Spotted knapweed (Centaurea maculosa Lam.)
Spotted knapweed is a highly aggressive invasive forb that reduces surrounding forage by releasing allelopathic compounds into the soil. It is typically a biennial or short-lived perennial that develops from a deep taproot.
In its first year, the plant forms a basal rosette. In subsequent years, it produces one or more erect stems that can grow up to 3 feet tall. Stems are often branched near or above the base, giving the plant a bushy appearance. The entire plant is covered with fine hairs, resulting in a grayish- to sage-green color.
Leaves are arranged alternately along the stems and are pinnately lobed. Basal leaves have petioles and are larger, while upper stem leaves become progressively smaller and less divided.
Flowering occurs from early summer through fall. Flower buds begin as rounded, bulb-like heads. Mature flowers are pink to purple (occasionally white) and larger than those of Russian knapweed. Flower heads are solitary at stem tips or may appear in small clusters. Each head is enclosed by stiff bracts tipped with dark brown to black margins, creating the characteristic “spotted” appearance.
Seeds are small (about 2.5 mm), oval, brown to black, and possess a short pappus that aids in limited dispersal.
Quick ID Guide
Life Cycle:
• Biennial or short-lived perennial
• Deep taproot
• Basal rosette first year
Height:
• Up to 3 feet tall
• Bushy, branched stems
Leaves:
• Grayish-green, hairy
• Alternate arrangement
• Deeply lobed basal leaves
• Smaller upper leaves
Flowers (Early Summer–Fall):
• Pink to purple (sometimes white)
• Larger than Russian knapweed
• Solitary at stem tips
• Bracts with dark “spotted” tips
Seeds:
• Small, oval
• Brown to black
• Short pappus at one end
Management Options
Prevention and strong cultural practices should be prioritized to limit the spread and establishment.
Because spotted knapweed develops from a taproot, small infestations can often be effectively managed by hand pulling or digging, provided the entire root is removed. Gloves and long sleeves should be worn when handling knapweed species, as they may cause skin irritation. Mowing will not eradicate established stands, but can help prevent flower and seed production if conducted before seed set. Do not mow once seeds are present.
Large or established infestations are typically managed with herbicides. Spring and fall applications targeting rosettes provide the most consistent control, though treatments applied to bolting plants may also be effective. Always read and follow the herbicide label before use. Contact Sublette County Weed & Pest for assistance with product selection, rates, and timing of application.
