Spotted Knapweed (Centaurea stoebe)

Taxonomy

Domain	Eukarya
Kingdom	Plantae
Phylum	Magnoliophyta
Class	Magnoliopsida
Superorder	Asteranae
Order	Asterales
Family	Asteraceae
Subfamily	Carduoideae
Tribe	Cardueae
Genus	Centaurea
Species	Centaurea stoebe

Scientific Name

Centaurea stoebe ssp. micranthos

Scientific Name Synonyms

Centaurea biebersteinii

Acosta maculosa

Common Name

spotted knapweed

Miller, J.H., E.B, Chambliss, N.J. Loewenstein. 2010. A Field Guide for the Identification of Invasive Plants in Southern Forests. General Technical Report SRS-119. Asheville, NC. United States Department of Agriculture, Forest Service. 126 p.

Plant

A bushy, winter-hardy, upright perennial forb living 3 to 5 years or sometimes longer, often in dense infestations. A deep taproot supports an initial rosette of bluish-green, woolly, dandelion-like leaves. Stem leaves pinnately dissected becoming smaller and less dissected toward the tips of multiple woolly, hairy stems. Midplant branches topped by a few to many pink-to-lavender thistle flowers constricted below the plume by distinctively fringed bracts with black tips (thus the common name “spotted”) to produce thousands of tiny bristle-topped seeds. Dead tops remain in winter with new sprouts in spring. A severe invasive in much of the U.S. and now invading the South.

Stem

Round, upright in multistemmed clumps, to 3.2 feet (1 m) tall, covered in dense woolly hairs and having ridges that extend downward from leaf bases. Upper stems wiry and slender with many alternate branches that end in flowers. Young plants may have only 1 stem with 1 flower while older plants can have hundreds of flower-tipped branches.

Leaves

Rosette leaves bluish green, hairy and covered with shiny specks interspersed with translucent dots, 4 to 8 inches (10 to 20 cm) long decreasing in size above midstem, alternate, spiraling and jutting out and upward. Basal leaves deeply divided into elliptic or linear lobes that can appear like leaflets along a wide whitish-to-purplish midvein. Lobes become more slender and fewer on upper leaves, with simple leaves near the flowers.

Flowers

June to November. Each branch topped by an egg-shaped flower bud covered with overlapping rows of dry, fringed (not spiny) green bracts with black tips. Bracts eventually part at top to allow a constricted pinkish-to-lavender thistle plume to radiate out and up, 0.75 to 1 inch (2 to 2.5 cm) wide and overall about 1 inch (2.5 cm) long, composed of hundreds of tiny perfect flowers with the outer ones being sterile.

Fruit and seeds

June to February. Tightly packed seed heads of oblong, brownish, hairy nutlets (achenes), 0.1 inch (3 mm) long, topped by short stubby bristles. Hairs and bristles for clinging. Upwards of 1,000 seeds per plant.

Ecology

Rapidly colonizes roadsides and disturbed lands, especially dry and droughty sites, to invade adjacent undisturbed prairies and open forests. A severe invasive spreading into the South by seeds equipped for dispersal by wind, water, livestock, wildlife, and human activity, with viability in the soil for many years. Seeds germinate throughout the growing season. The roots secrete allelopathic chemicals to inhibit other plant seeds from germinating.

Resembles

other thistles and knapweeds, but none have sharp spines, highly dissected leaves with narrow lobes or distinct black spots on the involucre.

History and use

Accidentally introduced from Europe into the Northwestern United States in the 1890s and later spread rapidly across the West and Midwest and into the Northeast and now the South. Listed in most Western States’ noxious weed laws.

Distribution

Found throughout the region except OK and TX.

Management strategies

• Bag and dispose of plants and seed heads in a dumpster or burn.
• Treat when new plants are young to prevent seed formation.
• Pull, cut, and treat when fruit are not present.
• Minimize disturbance within miles of where this plant occurs, and anticipate wider occupation when plants are present before disturbance.
• Manually pull when soil is moist to ensure removal of all roots.
• Prescribed burning is not recommended.

Recommended control procedures

• Thoroughly wet all leaves with one of the following herbicides in water with a surfactant: Milestone VM* at 5 to 7 ounces per acre applied at the spring bolting or fall rosette stages; Tordon K* ‡ at 0.25 to 0.5 pounds per acre will control spotted knapweed plants and seedlings for 2 to 3 years; Tordon K* ‡ should be applied either in fall when the plants are in the rosette growth stage, or in spring during the bud-to-bloom stage; or Vanquish* at 1 to 2 pounds per acre (acid equivalent, see label). May require annual followup treatment for a minimum of 2 years; apply Overdrive* as a 0.2-percent solution (0.6 ounce per 3-gallon mix) plus Tordon 101* ‡ as a 4-percent solution (2 pints per 3-gallon mix), or Garlon 3A as a 2- to 3-percent solution (8 to 12 ounces per 3-gallon mix) three to four times per year for 2 years.

* Nontarget plants may be killed or injured by root uptake.

‡ When using Tordon herbicides, rainfall must occur within 6 days after application for needed soil activation. Tordon herbicides are restricted use pesticides.

Images

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June

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