The appearance of localized, dark green circles or arcs in a lawn is a common observation that causes confusion for property owners. These patches stand out because they exhibit a distinct color and growth rate compared to the surrounding turf.
This visual effect is almost always symptomatic of a concentrated increase in available nitrogen within the soil directly beneath the affected area.
While the symptom is consistent—lush, darker green grass—the underlying cause can be attributed to distinct biological or chemical origins, each requiring a different diagnostic approach. Understanding the source of this nutrient spike is the first step toward effective management.
Fungal Activity: The Fairy Ring Phenomenon
The most frequent biological cause for these dark green rings is a subterranean fungus, known commonly as a Fairy Ring. This is often categorized as a Type II Fairy Ring, characterized by stimulated, darker green grass growth. The fungi are typically basidiomycetes that live deep in the soil and feed on decaying organic matter such as old tree roots, buried lumber, or dense thatch layers.
The dark green color is a direct result of the fungal network, or mycelium, breaking down complex organic compounds. As the mycelium grows outward in a circular pattern, it secretes enzymes that decompose this matter. This process releases concentrated levels of nitrogen into the soil at the ring’s expanding edge. The grass roots absorb this nitrogen, leading to the rapid growth and deep, rich green color.
These rings can be quite variable, ranging in diameter from a few inches to many feet, and they typically expand slowly over time. In some cases, the fungus may also produce mushrooms or puffballs along the perimeter, which are the visible fruiting bodies. A more severe Type I Fairy Ring is also possible, where the dense mycelium creates a hydrophobic layer that repels water, causing the grass within the inner ring to turn brown and die.
Localized Nutrient Concentration
Not all dark green circles are caused by fungal growth; similar symptoms can arise from concentrated chemical or organic deposits in the soil. These non-biological causes also result in a localized nitrogen spike that triggers the same growth response. This category generally involves external application or internal decay of nitrogen-rich matter.
One common non-fungal source is the accidental spillage of synthetic granular or liquid fertilizer. If a bag is left sitting on the lawn, or if a spreader is stopped while open, the concentrated chemicals dissolve into the soil. This creates a sharply defined circular or irregular patch of intensely dark green grass. This high concentration of salts can sometimes lead to an initial burn of the grass blades, followed by the dark green flush if the grass survives the chemical shock.
Another non-fungal cause originates from subterranean decay of buried organic matter, distinct from the fungal mycelium’s outward growth. This often occurs when large pieces of debris, such as construction materials, old tree stumps, or massive root systems, are buried beneath the turf and begin to decompose. Decomposition releases a plume of nutrients, predominantly nitrogen, into the soil directly above the decaying mass. Unlike a Fairy Ring, the resulting dark green patch tends to be stationary and corresponds to the size of the buried object.
Distinguishing Between Causes
Identifying the precise cause requires careful inspection, as the management strategy differs significantly for biological versus chemical issues. The physical appearance of the affected area offers the first set of clues for differentiation. Fairy Rings are typically circular or semi-circular and expand slowly and uniformly outward, creating a crisp line between the stimulated and normal turf. A fertilizer spill, by contrast, often presents an irregularly shaped patch and may have an initial ring of dead or scorched grass due to chemical overload.
Soil inspection provides the most definitive evidence to confirm the presence of a Fairy Ring. Using a soil probe or a small spade to lift a section of turf from the edge of the dark green ring can reveal a dense, white, cottony mat of fungal mycelium in the thatch layer or upper soil profile. If the cause is subterranean debris, probing the soil may encounter solid wood or other decaying matter instead of a fungal mat.
Odor can sometimes be a final indicator, particularly in cases of severe chemical spills. A strong, pungent ammonia smell suggests a massive overdose of nitrogen-rich fertilizer. Conversely, a fungal ring may produce an earthy or musty aroma, or no distinct smell, with the occasional appearance of mushrooms confirming its biological origin. If a fertilizer spill is suspected, heavily watering the area immediately can help dilute the concentrated salts, an action that would have minimal effect on an established fungal network.