Diffuse Knapweed (Centaurea diffusa Lam.)
Diffuse knapweed is a biennial or short-lived perennial (and occasionally annual) forb that develops from a deep taproot. In its first year, it forms a basal rosette. In subsequent years, it produces an erect main stem that typically grows up to 2 feet tall. The stem is often branched near or above the base, giving the plant a rounded, bushy appearance.
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The entire plant is covered with fine hairs, creating a grayish- to sage-green color. Leaves are deeply divided into narrow, linear lobes. Basal leaves have petioles, while stem leaves become progressively smaller and less divided toward the top of the plant.
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Flowering occurs from early summer through fall. Flowers are borne singly at the ends of stems, though they may occasionally appear in small clusters. Flower heads are usually white but may sometimes be pink to lavender. Each flower head is enclosed by bracts tipped with a prominent terminal spine and four to five smaller spines along the margins—an important distinguishing feature. This single terminal spine differentiates diffuse knapweed from spotted knapweed, which lacks a strong central spine.
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Seeds are small (approximately 0.5 mm), oblong, dark in color, and marked with fine longitudinal lines.
Quick ID Guide
Life Cycle:
• Biennial or short-lived perennial
• Deep taproot
• Basal rosette first year
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Height:
• Up to 2 feet tall
• Bushy, branched stems
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Leaves:
• Gray-green, hairy
• Deeply divided into narrow lobes
• Basal leaves with petioles
• Smaller leaves up the stem
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Flowers (Early Summer–Fall):
• Usually white (sometimes pink/lavender)
• Solitary at stem tips
• Bracts with one long terminal spine
• 4–5 smaller side spines
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Seeds:
• Small, oblong
• Dark with fine longitudinal lines
Management Options
Prevention and cultural control practices should be emphasized whenever possible to limit the spread and establishment.
Because diffuse knapweed grows from a taproot, it can often be effectively managed by hand pulling or digging if the entire root is removed. Protective gloves and long sleeves are recommended when handling knapweed species, as they may cause skin irritation. Mowing will not eliminate established stands but can reduce flower production if conducted before seed set. Do not mow once seeds are present.
Large or established infestations are typically controlled with herbicides. Spring and fall applications targeting rosettes are most effective, although treatments applied to bolting plants may also provide control. Always read and follow the herbicide label before use. Contact Sublette County Weed & Pest for guidance on appropriate product selection, application rates, and timing
