Anything you put down to kill those "weeds" will also kill off your Dwarf Mondo Grass. The only way to control them is mechanical removal of the "weeds" which can mean digging up some clumps of the Mondo grass and snaking out the roots of the "weeds".

Be advised that over time even mondo grass will become invasive with roots which grow and spread 18 inches deep beneath the soil.

Only root barriers that are wider than 18 inches wide on the ground surface and/or 18 inches deep will prevent the mondo grass from, over time, also becoming an invasive problem.

Thanks for the info.

"Mechanical" means works fine on the areas where the mondo grass is spreading, but it really doesn't work on the areas that are firmly entrenched. Its growing very tightly between large stone pavers. In theory healthy tight knit mondo grass should preclude weeds, right? I wish.

Katrina: I've never known the dwarf variety to be very aggressive. Maybe somewhere else, but not here in the south.

Since I have seen quack grass and Johnston grass roots growing through some very dense root systems of many plants a dense root system is not a protection against any invasive plant species. Someone gave daughter a clump of Daylilies once and the following year there was quack grass growing in the planting bed I had cleaned if all out of they year before. I found, upon digging up that clump of the Daylilies quack grass roots weaving all through the plants roots and only after much rinsing of clay off was I able to clear that root ball of all the invasive roots. I have had the same thing, often enough, that I always make sure any plants I give to others are free of any invasive roots.

You can mix 1oz. Of Roundup with 1 gallon of water in a sprayer. It will kill the weeds and NOT the mondo.