The GARDENA gardening expert
Dear Mr. P., what you describe as “mould” is in fact a fungal infection on the lawn, caused by the fungi Michrodochium nivale or Gerlachia nivalis. This type of fungi is sometimes called “snow mould”. It generally grows out as the lawn starts to grow again in spring, just as you described in the previous year.
It is correct to scarify the affected areas, and to hack away the infested pieces of lawn. You can re-sow the affected areas with the same lawn grass seeds to help the bald patches grow back better again. Ensure that you sow the appropriate seed mixture on ornamental lawns and sports turf, otherwise your lawn will look patchy!
You can combat Gerlachia nivalis and Michrodochium nivale in the same way as you would combat moss. We recommend an iron sulphate preparation, but in the fight against your fungi, you should use a dose of only 5 grammes per litre (50 grammes to a 10-litre watering can).
Michrodochium nivale or Gerlachia nivalis is a so-called secondary parasite. In future, you can prevent it by providing your lawn with a strengthening autumn fertiliser, but without overdoing things, and above all preferably containing slow-flowing forms of nitrogen (amide nitrogen, or at least ammonium nitrogen). The packaging will have “nitrogen slow-release fertiliser” written on it.
Finally, the following applies: Do not trim the lawn too short ready for winter – it doesn't have to be three centimetres high; four to five centimetres are sufficient. In this way, the stalks receive more light in this darker season, and their metabolism is strengthened – also against fungi.