Weeds of the Week: Knawel and Prostrate Knotweed

Posted on March 23, 2017

Peter H. Dernoeden, Ph.D

Knawel (Scleranthus annuus) also is 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 or where soil has been disturbed. Its grass-like appearance enables it to become established unnoticed. It is not invasive, and like all winter annuls, it flowers in spring and dies in response to heat in early summer. Leaves develop in whorls, are linear or grass-like, sharply pointed at the tip, and usually are bent downwards.  Grass-like leaves are borne in clusters along branching, spreading and hairy stems.  Flowers lack petals, are green, very small, and inconspicuous in spring.  Use of herbicides is seldom needed.

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. It 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.

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, slender leaves that appear grass-like and often are initially confused with crabgrass. However, crabgrass emerges several weeks later. Leaves alternate on short stalks (i.e., petioles) and become oblong to linear. 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.

Controlling compaction and diverting foot traffic help 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.