I’ve been trying to eradicate some pampas grass, and am told it will almost always return. I cut it this spring and when it started
to regrow I hit it with a heavy dose of Roundup, then covered it with a tarp the following day to allow the round up to fully be ingested by the plant. I'm thinking this will keep the plant from getting any sun or rain. It is now three months later and eve rything seems dead, how long can pampas grass remain dormant? Can it just be laying dormant and return next year?
It sure sounds like your strategy worked. With no green to do photosynthesis for an entire growing season, the roots have likely died. Add the RoundUp and I can't see any future for the pampas grass. It will take a number of years before the root mass deteriorates enough to plant something else in the area. You can speed this along by cutting into the mass with a sharpened spade, mattock or axe. Compost or soil conditioner over the plant may speed the process; however, don't expect to use the area next spring.