I had something nasty growing right through the middle of my favorite red-pink-white Rhododendron, and merely cutting it at ground level didn't discourage it enough.
So I saved a heavy plastic bag that had held 2 cubic feet of mulch and rammed as much of the nasty thing into it as i could.
Then I sprayed it well
inside the plastic bag, and left it tied up tight for a few hours.
Then I sprayed it again with a different herbicide and left it tied up overnight. I forget when I finally untied it, but it was dry enough that the Rhodie didn't complain.
One other time, I thought I could safely spray some Ranunculus without hitting a nearby Azalea. The Ranunculus had invaded from the neighbor's yard. it also engulfed my only tulip. Ate the bulb, as far as I can tell.
I didn't use a plastic barrier. There was no serious effect on the Ranunculus, but that Azalea still looks like it caught Harry Dresden's death curse.
<-- Rhodie did fine. ----> Azealea looks pink when the sun is too bright. No weeds shown in these photos.
Ranunculus
