Prostrate knotweed (Polygonum aviculare) is a low-growing summer annual commonly found in lawns, high-traffic areas, waste areas, and along driveways and sidewalks. Prostrate knotweed provides a food source to birds and mammals but is also a known host for many fungi, viruses, and nematodes. Prostrate knotweed quickly covers bare soil preventing native plant species from becoming established. Toxins from its roots and leaves alter the composition of the soil, making it unsuitable for native species.

 Prostrate knotweed is a low-growing plant with wiry stems that radiate out from a central point and grow up to 2 feet in length. Small leaves and tiny white or pink flowers emerge from nodes that are covered in a papery sheath. Flowers have 5 petals and green centers. 

Integrated Pest Management (IPM) Options:  

• Cultural – Prostrate knotweed thrives in compacted soils. Avoid compacted soils by aeration. Apply a 3-inch-deep layer of mulch to planting beds to reduce seed germination.

• Mechanical – Hand pulling plants before they go to seed. Rototilling or hoeing will also effectively eliminate plants.

• Chemical – A complete list of approved chemicals is available on www.pnwhandbooks.org. Type “prostrate knotweed” in the search, then select “Annual Broadleaf Weeds in New Stands” for recommendations. Always read and follow herbicide label directions!

Tom Jacobsen, University of Idaho Extension Educator in Fremont County. (208) 624-3102 [email protected] and Justin Hatch, University of Idaho Extension Educator in Caribou County. (208) 547-3205