When a lawn develops patches of growth that are significantly darker and lusher than the surrounding grass, it indicates highly localized, accelerated growth. This discoloration is not typically a symptom of disease, but rather an indication that a specific area is receiving a disproportionate supply of a growth stimulant. The visual difference occurs because the grass blades in these spots contain a higher concentration of chlorophyll, the pigment responsible for the deep green color and photosynthesis. Understanding the source of this localized stimulation is the first step in diagnosing and correcting the issue.
Patches Caused by Excess Nitrogen
The most frequent chemical explanation for a sudden, deep green patch is an uneven application of nitrogen, the nutrient responsible for vegetative growth and color in turf. Nitrogen is a component in all lawn fertilizers, and over-application in a small area acts like a concentrated growth booster. This can happen accidentally, such as when granular fertilizer spills while being loaded into a spreader or when a liquid application is sprayed too heavily. The excess nutrient causes the grass in that small area to grow rapidly and develop a darker hue almost overnight.
These patches often appear random, sometimes clustered near storage areas or walkways. Another cause of localized nitrogen spikes is the decomposition of buried organic material, which releases high concentrations of nitrogen into the soil as it breaks down. Subsurface items like old tree stumps or construction debris will slowly enrich the soil directly above them. This steady release of nutrients results in a persistent, dark green patch that may last for several seasons, unlike a fertilizer spill which is a single event.
Patches Caused by Specific Grass and Weed Types
Sometimes, darker patches result from a simple difference in plant species or variety, rather than a chemical spike. Certain types of turfgrass and common weeds naturally possess a darker green color or a different growth habit than the main lawn variety. A common example is a rogue clump of Tall Fescue, which is often substantially darker green and grows much faster than finer-bladed varieties like Kentucky Bluegrass. These patches are a genetic difference, meaning they will not respond to changes in the overall lawn’s nutrient levels.
Weeds that are nitrogen-fixing, such as Clover, can also create a patch of darker, healthier growth. Clover has a symbiotic relationship with specific bacteria in its root nodules, allowing it to pull gaseous nitrogen from the atmosphere and convert it into a usable form. This self-fertilization makes the clover appear lush and slightly enriches the surrounding soil. Unlike fertilizer spills, these biological patches are typically permanent and maintain a fixed size and shape unless actively removed or treated.
Localized Nutrient Spikes from Pet Waste
Pet urine is a common culprit behind a specific type of dark green patch, and the mechanism is distinct from general fertilizer over-application. Dog urine is high in nitrogen-containing compounds, specifically urea and other nitrogenous salts, which are byproducts of protein digestion. When deposited in high concentration, the salts cause osmotic stress that draws moisture out of the grass roots, resulting in a brown or dead spot at the center.
The edges of this concentrated spot receive a diluted dose of the nitrogen salts. This lower concentration acts as an effective, localized fertilizer, stimulating the grass to grow rapidly and produce more chlorophyll. The result is the formation of a distinct, dark green ring or halo of accelerated growth surrounding the central patch of dead grass. These spots are almost always circular or semi-circular and are found where a pet frequently relieves itself.
Repairing and Preventing Dark Green Patches
Addressing patches caused by excess nitrogen requires leaching the concentrated salts out of the root zone. This is accomplished by applying heavy, repeated applications of water to the affected area over several days. The goal is to flush the excess soluble nutrients deeper into the soil profile where the grass roots cannot readily access them, effectively diluting the concentration.
Repairing patches caused by a different grass type or weed variety requires a different approach, as nutrient adjustment will not solve a genetic difference. Clumps of undesirable grass, like Tall Fescue, must be physically dug out, ensuring the entire root ball is removed, or treated with a non-selective herbicide. Once the unwanted plant is gone, the soil should be amended and the area must be reseeded with the desired turf variety.
For pet waste spots, immediate action is the most effective prevention. Immediately flushing the area with water after the pet urinates will dilute the nitrogen concentration, minimizing the burning effect and the resulting dark green ring. To prevent future issues, ensure even fertilizer application across the lawn by using a calibrated spreader and carefully sweeping up any spills. Managing pet habits by designating a specific area for urination can also prevent the formation of these distinct patches.