Undesirable Bluegrasses: Poa annua & Poa trivialis
Weed grasses are among the most difficult weeds to treat and control in lawns. The difficulty arises in the fact that any herbicides that will kill these weed grasses will kill lawn grass as well. This means that a combination of weed control methods must be used and maintained over an extended period of time for continued control. There is no quick and easy fix for weed grass problems, particularly when those weed grasses are the undesirable bluegrasses. Let’s look at the most common weed bluegrasses: annual bluegrass (Poa annua) and rough bluegrass (Poa trivialis). Both types of bluegrass are widespread enough to consider them naturalized and they are more than likely already in your lawn.
Annual Bluegrass (Poa annua)
It is almost a certainty that you have Poa annua in your lawn: it is one of the five most widely distributed plants in the world.  With regular control measures, annual bluegrass can be contained but is rarely eliminated.
In winter it will be visible as green tufts in dormant warm-season lawns, and is usually easily visible in cool-season lawns, particularly as they stop growing during the coldest part of winter. Once lawn grasses are actively growing, the annual bluegrass will appear to be a lighter yellow-green in comparison to the turf. It produces many distinctive, light-colored flowers per plant and can do so even when mowed very short.
Annual bluegrass is a cool season annual grass that germinates in fall and grows slowly through the winter. In late spring it flowers and drops many seeds that lie dormant until the return of cool weather in the fall, starting the cycle again. In areas of stressed turfgrass, the annual bluegrass will out-compete  and thrive.
The annual bluegrass plant itself will die out when the heat of summer arrives, leaving bare patches in the lawn. When there are few small patches of annual bluegrass, these bare spots may be barely noticeable; as the population increases the patches get larger and more numerous as more seeds are left in the soil to germinate.
There are a few post-emergent herbicides that will selectively control annual bluegrass, but they are prohibitively expensive for most homeowners. The preferred herbicidal control method is to use a pre-emergent herbicide to prevent seed germination in fall, or non-selective herbicides. Cultural practices (more on that below) can also provide a bit of control.
Rough bluegrass (Poa trivialis)
Rough bluegrass (also known as rough stem bluegrass) is a perennial grass somewhat similar to annual bluegrass. It’s tolerance for shade makes it particularly aggressive in shady lawns. It also thrives in cool weather and moist soil and dies back as summer heat arrives, leaving bare soil. Unlike annual bluegrass, rough bluegrass just appears to be gone. In reality the plant’s stems survive underground waiting for cool weather to return and pops up again in fall.
Poa trivialis is a light green grass that creates thick mats of stems, choking out areas of lawn. Because of the off color and rapid growth it is aesthetically unappealing, as are the large bare spots it can leave when it goes dormant in summer. The timing and appearance of the die-out can look like a fungal infection, and is often mistakenly treated as such.
This weed grass is actually intentionally used in some applications as a winter overseed  (it makes a good winter putting surface for golf greens). Because of rough bluegrass’s intolerance of heat, it is not suitable for use in our area as a turf grass. Unfortunately it is often found unintentionally in shade grass seed mixes. Due to the size of the seeds, Poa trivialis cannot be removed in the screening process that normally removes unwanted seeds and has contaminated a large portion of grass seed shade blends. The seed is also often missed during visual seed quality testing due to its similarity to the seeds of perennial bluegrass. For these reasons it has become a common contaminant in shade grass seed blends and is likely to remain so for the foreseeable future. Even worse, there are not currently any selective herbicidal controls available for rough bluegrass.
There are a few cultural practices that can reduce the chances of weedy bluegrasses becoming established in your lawn, but they will not fix an existing problem. Primarily this means maintaining a thick and healthy lawn that can out-compete these grasses and other weeds. This requires being careful not to mow too short, as taller turf grass shades the soil and prevents weed seed germination (it helps with all weed seeds, not just annual bluegrass). Grass cut too short is also stressed and less able to compete against weeds. It also requires proper watering by watering deeply and infrequently, preferably once weekly; frequent, shallow watering benefits shallow-rooted annual bluegrass while inhibiting the development of deep, healthy roots on fescue and other desirable turf grasses. Low areas that remain wet for extended periods also encourage weed bluegrasses, as does a shady lawn (Both weed bluegrasses are more tolerant of wet soil and shade than desirable lawn grasses.) Finally, proper rates and timing of fertilizer applications will help strengthen your lawn while not overly stimulating undesirable weeds.
While there are no selective herbicide solutions for rough bluegrass, it can be controlled with a non-selective herbicide like glyphosate (Roundup) applied in spring, though this will kill desirable grass also. The resulting bare areas can be reseeded in spring, provided you have adequate irrigation to support young grass plants through summer.
Another option is to use herbicide in the fall, once the rough bluegrass has re-sprouted in the cooler weather. This should be accompanied by an application of a pre-emergent herbicide in late August to prevent any seeds in the soil from sprouting. If you use this method to control rough bluegrass, fall seeding must be skipped as the pre-emergent will also prevent lawn grasses from germinating.
Once you get control of weedy bluegrasses, regular pre-emergent applications are necessary until your lawn is thick and healthy enough to prevent any seeds from germinating and getting established. Because these weeds are so widespread, constant monitoring is required to keep them from becoming a problem again in your lawn.
Poa annua by Rasbak (Own work) [CC-BY-SA-3.0 )], via Wikimedia Commons
Poa annua flowers by Robert Vidéki, Doronicum Kft., Bugwood.org
Poa trivialis dieout by Phil Stilson, New Garden Select
Poa trivialis flowers by Joseph M. DiTomaso, University of California - Davis, Bugwood.org