Kentucky Bluegrass (Poa pratensis) is a perennial cool-season grass widely recognized for its dense, dark green appearance and durability in northern lawns. However, its vigorous growth habit makes it an aggressive weed when it invades flower beds or spreads into other turf types like warm-season lawns or fine fescue. The robust underground structure and physiological similarity to other cool-season grasses make it challenging to eliminate using selective chemical controls.
Confirming Kentucky Bluegrass Identification
Before starting any removal process, confirm the grass identity to ensure effective treatment. Kentucky Bluegrass blades have a unique boat-shaped tip and the young leaves emerge folded in half (folded vernation).
The blades are smooth on the upper surface and feature a prominent midrib or keel. Two clear, parallel lines are often visible alongside the keel. The most significant feature related to its aggressive spread is the presence of rhizomes, which are underground stems that allow the plant to spread horizontally and form a dense sod.
These rhizomes typically penetrate the soil two to four inches deep, allowing the plant to recover quickly after above-ground damage. This extensive network differentiates Kentucky Bluegrass from bunch-type grasses and makes complete eradication difficult.
Non-Chemical Removal Techniques
Manual Removal
For small patches or garden beds, physical removal is an effective non-chemical method, requiring meticulous effort to prevent regrowth. Manual digging must be thorough and extend deep enough to remove the entire network of rhizomes, as fragments can regenerate new plants. Using a shovel or specialized plugger tool helps ensure the deep-seated rhizomes are fully extracted.
Solarization
Solarization uses intense heat to eliminate the grass and its underground structures. This involves cutting the grass short and covering the area with a clear plastic sheet, which acts like a greenhouse to raise the soil temperature. The process typically takes one to two months during the warmest part of the year to kill the grass completely.
Smothering
For larger, non-turf areas, smothering is a viable option that avoids herbicides. This technique involves laying down materials like cardboard or several layers of thick newspaper directly over the grass. The covering should then be topped with a layer of organic mulch, such as wood chips, to block sunlight. This method starves the grass of light and requires several months before the area can be replanted.
Selective and Non-Selective Chemical Treatments
For widespread or deeply established infestations, chemical control is often the most practical option. The most certain method for complete eradication is using a non-selective herbicide, such as one containing glyphosate, which kills all plant tissue it contacts. This product is appropriate for spot-treating isolated patches or clearing an area completely before reseeding or planting.
When using a non-selective product, it is important to apply it directly to the foliage, ensuring a thorough coating without excessive runoff. Due to the robust rhizome system, a single application is often insufficient, and follow-up treatments may be necessary after a few weeks to eliminate regrowth from underground stems. After the grass is visibly dead, the area requires reseeding with the desired turfgrass, following the label instructions regarding waiting periods.
Selective herbicides are significantly more challenging to use when removing Kentucky Bluegrass from a lawn of another cool-season turfgrass, like tall fescue, due to their physiological similarities. Few products are labeled for this specific purpose, and those that exist, such as certain sulfonylurea herbicides, can injure the desirable grass if applied incorrectly. Treating a mixed stand often necessitates a full renovation, involving killing the entire area with a non-selective herbicide and then reseeding.
Preventing Regrowth and Future Infestations
A healthy, dense stand of turf is the best defense against the re-establishment of weed grasses. Maintaining a proper mowing height is an effective cultural practice; keeping blades between 2.5 and 3.5 inches encourages deeper root growth and helps the desirable turf shade out the soil. Taller grass blades reduce the amount of sunlight reaching the soil surface, making it difficult for new Kentucky Bluegrass seeds to germinate and establish themselves.
Proper watering techniques also play a significant role in long-term prevention. Lawns should be watered deeply and infrequently, aiming for about one inch of water per week, which encourages the development of deep, drought-tolerant roots in the desirable grass. Watering in the early morning is recommended to allow foliage to dry quickly, minimizing disease risk.
Overseeding the lawn with the desired turf variety, such as tall fescue or perennial ryegrass, increases the overall density of the stand, crowding out space for new invaders. Pre-emergent herbicides can also be used to prevent new seeds from germinating, particularly in early spring when soil temperatures reach about 55 degrees Fahrenheit. This creates a chemical barrier that prevents the establishment of new plants from seed.