Zoysia grass is a popular warm-season turf prized for its dense growth habit and resilience in high-heat conditions. This grass forms a thick, carpet-like lawn that withstands significant traffic once established. Despite its durability, Zoysia is not immune to chlorosis, the technical term for yellowing leaf tissue. Understanding the specific causes of this color change is the first step toward restoring the lawn’s characteristic emerald green appearance.
Identifying Nutrient Deficiencies
The most common nutritional cause of Zoysia turning a pale, uniform yellow is a lack of nitrogen (N), a mobile nutrient. When nitrogen levels are insufficient, the grass reallocates this element from older leaf tissue to support new growth. Consequently, older blades of grass typically show yellowing first, while the newest growth may temporarily retain some color.
Maintaining appropriate nitrogen levels is complex. Too little nitrogen can promote fungal diseases like rust and dollar spot, while too much can encourage brown patch fungus. Nitrogen is a foundational component of chlorophyll, the pigment responsible for capturing light energy. Correcting this deficiency usually involves applying a balanced fertilizer during the growing season.
Interveinal chlorosis is a distinct form of discoloration where the veins remain green while the tissue between them turns yellow. This symptom often indicates a micronutrient deficiency, specifically iron (Fe) or manganese (Mn). Iron is necessary for chlorophyll production, and its absence directly impacts the grass’s green hue.
Unlike nitrogen, iron is an immobile nutrient. Consequently, the symptoms of iron deficiency appear first and most severely on the newest grass blades toward the top of the plant. This distinct pattern helps differentiate iron deficiency from a general nitrogen shortage.
High (alkaline) soil pH strongly influences the availability of these micronutrients. Alkaline levels can chemically bind iron and manganese, making them insoluble and unavailable for root uptake. A soil test provides the most accurate data on pH and nutrient levels, guiding the application of an iron supplement or a soil acidifier.
Potassium (K) deficiency can also manifest as yellowing, typically observed as yellowing of the older leaves followed by tip dieback and scorching along the leaf margins. Zoysia benefits from a higher potassium ratio compared to other turf types, as this nutrient enhances its stress tolerance to heat and drought.
Hydration Stress and Soil Structure
Improper water management creates significant stress on Zoysia turf, leading to yellowing from two opposing extremes: drought and saturation. When the grass receives insufficient water, it enters a state of drought stress where cells desiccate and chlorophyll production halts. If the stress is prolonged, Zoysia may enter a temporary, protective dormancy, turning a uniform tan or brown to conserve resources.
Conversely, persistent overwatering or poor drainage causes the soil to become saturated, depriving the roots of oxygen for respiration. This anoxic condition suffocates the root system, leading to root rot. Root rot manifests as uniform yellowing or browning since damaged roots can no longer absorb water or nutrients. Zoysia is intolerant of standing water, making proper drainage paramount.
The underlying cause of poor drainage is often soil compaction, where the soil particles are pressed tightly together, reducing the pore space necessary for air and water movement. Compacted soil restricts the growth of deep, healthy roots, making the grass more susceptible to both drought and disease. Compaction also prevents the natural exchange of gases, trapping carbon dioxide and limiting oxygen availability to the roots.
Addressing this structural issue involves core aeration, a process that removes small plugs of soil from the lawn. This action immediately fractures the compacted layer, improving the infiltration rate of water and allowing oxygen to reach the root zone. Aeration is most effective when performed during the peak growing season, typically late spring to early summer for Zoysia.
Proper watering technique involves applying approximately one inch of water per week, delivered infrequently but deeply, rather than short, daily cycles. Deep, infrequent watering encourages the Zoysia roots to grow deeper into the soil profile, increasing the turf’s tolerance to heat and dry periods.
Pests and Fungal Diseases
Biological threats can cause localized patches of yellowing that often resemble drought stress or nutrient deficiencies. Among insects, the chinch bug is a common offender, using its piercing-sucking mouthpart to extract internal fluids from the grass blades. They also inject a toxic saliva that disrupts the plant’s vascular system, resulting in spreading patches of straw-colored turf in sunny, hot areas.
Another subsurface pest is the hunting billbug, whose larvae are legless grubs that bore into and feed on the lower stems and rhizomes. This internal damage hollows out the stem, severing the connection between the leaves and the roots. The resulting yellowing patches often pull up easily from the soil, distinguishing them from other causes.
Fungal pathogens also cause discoloration, with Zoysia rust being a visible example. Caused by the Puccinia fungus, this disease manifests as small, orange-yellow pustules that rupture the leaf surface. Rust is often linked to low nitrogen fertility or prolonged leaf wetness.
Dollar spot is another common Zoysia fungus, characterized by straw-colored spots on the blades that are about the size of a silver dollar. This disease is frequently associated with excessive thatch and low nitrogen levels, indicating a strong link between maintenance practices and disease susceptibility. Timely identification of these specific symptoms is necessary, as treatments for insects and fungi differ greatly.
Environmental and Chemical Stressors
External factors unrelated to maintenance practices can also trigger yellowing, the most significant being seasonal dormancy. Zoysia naturally ceases active growth and turns a uniform tan color when soil temperatures consistently drop below 55°F. This uniform change is a protective mechanism, not a sign of death or disease.
Differentiating this natural dormancy from other issues involves simple observation: dormant grass is firm and resists being pulled up, while dead grass is shriveled and easily uprooted. The grass will naturally green up when soil temperatures return to approximately 65°F in the spring.
Chemical exposure is a non-biological stressor that causes localized discoloration. Accidental overspray from non-selective herbicides can result in bleached, white, or yellow patches with distinct, sharp boundaries. Similarly, an overapplication of quick-release synthetic fertilizer can cause fertilizer burn, where high salt concentrations pull water out of the grass blades, leading to a scorched, yellow-brown appearance.