University of Maryland Extension

Algae - Lawns

black crust on grass blades

Back to Common Problems - Lawns

In poorly drained or compacted areas, a black or greenish crust may form on bare soil in areas of thin turf. This black or greenish substance is algae growth, which can be more severe in compacted soils or shaded areas. Algae may have a slimy appearance when the soil is wet. It becomes crusty as the soil dries.

To control algae growth, eliminate or reduce the conditions that encourage it to flourish. Increase drainage by redirection of water from downspouts and aerate compacted areas. Add soil or grade to eliminate low areas from the lawn. Increase sunlight in shaded areas by removing overhanging trees or branches, if possible. Select shade tolerant grass varieties in areas where shade cannot be reduced. Fertilize and overseed according to University recommendations to establish a thicker stand of turf.

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