“Winter grass” in Arizona’s low desert is an intentional landscaping practice called overseeding. This involves planting a cool-season turf, typically ryegrass, over a dormant warm-season base grass. The purpose is to maintain a vibrant, green lawn during the cooler months when the summer turf has turned brown. Understanding when this temporary grass dies off requires focusing on environmental triggers, not a specific date. The natural decline of the winter lawn is a critical part of the annual cycle, preparing the ground for the return of the summer-dominant grass.
Identifying Winter Grass in Arizona Lawns
The grass providing a lush, green appearance during an Arizona winter is most often annual or perennial ryegrass. This cool-season variety thrives in the mild desert winter temperatures. Ryegrass germinates quickly in the fall, establishing itself over the dormant summer turf before the coldest part of the season.
Perennial ryegrass is a common choice, favored for its darker green color and finer blade texture. The ryegrass grows as a temporary layer above the established warm-season turf, which is usually Bermuda grass. While the Bermuda grass is dormant, the ryegrass maintains a green color, providing continuous turf cover until the heat returns.
The Critical Soil Temperature Threshold
The death of winter grass is not determined by a set date but by the sustained rise in soil temperature. Ryegrass is a cool-season grass, meaning it cannot tolerate the intense heat that begins to build in late spring. The precise moment of decline is triggered when the soil temperature, measured at a depth of four inches, consistently reaches and exceeds the range of 65°F to 70°F.
Once this thermal threshold is crossed, the root system of the ryegrass experiences significant heat stress. The grass rapidly enters a state of decline, struggling to take up water and nutrients in the increasingly hot soil. In the Phoenix and Tucson areas, this temperature-driven decline typically begins in late April and becomes noticeable throughout May. A warmer-than-average spring can accelerate this process, causing the die-off to begin earlier.
The sustained high temperatures are lethal to the cool-season grass structure. This environmental failure mechanism forces the ryegrass to retreat as the soil warms up. Soil temperature is the most reliable indicator for the start of the die-off, as the timing is variable year-to-year.
Facilitating the Transition Process
Homeowners often take specific actions to manage the transition, ensuring the dying winter grass does not hinder the returning summer turf. This managed die-off process is sometimes referred to as “scalping” or “power-raking.” The goal is to weaken the ryegrass canopy while simultaneously encouraging the Bermuda grass underneath to break dormancy.
Water Reduction
One of the first steps involves dramatically reducing the amount of water applied to the lawn, which starves the cool-season ryegrass. The reduced water stress speeds up the ryegrass’s natural decline, which is already triggered by the rising soil temperatures. This technique also avoids over-saturating the soil, which can harm the Bermuda grass as it begins to wake up.
Fertilization Strategy
A change in fertilization strategy is also implemented to facilitate the transition. Homeowners switch from the balanced fertilizers used in winter to high-nitrogen formulas, which are beneficial for the emerging Bermuda grass. The nitrogen boost helps the warm-season turf green up and spread, while the high concentration stresses the already weakening ryegrass.
Scalping and Mowing
Finally, the mowing height is lowered in a process called scalping. This involves gradually decreasing the blade setting to cut the ryegrass down to a very short height, often less than an inch. This action removes the canopy of the ryegrass, exposing the dormant Bermuda grass to the maximum amount of sunlight and warmth. Removing the ryegrass cover is necessary to allow the Bermuda grass to absorb the energy needed to begin its active growth phase.
Warm-Season Turf Activation
The completion of the ryegrass die-off signals the full activation of the warm-season turf base, usually Bermuda grass. The death of the cool-season grass removes competition for sunlight, water, and soil nutrients. This removal provides an ideal environment for the underlying Bermuda grass to thrive. As soil temperatures remain high, the Bermuda grass breaks dormancy and begins to spread aggressively. Active growth typically starts in late May and continues through the summer, ensuring the lawn remains dense and green throughout the intense Arizona summer.