Peter H. Dernoeden, Ph.D

Identification and Characteristics of Knawel 

Knawel (Scleranthus annuus) is also known as German Knotweed. It is a winter annual that has a grass-like appearance.

Knawel is mostly found in areas where there is little turf competition and soil has been disturbed. Its grass-like appearance allows it to go unnoticed. It is not invasive, and like all winter annuals, it flowers in spring and dies in response to heat in early summer.

Leaves are linear or grass-like, sharply pointed at the tip, and usually bent downwards. Grass-like leaves are born in clusters along branching, and hairy stems. Flowers lack petals, are green and very small, and inconspicuous in spring.

The use of herbicides is seldom needed.

Where to Find Prostrate Knotweed

Prostrate Knotweed (Polygonium aviculare) is one of the first broadleaf annual weeds to emerge in spring and becomes highly competitive with cool-season turfs in the summer.

Knotweed is found throughout the U.S. and proliferates in compacted soils, and is especially common adjacent to sidewalks, curbs and cart paths.

Knotweed is found in compacted lawns and highly trafficked areas, such as sports fields. It is one of the nastiest summer weed problems.

Behaviors and Defining Traits of Prostrate Knotweed

Knotweed seedlings emerge soon after soils begin to thaw in early spring, usually in March “During March Madness Week” in the Mid-Atlantic. Seedlings have long leaves that appear grass-like and are often initially confused with crabgrass. However, crabgrass emerges several weeks later. Leaves darken and become dull-green or blue-green with age.

Knotweed develops slowly, but by mid-summer it produces leafy stems that eventually form dense, low growing and wiry mats.

Stems become “knotty” with swollen nodes. Each node has a paper-like white sheath, but nodes do not form roots. Inconspicuous, white flowers form in leaf axils in early summer and are produced until an autumn frost. Frosts turn knotweed brown and plants die, but plant remnants can be found throughout winter. Knotweed survives as seed.

Reducing Knotweed Invasiveness

Controlling compaction and diverting foot traffic helps to reduce invasiveness of knotweed. Knotweed is best controlled in early summer prior to the time that plants develop their mats of tough wiry stems.

Dicamba in a 2 or 3 combination with either 2,4-D, MCPP, clopyralid (not for lawns), fluoxypyr, or triclopyr are effective.

In late summer, heat-hardened knotweed mats are extremely difficult to control, even with multiple applications of herbicides.