Perennial grassy weeds are considered to be the most difficult weed problems to deal with in lawns. Control options are limited because the weed species are very similar to the lawn species. In fact, many perennial grassy weeds are not considered weeds, but are considered desirable grasses when growing by themselves under a different set of conditions.
For example, several common perennial grasses, when growing in Kentucky bluegrass lawns, are considered weeds because they differ greatly in leaf width, color, or growth habit. Tall fescue is more coarse and grows in distinctive clumps when it occurs with Kentucky bluegrass. Creeping bentgrass, a very desirable turf species for golf courses, becomes a weed in bluegrass lawns because it appears as patches of finer grass, usually lighter in color. Zoysiagrass, a warm season turf species, appears as patches of thick grass, dormant (straw-colored) for much of spring and fall in Kentucky bluegrass or other cool-season grass lawns.
There are additional perennial grasses that are frequent weed problems. Quackgrass, a coarse species with thick underground stems (rhizomes) can be a major problem in lawns. Nimblewill, a creeping warm season species, often appears as light colored patches in lawns.
One way to distinguish perennial grasses from annuals is the time of the year established plants are present. Perennials (other than nimblewill and zoysiagrass) will appear as established green grasses early in spring; whereas most annual grasses like crabgrass don't appear until late spring or early summer. Likewise, most annuals die off quickly in fall, but perennials do not.
Removing these weed patches by hand is one control option. It's important to get all of the plant, as many have underground or above ground stems (rhizomes or stolons). These stems enable these species to spread quite readily, so if broken or cut, they regrow.
Selective chemical control is not an option with most perennial grassy weed species. Unlike selective herbicides used on annual grasses (i.e. crabgrass), nonselective herbicides used to control perennial weed grasses may also damage the lawn species. For this reason, spraying over the lawn is not suggested unless the problem is severe enough that all grasses need to be killed and the lawn reestablished. Using a nonselective herbicide, such as glyphosate, patches of the undesirable species can be spot treated. After weeds and portions of lawn die, reseed with desirable grass species.
Treating in mid-August is generally thought of as the best timing (late July to early August to control nimblewill and zoysiagrass), as late August into early September is the most favorable time for reseeding. If resodding the area afterwards, there is a longer period of time to treat the weeds. Keep in mind the weed species need to be actively growing to be controlled by glyphosate, however. Allow 10 to 14 days to determine if weeds have been completely controlled.