Certified GKH Gardening Expert
You have several options: I would recommend pouring boiling water on the areas where the crabgrass was growing. Boiling water will kill whatever it comes in contact with, but you'll have to protect the monkey grass. Alternatively, you could cover monkey grass clumps with cardboard boxes or use a gallon milk jug with the bottom cut off as a shield. Spray the grass with an herbicide during hot weather when the grass is actively growing. One application won't work, as it's going to take several. If a little herbicide touches the monkey grass, don't despair. It will only kill the little sprout, not the clump. Finally, you can use the old standby--hand pulling all the unwanted grass. If hand pulling is not viable, you can try using Ortho Weed-B-Gone. This will kill the crabgrass but not harm the monkey grass.