Quackgrass (Elymus repens) is the hardest turf weed to control that I know of. You can identify it by the long clasping auricles on its collar (Fig. 1) and by its extensive rhizome system (Fig. 2). I get the question occasionally whether the new herbicide Tenacity (mesotrione) will selectively control this species. The answer is “no”.
Figure 1. Auricles
Figure 2. Rhizomes
We just treated the Kentucky bluegrass (Poa pratensis) turf at the research station shown in Figure 3. The result is that the quackgrass turns white, as can be seen in the picture. That always gets peoples hopes up, including mine when I first started working with this product. Unfortunately, the quackgrass always recovers and comes back as bad as ever. Repeat applications do not work either. I tried for 4 seasons to kill patches in my own lawn, hoping that my tenacious applications of Tenacity would kill it. I lost, and the quackgrass was not even reduced in severity by my repeated applications.
Figure 3. Quackgrass turned white by Tenacity.
The only way to control it remains non-selective applications of Roundup (glyphosate). The rhizomes are very hard to kill and repeated applications of Roundup will be necessary. If you have this problem and want to get rid of the quackgrass, start now in May by killing the infested areas. Then repeat apply every time the quackgrass comes back from rhizomes. You should set a goal of reseeding in mid-August.
Even a better solution would be to sod over the dead areas. The rhizomes have a harder time emerging through sod than they do into a newly seeded area.