Turfmate field notes · Lawn problems

Algae in lawns and turf

Cyanobacteria, including Nostoc species, and other turf-colonising algae.

Dark, slimy patches can form when algae or cyanobacteria colonise wet, thin turf. A black film may spread across the surface during prolonged damp weather, affecting drainage and turf quality. Start by investigating persistent moisture and poor grass cover before choosing a treatment.

Dark algae and cyanobacterial scum across the surface of a golf green.
Dark algae and cyanobacterial scum across the surface of a golf green.
Photo: Syngenta GreenCast · Full photo ↗

What to look for

  • Black or dark green, splotchy surface growth, particularly noticeable when the turf is wet.
  • A slimy film or gelatinous material sitting on the soil or thatch between grass plants.
  • Recurring patches in low spots, shade, compacted ground or areas that remain wet after rain.

Check the grass and site

Can occur in any turf type where damp, exposed surfaces allow growth. Closely mown greens and thin, worn lawn areas are useful places to inspect.

Confirm it before you treat it

Look at the material close up and compare the affected area with nearby healthy turf. Record where water collects and how long the surface stays damp. If the growth is unfamiliar, photograph it before disturbing it and ask for identification.

What to do next

  1. Review irrigation and clear obvious surface-drainage obstructions so water does not keep collecting in the same place.
  2. Address compaction and excessive thatch when the grass can recover, using a suitable aeration programme.
  3. Improve light, airflow and grass cover where practical; adjust nutrition to the turf’s actual needs.
Seasonal watch

Conditions matter more than dates.

Persistent surface moisture, shade, limited airflow and weak grass cover favour colonisation. Extended rainy, cloudy weather can also produce problems on otherwise open sites. Surface scum may then further restrict water movement and soil aeration.

Does black surface growth mean the lawn needs more fertiliser?

Not necessarily. First check moisture, drainage and why the grass has thinned. Feeding alone will not correct a persistently wet surface; use soil information to guide any nutrition changes.

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Reviewed 18 October 2026 · Turfmate. Identification guidance is not a laboratory diagnosis or a spray calendar.

Technical sources & image credits

Syngenta photographs are reproduced in full with attribution. Source grass species and settings are identified where supplied; photographs support identification and do not replace diagnosis.

A Turfmate product link does not imply endorsement by a photographer or source.