Japanese knotgrass was originally imported to new countries for its use as both a decorative plant and for erosion control. Unfortunately, the plant became an invasive pest that quickly spread, partially due to ineffective control measures. One of the best techniques for Japanese knotgrass removal combines chemical herbicides and manual removal. If Japanese knotgrass invades your property, you can permanently take care of the problem in as little as a week, but it may take much longer, depending on the size of the knotgrass.
Prepare a glyphosate herbicide to spray on the knotgrass. Read the directions on your chosen product, as some herbicides must be diluted with water, while others can be used straight out of the package.
Cut the stem of the Japanese knotgrass with a pruning shears, but let 3 inches of it stick up above the ground.
Spray the herbicide directly on the open stem of the knotgrass. Do this immediately after you cut the stem, as the knotgrass makes a protective barrier over the fresh cut within a few minutes.
Repeat the process in early fall if necessary. If the knotgrass doesn't die after the second application, repeat the process the following year.
Cut the stem between the herbicide applications if desired. Use rubber gloves when mixing the herbicide, to protect your skin.