I have a long border of Lo-Gro Sumac where grass grows up in between and amongst the sumac. The sumac isn't dense enough to shade out the grass from growing in between it and it's about impossible to thoroughly remove, so it's a hassle more often than it should be to keep clean.

I'm not generally in favor of herbicide when hand weeding/mulch can take care of the problem or at least keep it under control, but this is just one of those times when I need to resort to a herbicide.

What herbicides can I safely use to remove the grass without damaging the Lo-Gro Sumac?