Blue-green algae

If black or dark green-black spots appear on the lawn, they are very often caused by blue-green algae (cyanobacteria). These tend to appear mainly during the cooler months and are a clear sign that soil conditions are not optimal.

The most common causes are an unbalanced nutrient supply—particularly a potassium deficiency—as well as waterlogging and soil compaction. These factors create ideal conditions for the spread of algae.

Factors that promote algae growth

– Soil compaction
– Excess water, e.g., due to heavy rainfall, overwatering, or poor drainage
– Lawn thatch
– Mowing the lawn too short
– Areas with heavy shade
– General nutrient deficiency

What should you do if you have an active algae infestation?

If there is an existing algae infestation, we recommend a mechanical approach:

– Gently remove the layer of algae mechanically, for example, with a rake or shovel
– Then thoroughly roughen the affected surface
– Sand the area to improve the soil structure
– At the same time, apply a reseed to quickly fill in any gaps that have formed

Overseeding prevents weeds from taking root while the lawn regains its density.

Why Sand Is So Important

The sand makes the soil surface more porous. This improves air and water permeability, allowing excess moisture to dry out more quickly and enabling the grass to close the turf again. This permanently deprives the algae of their means of survival.