Kentucky bluegrass (\(Poa\) \(pratensis\)), often abbreviated as KBG, is a cool-season turfgrass highly prized for its rich, dark-green color and its ability to form a dense, resilient sod layer. This desirable quality is due to its aggressive growth habit, primarily spreading through horizontal underground stems known as rhizomes. However, this same trait makes it a persistent and invasive weed when it infiltrates other turf varieties, such as tall fescue or perennial ryegrass, or when it invades garden beds. The extensive network of rhizomes allows it to aggressively outcompete other plants, forming thick patches. Successfully removing the unwanted grass requires targeting this tenacious root system.
Non-Chemical Eradication Methods
For smaller areas or for those who prefer to avoid herbicides, physical removal methods focus on depriving the plant of resources. Manual removal is only practical for very small, isolated patches of Kentucky bluegrass, as the entire root structure must be carefully extracted to prevent regrowth. Any fragments of the rhizomes left behind can sprout new plants, making this process highly labor-intensive.
A more effective non-chemical approach for larger areas is soil solarization, which uses the sun’s heat to sterilize the soil. This process involves mowing the area as low as possible, thoroughly watering the soil, and then covering it with clear plastic sheeting. The plastic traps solar radiation, raising the soil temperature to lethal levels, often exceeding 140 degrees Fahrenheit in the top layer, which is sufficient to kill KBG plants and many weed seeds. For the best results in hot weather, this treatment must be maintained for four to six weeks.
Alternatively, a technique known as smothering can be used by covering the infested area with thick black plastic or several layers of cardboard to completely block sunlight. This method starves the Kentucky bluegrass by preventing photosynthesis, but it requires a much longer time commitment than solarization. Depending on the climate and the thickness of the material used, smothering can take several months to ensure the complete decay of the rhizome system.
Chemical Control Strategies
For widespread or deeply entrenched Kentucky bluegrass invasions, chemical control is often the most reliable method for achieving complete eradication. The most common and effective option is a non-selective herbicide, such as those containing glyphosate, which kills all vegetation it contacts. Because KBG spreads through rhizomes, a single application may not be sufficient to kill the entire root system, necessitating a second application approximately two to four weeks after the first treatment.
The optimal timing for applying non-selective herbicides against Kentucky bluegrass is during the spring when the plant is actively growing and rapidly translocating nutrients down to its roots. Research indicates that spring treatments are generally more effective for KBG control compared to summer or fall applications. Following the application, the dead grass will begin to yellow within a few days, but complete death of the plant can take up to two weeks.
The use of selective herbicides to remove KBG from a desirable cool-season lawn is significantly limited and often impractical. Since Kentucky bluegrass is itself a cool-season grass, most selective herbicides designed to kill grassy weeds in a lawn will also severely damage or kill the desirable turf. Always follow all label instructions precisely regarding application rates, weather conditions, and personal protective equipment (PPE) to ensure safety and effectiveness.
Post-Treatment and Preventing Regrowth
Once the Kentucky bluegrass has been confirmed dead, the area must be prepared for replanting. Begin by thoroughly raking out all the dead plant material and thatch, which is essential for maximizing seed-to-soil contact for new plantings. If the soil is heavily compacted or contains large clumps, a light tilling or raking to loosen the top layer will create a more hospitable seedbed.
The ideal time to reseed the area is in the early fall, as cooling temperatures and increased moisture provide the best conditions for germination and root development. Seeding in the fall allows the new grass to establish a robust root system before the stress of the following summer’s heat. If a non-selective herbicide was used, check the product label for the specified waiting period, which can range from a few days to several weeks, before safely replanting.
Long-term prevention of Kentucky bluegrass regrowth involves maintaining a thick, healthy lawn to naturally crowd out future invaders, along with strategic use of pre-emergent herbicides. Applying a pre-emergent herbicide in late summer or early fall will create a chemical barrier that stops any remaining KBG seeds from germinating. This proactive step is crucial, as Kentucky bluegrass seeds can remain viable in the soil for decades.