Knotweed is sometimes also called prostrate knotweed because it creeps along the ground rarely achieving more than 4 inches in height. The long tendrils creep out in a spider-like pattern and have tiny leaflets that resemble knots along the stems. The plant is an annual, but each plant disperses an abundance of seeds that germinate the following spring. Killing prostrate knotweed requires the use of both post-emergent and pre-emergent herbicide solutions.
Fill a pump-style garden sprayer with an herbicide that contains two of the following ingredients dicamba, 2,4-D, triclopyr, MCPP and clopyralid. Do not mix multiple products. Instead, find an herbicide that already contains multiple active ingredients.
Add water to the sprayer and then close it up by screwing on the top. Shake the sprayer gently to mix the solution and then pump the handle of sprayer up and down until you feel resistance.
Spray the mixture onto all areas of the prostrate knotweed, making sure to saturate the plants. If the knotweed plants are still young, less than 3 inches in diameter, one application of herbicide will kill them. For larger knotweed plants, wait one week and apply a second dose of herbicide to the plants if any green coloring is detected.
Wait until the following February before the grass and weeds start producing green growth. Apply a pre-emergent herbicide to the entire lawn that contains one of the following ingredients, benefin, isoxaben, oryzalin, dithiopyr, prodiamine, pendimethalin or trifluralin. Use the amount specified on the herbicide bottle. This prevents any remaining prostrate knotweed seeds from germinating.