The appearance of dark green or black slimy patches across a lawn often prompts immediate concern from homeowners about the safety of their turf and family. This organism, commonly referred to as lawn algae, is a widespread biological issue that signals underlying problems with the lawn’s environment. This article will explore the organism’s identity, its impact on your yard, and the specific precautions necessary to protect both your turf and the people who use it.
What Lawn Algae Is and Why It Appears
The slimy, gelatinous material seen on lawns is not technically an alga but a type of cyanobacterium, often belonging to the genus Nostoc. These photosynthetic bacteria form colonies, appearing as rubbery, dark blue-green or black mats when wet. When dry, they shrivel into an inconspicuous black crust. Nostoc can survive long periods of desiccation, quickly rehydrating and expanding into its noticeable, slippery form after rainfall or irrigation.
This cyanobacterium thrives specifically in environments where turfgrass struggles to grow, acting as an indicator of poor cultural conditions rather than the primary cause of turf decline. The most common triggers are excessive soil moisture from poor drainage or overwatering, high humidity, and shade. Compacted soil also creates an ideal habitat because it prevents water from permeating, leaving the surface saturated for extended periods. High levels of phosphorus in the soil can also stimulate the growth and development of Nostoc colonies.
Assessing the Risks to Health and Turf
Physical and Structural Risks
The most immediate danger posed by a Nostoc infestation is the physical hazard it creates on walking surfaces. When the colonies are hydrated, the gelatinous mats are exceptionally slick, creating a significant slipping risk for people and pets on lawns, patios, and sidewalks. This slipperiness can lead to serious falls.
The direct impact on turfgrass is primarily structural, as the cyanobacterial mat forms a dense layer over the soil surface. This covering effectively smothers the underlying grass, blocking sunlight and preventing the necessary exchange of air and water into the root zone. If the mat dries out, it can form a hard, water-repellent crust that further inhibits water penetration, leading to localized turf decline and death.
Health Risks (Toxicity)
While the most common terrestrial species, Nostoc commune, is often characterized as harmless, it is part of a group of organisms known for producing potent cyanotoxins. Cyanobacteria, in general, are known to produce neurotoxins and hepatotoxins like microcystin, which can be extremely dangerous if ingested, particularly in aquatic environments. Terrestrial Nostoc species have been shown to produce microcystins and other bioactive compounds, though often in very low concentrations compared to aquatic blooms.
The potential for toxicity warrants caution, especially around children and pets who might inadvertently contact or consume the slimy material. Animals, particularly dogs, can be exposed by licking paws or grooming themselves after walking across the mats. Any ingestion should be avoided, and exposure should be treated with immediate rinsing of the affected area.
Effective Strategies for Removal and Prevention
The initial step for managing an outbreak is physical removal, which is easiest when the cyanobacterium is dry and crusty. Carefully raking or skimming the dried mats off the soil surface with a shovel can remove the bulk of the organism without damaging the underlying turf. For more persistent outbreaks, chemical treatments containing copper-ammonia complex or potassium salts of fatty acids prove effective against cyanobacteria.
Long-term management focuses on changing the underlying environmental conditions that allowed the Nostoc to establish itself. Since excessive moisture is the primary driver, improving soil drainage is paramount. Core aeration is an effective practice that reduces soil compaction, allowing water to percolate deeper and improving air circulation at the surface. Reducing the frequency of irrigation, especially in shaded or low-lying areas, will help dry out the soil surface. Increasing light penetration by pruning low-hanging tree branches or thinning dense shrubs will also make the environment less favorable.