How to Remove Knotweed From Your Lawn

Prostrate knotweed (Polygonum aviculare) is a common lawn weed in many states that’s an early summer annual or a short-lived perennial. Knotweed can be found in compact, high traffic areas like along the edges of the sidewalk or in areas where the lawn has been overwatered. This weed begins germinating in the late winter—as early as the end of February—and establishes a large tap root that allows it to survive throughout the summer and into the early fall. As it grows, its many wiry stems form a mat along the ground that can avoid the blades of the mower.

Knotweed can be identified by its blue-green oval shaped leaves and the swollen nodes on its stem that look like small knots, giving the plant its name. Flowers on the plant can range from light green to light pink and each plant can disperse an abundant number of seeds that will germinate the following spring. This weed looks similar to spurge, but it can be distinguished when the stem is broken, as spurge will produce a milky white sap, differentiating it from knotweed.

Knotweed Non-Chemical Control

To eliminate knotweed in your lawn, cultural controls can be utilized before chemical controls. Since the weed prefers compact soils and weak areas of the lawn, aerification and proper fertilization can ensure that the ideal growing conditions for knotweed never occur. Following a proper fertilizer schedule will keep your lawn thick so that it chokes out weeds. Here are some of our favorite fertilizer selections:

Watering on a proper schedule can also help to keep the lawn healthy and limit knotweed development as it can thrive in areas that are overwatered and have low soil oxygen. If the problem is minimal, knotweed can be pulled by hand; however, this is not always an effective control as the weed will grow back if any portion of the taproot is missed. For this reason, the best cultural controls involve maintaining a healthy, dense turfgrass canopy so that knotweed does not have a chance to germinate. Learn more in How to Thicken Your Lawn.

Photo credit: NC State Extension.

Knotweed Chemical Control

When cultural controls are not enough, there are many chemical options to eliminate knotweed. Pre-emergent and post-emergent herbicide options are available for over-the-counter control of knotweed. However, there is a difference between these two herbicides. Pre-emergents are designed to prevent weeds from breaking through the surface of the soil. Post-emergents, on the other hand, kill off or control any currently existing weeds.

Typically, pre-emergent herbicides are a more effective control against knotweed as it can be more difficult to control when the plant is fully established due to the extensive taproot system below the soil’s surface. One common pre-emergent is Dimension 2EW (active ingredient Dithiopyr). This is combined with water to create a solution that can be sprayed on the entire lawn or the affected area. Another popular pre-emergent for knotweed is Prodiamine 65 WDG (active ingredient Prodiamine). This is mixed in a spray tank and applied to the entire lawn and provides long lasting control with just one application. Since knotweed can begin germinating as early as February, a pre-emergent must be applied before the growing window or they will not be effective.

Post-Emergent Control of Knotweed:

If post-emergent control is needed, PastureGard (active ingredient Triclopyr and Fluoxypyr) can be used. PastureGard can be mixed in a backpack sprayer and applied as a spot spray to the leaf tissue of any knotweed plants. If the plant is not totally killed, an additional application may be needed 4–6 weeks later. Other herbicides with proven effectiveness over knotweed include both SpeedZone Broadleaf Herbicide and SpeedZone Southern Herbicide.

It’s recommended to wait until you’ve mowed new grass at least 3–4 times before applying any herbicides or other control products. This waiting period allows the new grass to establish roots and reduces the risk of stress from chemical applications.

Some of the herbicides listed above should not be used on St. Augustine or centipede grass as these types of turf are sensitive to chemicals like MSMA and 2,4-D. These chemicals have the potential to severely damage these types of grasses. Use an Atrazine-based post-emergent herbicide for both centipede and St. Augustine such as Hi-Yield Atrazine Weed Killer.

If knotweed develops in your lawn and cultural practices cannot completely control it, chemical controls can be an effective tool; however, the herbicide labels must be read and followed carefully to ensure their compatibility and effectiveness with your lawn. If used incorrectly, chemical applications can damage your lawn.

For more information on weed control fundamentals, check out our Weed Control and Identifying Common Lawn Weeds blogs.