Can anyone please help with the identification of this weed grass in my lawn? I live in the Pittsburgh area. It is like a lime green with purple stems (see Photos). The stems are thick and course. I have tried tenacity on this but no luck. I can not get a solid answer as to what this is. Any help would greatly be appreciated.
It looks like Elymus repens, common names "couch grass" (pronounced cootch) or "twitch" in the UK. In the UK the young stems can be light purple coloured but not usually as dark as in your picture, but I think it is the same species.
The bad news is there is no selective herbicide that will affect it. You could try a gel formulation of glyphosate or similar weedkillers. Even non-selective herbicides may take up to 3 weeks to kill it, and may need repeat applications.
The only non-chemical way to eradicate it is completely remove the roots (which are actually rhizomes, not roots). If you leave any root at all in the soil it will regrow.
The "nuclear option" of completely removing the turf layer and burning it (or disposing of it if off-site) and then frying any remaining roots in the soil with a flame-thrower will probably work, but is not 100% guaranteed. This thing is a survivor.
Its only "good" quality is that it does not have fertile seeds, so it only reproduces and spreads vegetatively - but that is little consolation if you want to get rid of it.