Common knotweed (prostrate knotweed) (Polygonum arenastrum)
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Common knotweed (prostrate knotweed) is a short-lived perennial broadleaf plant that sometimes lives as an erect annual. It is found throughout California up to 8200 feet (2500 m). Common knotweed can thrive even on poor and compacted soil and inhabits agricultural land, nursery grounds, and other disturbed areas. This plant often attracts predatory insects. Common knotweed seeds serve as forage for songbirds and small animals.
Roadsides, fields, agronomic croplands (especially alfalfa), orchards, vineyards, yards, turf, gardens, landscaped areas, nursery grounds, paths, walkways, and disturbed, unmanaged places.
Cotyledons (seed leaves) are long, about 1/6 to 3/5 of an inch (4–15 mm) in length, very narrow, rounded at the tip, bluish green with a white powdery coating—especially on the lower surface, and continue to elongate over time. They are fused together at the base and the first true leaf emerges from the "cup" formed between them. True leaves are much broader than the cotyledon, stalkless, are alternate to one another along the stem, and taper toward the base.
Common knotweed is a prostrate annual plant with numerous slender, wiry stems that are highly branched and form mats. The extensive branching gives it a zigzag appearance. Stems are round in cross-section, can reach 4 feet (1.2 m) long, and have longitudinal ribs that are often slightly swollen at the joints (nodes). Leaves are linear to narrowly football shaped, stalkless, are alternate to one another along the stem, and are about 1/5 to 4/5 of an inch (0.5–2 cm) long.
Flowers bloom from May through November. The inconspicuous flower heads are found at the top of short stalks that grow from the bases of leaves. They consist of a cluster of two to eight tiny, green flowers with white or pink edges.
Fruits are tiny, about 1/8 of an inch (3 mm), one seeded, and enclosed by the flower petals and sepals. Removing the flower petals and sepals, reveals a fruit that contains one seed, is somewhat egg shaped, three sided, dull or slightly glossy, and dark brown.
Reproduces by seed.