Overseeding is a common practice in the low-elevation desert regions of Arizona to maintain a green lawn through the cooler months. The primary warm-season turfgrass, Bermudagrass, becomes dormant when soil temperatures fall, causing the lawn to turn a straw-like tan or brown color. Overseeding involves planting a cool-season grass, typically ryegrass, directly into the existing turf before the Bermudagrass goes fully dormant. This process ensures a consistently green and aesthetically pleasing lawn surface from late fall through the spring.
Determining the Right Seeding Window
The optimal window for overseeding is when the nighttime air temperatures consistently fall below 65°F. In the Phoenix and Tucson areas, this window opens in late September and extends through the first two weeks of October. Seeding during this time ensures the soil remains warm enough for the cool-season grass to germinate quickly while the warm-season Bermudagrass is slowing its growth.
Seeding too early in September allows the still-active Bermudagrass to aggressively compete with the new seedlings for water and nutrients, leading to a thin and patchy winter lawn. Conversely, seeding too late in November risks exposing seedlings to potential frost damage, which can stunt growth and hinder establishment. Targeting the period when nighttime temperatures are consistently in the low 60s and daytime highs are around 80–85°F provides the ideal environment for rapid establishment.
Preparing the Lawn and Seeding Application
Preparing the existing warm-season lawn ensures the new ryegrass seed makes contact with the soil. Preparation begins by stopping nitrogen fertilization of the Bermudagrass four to six weeks before the planned overseeding date. The existing turf must be “scalped,” meaning it is mowed down to a very low height, generally between one-quarter and one-half inch, to remove the majority of the green leaf tissue. This scalping process reduces competition and exposes the soil surface, which is necessary for germination.
Dethatching or vertical mowing may be needed to lightly loosen the thatch layer and promote direct seed-to-soil contact. Once the lawn is scalped, all clippings and debris must be removed to prevent smothering the newly sown seed. Perennial ryegrass is the standard choice for Arizona winter grass, offering a dark green color, finer texture, and better overall quality compared to annual ryegrass.
The seeding rate for a typical home lawn should be between 12 and 15 pounds of perennial ryegrass seed per 1,000 square feet. To achieve uniform coverage, the seed should be applied using a broadcast spreader.
- Split the total amount into two equal portions.
- Spread the first portion by walking in one direction.
- Apply the second portion by walking in a perpendicular direction across the lawn.
- Lightly rake the seed or cover it with a thin layer of top-dressing material, such as compost or mulch, to retain moisture and protect it from wind or birds.
Initial Care for Successful Germination
The first 7 to 14 days following seeding require an intensive watering schedule to ensure successful germination. During this period, the goal is to keep the top inch of the soil consistently moist without creating puddles or runoff. This is achieved by watering in short, frequent cycles, three to five times per day, with each cycle lasting only five to ten minutes. The frequent, shallow watering keeps the seedbed hydrated, which is necessary for the seed coat to break and the sprout to emerge.
Before the initial watering begins, apply a starter fertilizer high in phosphorus, such as a 6-20-20 or 15-15-15 blend, to provide immediate nutrients for the new roots. Once the seedlings emerge, the watering frequency must be gradually reduced to encourage deeper root growth. The schedule can be decreased to two or three short cycles per day for the next two weeks, then eventually reduced to one longer, deeper watering session per day as the grass establishes.
Delay the first mowing until the new ryegrass reaches a height of approximately three inches. Mow the lawn with the blade set high, cutting the grass down to about two to two and a half inches. Remove no more than one-third of the grass blade height at any single mowing to prevent stressing the young plants. After the first or second cut, the lawn can transition to a regular winter maintenance schedule, including weekly mowing and deeper, less frequent irrigation.