Yellowing in turfgrass, known as chlorosis, is a symptom indicating that the grass is not producing enough chlorophyll. Chlorophyll is the pigment responsible for capturing sunlight and giving grass its healthy green color. Diagnosing the specific cause requires careful observation of the yellowing pattern, as the issue could be related to soil chemistry, maintenance routines, or the presence of biological agents.
Nutrient Deficiencies
The most common reason for widespread yellowing is a shortage of essential elements required for chlorophyll synthesis. Nitrogen (N) deficiency often presents as a uniform, pale yellow-green color across the entire lawn. Because nitrogen is a mobile nutrient, the plant relocates it from older tissue to new growth, causing the oldest, lower leaves to lose their color first.
Iron (Fe) deficiency, known as iron chlorosis, looks distinctly different and is often mistaken for a nitrogen shortage. Iron is immobile within the plant, so symptoms appear first and most severely on the youngest leaves at the top of the plant. The grass blades will show interveinal chlorosis, where the tissue between the veins turns pale yellow or white while the veins themselves remain a darker green. This deficiency is common in alkaline soils (pH above 7.0), which makes the iron chemically unavailable for the grass roots to absorb.
Water Stress and Soil Conditions
The amount of water available to the turfgrass roots directly influences its health and color, and both too little and too much water can lead to chlorosis. In cases of drought or underwatering, the grass attempts to conserve moisture, and the blades may first turn a dull, bluish-gray before progressing to yellow and eventually brown. A simple test is to walk across the lawn; an underwatered lawn will retain visible footprints for an extended period because the grass blades lack the turgor pressure to spring back quickly.
Conversely, overwatering causes yellowing because it saturates the soil, displacing the air pockets necessary for root respiration. This lack of oxygen essentially drowns the roots, preventing them from properly taking up nutrients and water, which mimics drought stress. Soil compaction exacerbates both problems by hindering water infiltration and air exchange. Compacted soil restricts the growth of a deep, healthy root system and makes the grass more susceptible to both drought and waterlogging.
Damage from Pests and Fungal Diseases
Biological stressors, including insects and fungal pathogens, can cause yellowing in specific, often patterned, areas of the lawn. Grubs, the larvae of beetles, feed directly on the turfgrass roots, causing the grass to yellow, wilt, and die in irregular patches. A lawn with a severe grub infestation can often be rolled back like a piece of carpet because the root system is destroyed.
Chinch bugs use piercing-sucking mouthparts to inject a toxin into the grass blade while feeding, which blocks the flow of nutrients and water. This damage typically appears as irregularly shaped yellow or brown patches. Fungal diseases also frequently manifest with yellow or discolored patches. Dollar spot creates distinct, small, circular patches of yellow or straw-colored grass that can merge to form larger areas. Rust disease causes the grass blades to turn yellow or reddish-orange, and a closer look reveals powdery, rust-colored spores that easily rub off onto shoes or clothing.
Chemical and Application Errors
Yellowing can be the direct result of incorrectly applied chemicals, a condition known as phytotoxicity. Fertilizer burn occurs when excessive amounts of nitrogen fertilizer, particularly those with high salt content, are applied to the lawn. The high salt concentration draws moisture out of the grass cells, causing rapid dehydration and resulting in bright yellow or brown streaks and patches that follow the pattern of the spreader.
Herbicide misapplication is another frequent cause of localized yellowing. Non-selective herbicides, designed to kill all plant life, will cause rapid yellowing and death if they drift onto the turfgrass. Even selective herbicides can cause temporary yellowing, or chlorosis, if applied in excess or when temperatures are too high. Accidental spills of household chemicals, such as gasoline, oil, or cleaning agents, lead to immediate, severe yellowing and necrosis in a very defined, localized spot.