Prostrate knotweed (Polygonum aviculare) is a common and aggressive annual weed that often invades lawns, sidewalks, and high-traffic areas, frequently mistaken for crabgrass in its early stages. This persistent broadleaf weed forms dense mats that choke out desirable turf, leading to frustration for many homeowners. Getting rid of this plant requires a comprehensive approach that targets its specific life cycle and preferred growing conditions.
Identification and Growth Habits
Prostrate knotweed is a summer annual that is one of the first weeds to germinate in early spring, sometimes appearing as soon as the snow melts. Its characteristic mat-forming growth habit features slender, wiry stems that radiate outward from a central taproot. These stems are noticeably swollen at the nodes, where a thin, papery sheath called an ocrea encircles the joint.
The small leaves, which are blue-green and oval or lance-shaped, alternate along the tough stems. A single plant can produce thousands of seeds, which remain viable in the soil for years. This weed is considered an indicator species because it thrives in compacted soils, a condition most turfgrasses cannot tolerate, often appearing along driveways, sidewalks, and heavily trodden paths.
Cultural and Manual Removal Strategies
For small infestations, manual removal is a non-chemical solution, provided the entire taproot is extracted. Removal is easiest when the soil is thoroughly moist, which helps loosen the soil around the taproot and prevents the plant from regrowing. This method is most effective on young plants before they establish tough, wiry stems.
Alleviating soil compaction is an effective cultural control method. Core aeration, typically performed in the fall, loosens the soil, improves drainage, and creates a favorable environment for healthy turfgrass competition. Maintaining a taller turf height also helps manage this weed by shading the soil and inhibiting seed germination.
Encouraging deep, infrequent watering favors the turfgrass root system over the shallow-rooted knotweed. Knotweed thrives in overwatered areas with poor soil oxygen. Reducing soil compaction and promoting a dense lawn actively discourages the conditions that allow knotweed to flourish.
Chemical Control Options
Chemical intervention is effective for managing widespread infestations. Pre-emergent herbicides are most successful because they prevent seeds from germinating, effectively breaking the weed’s life cycle. These products must be applied very early in the spring, well before the seeds begin to sprout, often as early as late February or early March in some regions.
Active ingredients like prodiamine, dithiopyr, and pendimethalin are commonly used in pre-emergent products labeled for knotweed control. Once the weed has emerged, post-emergent herbicides are necessary, though knotweed is tougher to eliminate at this mature stage. Selective liquid broadleaf weed killers containing combinations of 2,4-D, dicamba, or triclopyr are effective.
Application of post-emergent products should occur when the weeds are small, ideally less than three inches in diameter. Multiple applications spaced several weeks apart may be required to control mature plants. Always read the product label carefully to ensure the herbicide is safe for your specific type of turfgrass and follow all safety instructions, including wearing protective gear.
Long-Term Prevention and Maintenance
Maintaining optimal soil health is the primary long-term strategy for preventing knotweed recurrence. Since the weed is an indicator of poor soil conditions, continuous monitoring and amendments are necessary. Regular core aeration in the fall should be a standard practice to keep the soil structure loose and well-drained.
A thick, healthy lawn is the most effective way to out-compete the weed. Overseeding thin areas with locally adapted grass seed in the fall creates a dense canopy that blocks sunlight, preventing knotweed seed germination. Consistent fertilization, timed appropriately for your turf species, promotes a vigorous lawn that resists weed establishment.
Diligent monitoring for new seedlings, especially in early spring, is necessary. Immediate hand-pulling of any newly emerged plants before they can establish a deep taproot and produce seeds will prevent future generations from infesting the area.