Bermuda grass is the dominant warm-season turf for Oklahoma lawns, but its natural dormancy leaves the landscape brown during the cool season. Homeowners often overseed this turf with a cool-season grass, typically annual or perennial ryegrass, to maintain a vibrant green color throughout the winter. This temporary cover provides aesthetic appeal and protects the soil from erosion while the Bermuda grass is inactive. Successful overseeding requires precise timing and preparation specific to Oklahoma’s transition zone climate.
Optimal Timing for Overseeding
The success of overseeding relies on planting the ryegrass seed when the soil temperature is in its narrow ideal range. Ryegrass seed germinates best when soil temperatures are consistently between 65°F and 70°F. This window typically opens in late September and extends through mid-October across Oklahoma, with southern areas sometimes allowing for a slightly later date.
It is crucial to monitor the five-day average soil temperature, rather than relying on a calendar date, to ensure the best results. Overseeding too early, when the soil is still warm, causes the aggressive Bermuda grass to outcompete the new ryegrass seedlings for light and water. Waiting too long risks a hard freeze that can kill tender, newly sprouted seedlings before they establish a strong root system.
Planting should occur when the Bermuda grass canopy is beginning to thin due to cooling temperatures, providing the necessary sunlight exposure for the ryegrass seed to reach the soil surface. This timing allows the ryegrass to establish itself while the Bermuda grass is slowing its growth cycle but is not yet fully dormant. Delaying the process past late October significantly reduces the likelihood of successful germination and establishment.
Essential Lawn Preparation Steps
Preparing the existing Bermuda grass turf is necessary to maximize seed-to-soil contact and reduce competition. The first step involves scalping the lawn, which means mowing the Bermuda grass significantly lower than its normal height, ideally to approximately one-half inch (0.5″) or less. This extreme cut removes the dense canopy, allowing sunlight to penetrate directly to the soil surface where the ryegrass seeds need to germinate.
After scalping, perform a light dethatching or aeration to further improve the seedbed. Removing excess thatch and creating small holes ensures the seed falls directly into contact with the mineral soil, boosting germination rates. This also helps with oxygen and water infiltration necessary for the ryegrass’s new root development.
Immediately before or after broadcasting the seed, apply a starter fertilizer that is formulated to encourage root growth. This fertilizer should have a higher concentration of phosphorus, such as a ratio like 16-20-0 or 18-24-12, to support the energy-intensive process of seedling establishment and promote a robust root system. Pre-emergent herbicides must not have been applied to the lawn for at least 6 to 8 weeks prior to overseeding, as these chemicals will prevent the ryegrass seed from germinating.
Post-Seeding Care and Spring Transition
Initial care after seeding focuses on maintaining continuous moisture in the top layer of the soil to facilitate germination. New seeds must be kept consistently damp, requiring light, frequent watering, typically three to five times per day for short intervals (5 to 15 minutes each) for the first 6 to 10 days. Once seedlings emerge, gradually reduce the watering frequency while increasing the duration to encourage deeper root growth.
The first mowing should occur when the ryegrass seedlings reach a height of approximately 2.5 to 3 inches. Only cut off the top one-third of the blade to avoid stressing the young grass. Throughout the winter, the established ryegrass benefits from a maintenance fertilization schedule using a high-nitrogen product, applied lightly (about 0.5 pounds of nitrogen per 1,000 square feet) every four to six weeks.
The overseeding cycle concludes with the spring transition, which removes the ryegrass so the Bermuda grass can emerge from dormancy without competition. This transition typically begins in late April or early May in Oklahoma, when the five-day average soil temperature consistently climbs above 60°F or 65°F. Cultural practices to aid this include lowering the mowing height to between 0.5 and 1 inch and reducing irrigation to stress the cool-season ryegrass.
For a quicker, more uniform transition, turf professionals utilize chemical methods with specific transitioning herbicides safe for Bermuda grass. Active ingredients such as Metsulfuron-methyl (MSM), trifloxysulfuron-sodium, or foramsulfuron are effective for selectively controlling the ryegrass. Timing the herbicide application correctly, just before or during the Bermuda grass’s green-up, ensures the warm-season turf has the best opportunity to recover and thrive.