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Weed Identification

spotted knapweed

Family

Asteraceae

Scientific Name

Centaurea stoebe ssp. micranthos

Other Common Names:

knapweed

Synonyms (former Scientific Names):

Centaurea biebersteinii
Acosta maculosa
Centaurea maculosa

Habit

An erect growing biennial or short lived perennialthat grows to 1.2 m, is freely branching with rough hair (like sandpaper) and has purple to pink to white flowers.

Leaves

Produce rosette leaves in the first year that are composed of compound leaves with several irregularly lobed segments. The leaves of the second season are alternate, 5-15 cm long, moderately hairy with translucent dots covering the leaf surface. If basal leaves are present they are long stalked.

Identifying Characteristics

Has floral bracts, like leaves directly below the flower, that are black tipped, with purple to pink (sometimes white) flowers, and leaves are covered with translucent dots. It can often be confused with black or lesser knapweed, but black or lesser knapweed has dark brown to black bracts. It can also be confused with meadow knapweed, which has dark brown to tan bracts, corn cockle and cornflower. However, the flowering leaves of spotted knapweed are more lobed. Stems are wirey and has hairs.

Flower Seed Head

Has purplish to pink flowers that are 6-10 mm wide and 8-12 mm long. Flower heads are oval in shape before they open and occur at the end of the stalks. Bracts around the flower appear brown with black tips often.

Seed Fruit

Each floret produces a single seeded achene. Seeds are around 2 to 3 mm long, oblong, and grayish brown in color. Seed surfaces will have 10 to 15 faint ridges and several white hairs about 0.2 mm long.

Where Found

Found in the northeastern United States and along the pacific coast and as far east as Virginia. It is not a serious problem in cultivated crops; however, its early spring growth allows it to become a problem in pastures. This plant also produces a chemical that prevents other plants from growing in close proximity to it. This very competitive plant settles itself in any disturbed soil robbing other plants of moisture and nutrients.

Growth Habit

upright and nonwoody

Thorns or Spines

not present

Approximate Flower Diameter

Varies: 
pencil
dime

Dominant Flower Color

Varies: 
purple
pink

Flower Symmetry

radial symmetery

Leaf Hairs

has hairs

Leaf Shape

Varies: 
lance
oval

Leaf Arrangement

Varies: 
alternate
rosette

Leaf Margin

lobed

Leaf Structure

Varies: 
simple
pinnate

Leaf Stalk

Varies: 
none
shorter than leaf

Stem Hairs

has hairs

Stem Cross Section

Varies: 
round or oval
square or multi-edged

Milky Sap

not present

Root Structure

taproot

Life Cycle

perennial

Ochrea

not present

Plant Type

Herb