines go in deep and then wiggle it back and forth to enlarge the holes. Then you need to pour on bags of sharp sand - not wet builders' sand - and brush it across the lawn to fill those holes. This will provide air to the roots and allow better drainage.
By the time you've done that it should be mid April which is a perfect time to apply a spring weed, feed and moss treatment. Follow the instructions on the pack and be prepared for parts of your lawn to turn black after a week or so. Buy or hire a scarifier and use this to rake up all the dead stuff and any loose stuff from previous cuttings (thatch). Alternatively, rake it up with a spring tined fork - see Wolf tool heads.
Loosen any bare earth with a normal rake head then sow appropriate grass seed to fill the gaps. Water with a sprinkler or gentle hose pipe spray if it doesn't rain. Do not walk on the newly sown areas.
Leave it to grow to at least 2 inches before its first cut and then never cut it shorter than one inch as this allows enough leaf to feed and maintain healthy roots and thus compete better for nutrients against any weeds.
Apply an autumn weed and feed in September
Repeat as necessary over the coming years.