Rough bluegrass (Poa trivialis) and Canadian bluegrass (Poa compressa) are perennial grasses that prefer cool, wet conditions and are considered invasive weeds in Virginia. They spread by short rhizomes and also by seed and are most vigorous when weather conditions permit (extended periods of cool, wet weather). Under these conditions, identification can be difficult and the appearance of the lawn will seem unaffected (the leaf resembles Kentucky bluegrass; boat shaped tip and prominent mid vein). As the weather warms, roughly circular, light green patches of the turf will appear and begin to grow slightly faster than surrounding grass. These typically appear in poorly-drained, shady areas, but can also appear in full sun. If the weather warms quickly, these patches will brown-out severely, except at the tips, and mat down. This will smother all the desirable turf in these patches. If there is a more gradual warming, the patches will yellow and die back more slowly, allowing desirable turf to fill in. This it is not a fungus that will spread throughout the lawn (a common misconception).
Chemical control is not recommended. Allow enough time between mowings so that about 1/3 of the total plant is cut (avoid frequent ‘trimmings’). The spots should be thinned once per week. This will prevent matting, as the unwanted bluegrasses die back with warmer weather. Raking through the affected areas will remove some of the offending grass each week and allow the desirable grass to fill in with little or no seeding.
Got rough/Canadian bluegrass questions? Call Bio Green at 703-450-0034 or 703-361-0313!