Prostrate Knotweed

About

Prostrate knotweed is a tough, wiry, mat-forming weed of trampled ground, from sidewalk cracks to worn lawn paths. Its presence usually means the soil is badly compacted.

Also called common knotweed, knotweed, wiregrass, wireweed, matweed

How to identify it

Look for

Thin, wiry stems radiate flat from a central taproot and form a mat, with small, dull blue-green leaves set one at a time along the stem. Each leaf joint is slightly swollen and wrapped in a thin, papery sheath. Tiny white to pinkish flowers sit in the leaf joints, and young seedlings have long, narrow seed leaves that look a lot like grass.

Look-alikes

Spotted spurge also forms flat mats, but its broken stems bleed milky sap and its leaves grow in pairs; knotweed has no milky sap.

Where it grows

Compacted, heavily walked soil: paths, driveway and sidewalk edges, cracks in pavement, sports fields and worn lawns.

When

It is one of the first summer weeds to sprout, in late winter or early spring, and flowers from about midsummer into fall.

How it spreads

By seed only; the seeds are probably long-lived in soil.

Most common in

What to do, gentlest first

Start at the top. Weed killers are a last step; always read and follow the label.

1. Relieve compaction and spread out traffic Prevent

   Core-aerate compacted lawn areas, keep turf dense, and add a path or stepping stones where people keep cutting across.

2. Hoe or pull while young By hand

   Young plants come out easily with a swivel hoe or hand tool; get them in spring before they set seed, since the taproot can grow over 18 inches deep.

3. Mulch or fabric in beds Barrier

   In beds, 3 to 4 inches of organic mulch or landscape fabric blocks seedlings; top it up every year.

4. Fatty-acid or vinegar burndown on seedlings Organic spray

   Pelargonic acid (a fatty-acid spray) or horticultural vinegar can burn down small plants, but the crown may resprout.

   Strong vinegar (over about 11% acetic acid) can burn skin and eyes, so wear gloves and eye protection. It browns any leaves it touches, grass included.

5. Pre-emergent in late winter or an early spot spray Conventional spray

   A pre-emergent such as dithiopyr, pendimethalin or prodiamine must go down before seeds sprout in late winter. On lawns, spot-treat young plants (under about 3 inches) with 2,4-D or dicamba in late spring or early summer.

   Read and follow the label. Many lawn weed killers also harm vegetables, flowers and shrubs, so don't let spray drift into beds, and keep dicamba products off the root zones of trees and shrubs.

Uses, wildlife and pets

Pets

Not listed by the ASPCA.