Russian knapweed (Acroptilon repens)
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Russian knapweed, a widely distributed perennial broadleaf plant, is listed as a Class B noxious weed in California. Except for the wettest areas of the northwest and driest regions of the Great Basin and deserts, it is found throughout California to about 6200 feet (1900 m). Russian knapweed inhabits agricultural land and other disturbed places. Aggressively competitive, this plant can rapidly colonize an area and develop dense stands. Extensive roots systems enable some populations to be extremely long lived. Like yellow starthistle, Russian knapweed is toxic to horses and can cause "chewing disease" to animals when certain levels are consumed.
Fields, rangeland, cultivated sites, orchards, vineyards, roadsides, ditchbanks, and disturbed, unmanaged places.
Seedlings are uncommon in the field because Russian knapweed reproduces mostly by creeping roots that send off new shoots. When seedlings are found look for egg- to spoon-shaped cotyledons (seed leaves) with a scaly coating on the lower surface. The first true leaves are pointed and the first several rosette leaves are football shaped to egg shaped and, in each case, are covered with a white powdery coating. Later developing rosette leaves are irregularly lobed with wavy edges.
The erect, thistle-shaped, open-branched, leafy plant is about 1 to 3 feet (30–90 cm) tall. Leaves are alternate to one another along the stem and vary in shape. Upper stem leaves are narrowly lance shaped to linear, smooth edged or toothed, and 2/5 to 1-1/5 inches (1–3 cm) long. Lower leaves are 1-3/10 to 4 inches (4–10 cm) long, deeply lobed or have wavy edges. After flowering in summer, stems die back in the late fall and new shoots emerge in spring.
Flowers bloom from May through September. Flower heads are produced at the ends of branches. They consist of about thirty white, pink, or lavender-blue (disk) flowers interspersed with bristles gathered above a green, somewhat spherical base with overlapping bracts (reduced leaflike structures) that look like scales.
The small, 1/10 to 1/6 of an inch (3–4 mm) long, one-seeded fruit is white or pale gray, narrowly egg shaped, and attaches to a tuft of soft white bristles (pappus) that are unequal in length and fall off at maturity.
Seeds are white or pale gray, narrowly egg shaped, and have a scar at the base where a tuft of bristles was formerly attached.
Reproduces mostly by shoots budding from creeping roots and less often by seed.