Common knotweed, Polygonum arenastrum, is also known as wiregrass, wireweed, matweed or doorweed. It is an annual species that is native to Europe that has established itself throughout most of the United States and Canada. It is found in field crops, row crops, orchards, yards, gardens and turf. It is tolerant of soils compacted by trampling with foot traffic and is therefore frequently found along paths and walkways. This weed is particularly well adapted to the winter and early spring rainfall pattern throughout California. It gets a good start with the early moisture and establishes a taproot, which allows it to survive the summer drought.
Common knotweed is a prostrate annual or short-lived perennial plant with numerous slender, wiry stems that are highly branched to form prostrate mats. However, in cultivated conditions it may grow slightly erect to 4 to 8 inches. Seedlings are initially upright with strap-shaped, embryonic or cotyledon first leaves that are 1/4 to 3/4 inch long. There is a single taproot that can penetrate to more than 18 inches. Leaves are bluish green in color with blades narrowly ovate in shape (about 1/5 to 4/5 inch in length). The leaf stalk is short and stem nodes are encircled by papery leaf stipules. These stem nodes are slightly swollen giving the typical “knot”-like appearance from which the common name is derived. Flowers are small and inconspicuous; they are borne in the upper leaf axils. The colors of the flowers range from white to green, and they may have a pinkish tinge. The seed is part of an achene or simple fruit that is three-sided, dark brown, not shiny, and about 1/8 inch long.
Silver-sheathed knotweed, Polygonum argyrocoleon, is similar to common knotweed, but has a more erect growth habit reaching 12 to 20 inches in height. It may be distinguished from common knotweed by its long leafless, rose-colored flower spikes and its shiny seed. Silver-sheathed knotweed is most common in southern California.
Common knotweed can be confused with spotted spurge in gardens and mowed areas. An easy way to distinguish them is by the white milky sap that is exuded from broken stems of spotted spurge.