Rough bluegrass, scientifically known as Poa trivialis, is a troublesome perennial grass that often invades cool-season turfgrass lawns. This cool-season species actively grows during the milder temperatures of spring and fall. Poa trivialis is notorious for its bright, lime-green color, which creates an unsightly contrast against the darker green of desirable turf like Kentucky bluegrass or tall fescue. The plant spreads rapidly across the ground using above-ground runners called stolons, quickly forming dense, irregular patches. It is difficult to control because it is genetically similar to the turfgrass it invades and thrives in moist, shady conditions.
Recognizing the Target
Identifying Poa trivialis accurately is the first step before any treatment begins. The most noticeable characteristic is its distinct, vibrant yellow-green or lime-green color that stands out sharply against the rest of the lawn, especially during the spring green-up. Unlike the desirable turf, the leaves of this weed have a noticeably glossy underside.
A closer look reveals other key structural details that confirm its identity as rough bluegrass. The leaf tips of Poa trivialis are boat-shaped, a trait common to all Poa species. The plant spreads horizontally by producing stolons, which root at the nodes to form new plants, leading to its characteristic mat-like growth. It possesses a shallow, fibrous root system, which can make entire patches easy to lift from the soil.
This weed is often mistaken for a patch disease, particularly during the stresses of summer. When temperatures rise or drought conditions occur, Poa trivialis has a low tolerance and will enter dormancy, turning brown or reddish-purple. This dormancy creates dead-looking patches, but the plant’s stolons and stems remain viable underground, ready to sprout again when cooler, moist weather returns.
Manual and Cultural Removal Strategies
Before resorting to chemical applications, manual removal and adjustments to lawn care practices can effectively manage small infestations. For individual, scattered patches, physical removal is an immediate solution involving a hand trowel or shovel to dig out the entire patch of the weed.
It is essential to remove several inches of the surrounding desirable grass to ensure all the stolons and shallow roots are completely extracted. Missing even a small section can allow the patch to regenerate because the stolons can survive in the soil for a long time. Once the weed is removed, the resulting bare spot should be filled with quality topsoil in preparation for reseeding.
Altering the lawn’s environment to favor desirable turfgrass will discourage Poa trivialis from establishing new areas. Since rough bluegrass thrives in high-moisture conditions, adjusting irrigation practices is highly effective. Instead of frequent, shallow watering, switch to deep, infrequent watering, aiming for about one inch of water per week. This practice encourages the deeper root growth of desirable grasses, making the surface less hospitable to the shallow-rooted weed.
Additionally, increasing the mowing height to between three and four inches helps the desirable turf compete. Taller turfgrass blades shade the soil, which naturally suppresses the germination of weed seeds and makes it more difficult for the low-growing Poa trivialis to spread. Addressing poor drainage through aeration or minor grading corrections is also important, as wet, compacted soil creates the perfect environment for this weed to flourish.
Selective Herbicide Treatments
Chemical control is often necessary for established or widespread infestations, though it is challenging because Poa trivialis is a grass growing within a grass lawn. The most straightforward, non-selective method involves using a product containing glyphosate. Glyphosate is a broad-spectrum herbicide that kills nearly all vegetation it contacts, making it suitable for spot treatments or a full lawn renovation.
When spot-treating with glyphosate, careful application is paramount to minimize damage to the surrounding healthy lawn. Applying the product with a sponge applicator or a very targeted sprayer helps prevent overspray onto the desirable turf. Due to the perennial nature and robust stolons of rough bluegrass, multiple applications spaced 7 to 14 days apart may be necessary to ensure the complete death of the plant.
Newer selective options have emerged that can reliably kill Poa trivialis without damaging the surrounding cool-season turf. These include products containing the active ingredient bispyribac-sodium, often sold under the trade name Velocity PM. This post-emergent herbicide offers selective control in specific cool-season grasses like tall fescue and perennial ryegrass, though it requires caution on Kentucky bluegrass cultivars.
Selective herbicides are most effective when the Poa trivialis is actively growing, typically during the cooler temperatures of the spring and fall. A typical treatment program involves two to three applications spaced 14 to 21 days apart, as the herbicide works slowly, often taking up to three weeks for visible results. Other selective options include products containing methiozolin, which works by disrupting the plant’s cell wall biosynthesis. Regardless of the product chosen, the application timing and rate must strictly follow the manufacturer’s directions to achieve control while protecting the desirable turf.
Restoring the Turf and Long-Term Prevention
Once Poa trivialis has been successfully eliminated, the resulting bare soil patches must be addressed to prevent other weeds from colonizing the area. The dead grass and debris should be raked out, and the soil should be prepared for planting. Reseeding or sodding is the necessary next step to restore the lawn’s uniformity.
When overseeding the repaired areas, it is crucial to use high-quality, certified grass seed. Seed with a Blue Tag Certification guarantees a high level of purity, ensuring the new seed mix is free from Poa trivialis contamination. The best time to reseed or renovate is in the late summer or early fall when soil temperatures are ideal for cool-season grass establishment.
Long-term prevention relies on maintaining the cultural practices that favor the desirable turfgrass. Continuing deep, infrequent watering and mowing at a taller height will keep the lawn dense and competitive, naturally suppressing weed growth. Pre-emergent herbicides, typically applied in the late summer or early fall, can be a component of a prevention strategy by preventing any new Poa trivialis seeds from germinating. However, pre-emergents will also prevent the germination of any desirable grass seed, so timing is important, and they should not be applied when actively overseeding.