Japanese and common knotgrass are both invasive weeds. Japanese knotgrass grows from a thick, woody stem. It may reach heights of more than 10 feet tall. Common knotgrass is much smaller than Japanese knotgrass, usually reaching only 8 inches in height. Common knotgrass grows in thick mat-like patches on grass and cement. It can be extremely difficult to eradicate established knotgrasses from a lawn or garden. You will need to be persistent with your methods over a few years. Common knotgrass can pose a dangerous tripping hazard if left to grow on a lawn or walkway.
Cut as much of the giant or Japanese knotgrass away using a handsaw. Common knotgrass may be pulled out by hand. Wait until late fall or winter when the plant will not grow back immediately.
Dig out any rhizomes--or roots--of the knotgrass plant. Giant or Japanese knotgrass will have an unusually large rhizome system, and it may not be possible to remove all of it. Remove as much of the root system as possible, using a shovel. Common knotgrass will not have as large of a rhizome system as Japanese knotgrass and will be easier to remove.
Place a tarp or landscape fabric over the area where the knotgrass is present. Be sure to extend the tarp or fabric 12 inches in all directions beyond the affected area. Knotgrass will send out sprouts a considerable distance to find light to grow.
Hammer down landscape fabric stakes into the corners of the landscape fabric or tarp. If you need to use multiple pieces of fabric overlap each piece 12 inches.
Cover the tarp with a 2-inch layer of mulch.
Remove the mulch and tarp in the spring. Continue to pull out or cut any knotgrass growth throughout the growing season. Repeat the cutting and covering process the following fall or winter. This may need to be done for two to three years before the knotgrass is completely gone.
Spraying an herbicide that contains glyphosate on the knotgrass is another effective measure for killing knotgrass. Do this in the fall, and follow the manufacturer's instructions to ensure safety.