Prostrate Knotweed
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Pennsylvania weed ID · Lawn broadleaf
Prostrate Knotweed
Polygonum aviculare · Polygonaceae (buckwheat family)
- Summer annual
- Broadleaf
If a tough, wiry mat of tiny blue-green leaves keeps filling the worn strip along your sidewalk or driveway, you're looking at prostrate knotweed, and it's telling you something about your soil.
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What is prostrate knotweed?
Prostrate knotweed is a low, mat-forming summer annual that shows up where the ground gets walked on, parked on or packed down. Quick prostrate knotweed identification is useful because this weed is really a sign of compacted soil. Fix the soil and the weed has a much harder time.
It’s one of the earliest summer annuals to sprout, often coming up in late winter when the soil is still cold. By summer it forms a tough, wiry mat that survives foot traffic, mowing and dry weather better than most lawn grasses.
Despite the name, it isn’t related to Japanese knotweed, the tall invasive shrub-like plant. Prostrate knotweed isn’t on Pennsylvania’s noxious weed list.
Prostrate knotweed identification: what to look for
Get down close, because everything on this plant is small.
- Growth habit: flat, spreading mats of wiry stems radiating from a central taproot.
- Leaves: small, bluish-green, oblong to lance-shaped, alternating along the stem.
- Nodes: each joint has a thin, papery, silvery sheath wrapped around the stem, a feature of the buckwheat family. The swollen joints give the plant its “knot” name.
- Flowers: tiny pinkish-white or greenish flowers tucked where leaves meet the stem.
- Seedlings: very narrow seed leaves on a reddish stem, easy to mistake for a grass sprout.
Where it grows in Pennsylvania
Prostrate knotweed favors packed, low-oxygen soil. Look along sidewalk and driveway edges, the path the dog wears along the fence, ball fields, gravel shoulders and cracks in older pavement.
Our central PA clay soils pack down easily, especially when they’re walked or driven on while wet. Areas that flood or hold water and then bake dry are also prime spots. Wherever grass thins out from traffic, knotweed moves into the gaps.
It’s also common on new construction lots, where heavy equipment packed the subsoil and only a thin layer of topsoil went back on top. Lawns on those lots can struggle for years, and knotweed is often one of the first weeds to show it.
Prostrate knotweed look-alikes
Spotted spurge grows in the same hot, packed spots and also forms flat mats. Break a stem: spurge bleeds milky sap and knotweed doesn’t. Spurge leaves are also in opposite pairs, often with a reddish spot.
Common purslane has thick, succulent leaves and yellow flowers, and carpetweed has leaves arranged in whorls around each joint. The papery sheath at each node is the giveaway for knotweed.
How to get rid of prostrate knotweed
Pulling works on small patches, and early in the season is easiest. Once the stems get wiry and the taproot sets, plants tend to break off at the surface.
- Pull or dig young plants: in spring, a hand weeder slides under the taproot and lifts out the whole mat. Moist soil helps.
- Pavement cracks: a crack weeder or stiff wire brush clears joints. Refilling joints keeps them from reseeding.
- Remove before seed: pulling by midsummer keeps seed from building up.
Cultural control. This is the part that actually solves the problem. Core-aerate compacted lawn areas, ideally in early fall, and overseed them so turf can fill in. Redirect foot traffic with a stepping-stone path or a proper walkway where a dirt track has formed. Where low spots flood and then crust over, improving drainage gives grass a fighting chance. Mow at a taller height so turf shades the soil.
Chemical control. Because it sprouts so early, preemergence products have to go down early too. Purdue’s turf program reports the best results from isoxaben applied in late fall, since spring applications often go down after germination has started. For plants already up in cool-season lawns, broadleaf herbicides that combine 2,4-D with dicamba or triclopyr work better than 2,4-D alone, applied in spring while plants are young. Read and follow the label, which is the law, and confirm the product is labeled for your lawn type or hardscape area. Applying herbicides for hire in Pennsylvania requires a PA Department of Agriculture pesticide applicator certification, so larger programs should go through a licensed lawn-care company.
Keeping it from coming back
As long as the soil stays compacted, knotweed will keep coming back, so the long-term fix is about traffic and soil, not just weeds. Aerate high-traffic lawn areas each year, keep cars and equipment off wet turf, and give people a better place to walk.
Along patios and walkways, keep paver joints full so there’s no loose soil for seed to land in. Seed thin turf in early fall, and by the next spring the grass will be competing for the same ground knotweed wants.
If a spot simply gets too much traffic for grass to survive, like the corner where everyone cuts across to the driveway, it’s often smarter to stop fighting it. A gravel strip, stepping stones or a short paved path will look better and hold up for years.
Questions we get about prostrate knotweed
Is prostrate knotweed the same as Japanese knotweed?
No. Prostrate knotweed is a low, mat-forming annual lawn weed. Japanese knotweed is a tall, bamboo-like invasive perennial. They're in the same plant family but are completely different problems.
Why does knotweed grow along my sidewalk?
It thrives in compacted, low-oxygen soil where foot traffic has thinned the grass. Aerating and overseeding the strip usually helps more than any weed killer.
When should I put down a preemergence for knotweed?
Earlier than you think. It sprouts in late winter, so research from Purdue favors a late-fall application. A spring crabgrass application usually goes down too late for knotweed.
Is prostrate knotweed toxic to pets?
It isn't considered toxic to people or pets.
Weeds taking over a bed, slope or wet spot?
If knotweed keeps filling a worn path across the lawn, Muthler can build a paver or stone walkway so foot traffic has somewhere better to go. We rebuild beds, regrade and drain wet ground, clear overgrown brush and build patios and walls across Clinton, Lycoming, Centre and Union counties.
Get a free estimate(570) 397-9800Similar weeds
Further reading: Penn State Extension
Herbicides: always read and follow the product label. Applying pesticides for hire in Pennsylvania requires PA Dept. of Agriculture certification.