Identification and Life History

Identification

Basal leaves are whorled from the central crown of the elongated taproot to form a low-lying rosette. Rosette leaves grow up to eight inches long (20 centimeters) and two inches (5 centimeters) wide, have short stalks, and are deeply divided into lobes on both sides of the midrib. Stem leaves are similarly divided but sessile, alternately arranged on the stem, and become progressively smaller and less divided higher up the stem. The uppermost leaves are small and bract-like. Stems grow from one to three feet tall (0.3 to 0.9 meters), with numerous branches giving the plant a ball-shaped form that tumbles in the wind when broken off. The flowerheads are urn-shaped, about 0.2 inch (3 to 6 millimeters) wide and up to 0.63 inch (16 millimeters) long. The bracts surrounding the heads are yellowish-green with buff or brown margins fringed with spines, and ending with a long spreading, stiff spine. Heads are solitary or in clusters of two or three at the branch ends. Most flowers are white, but pink or purple flowers are not uncommon. The achenes (seeds) are 0.12 inch (3 millimeters) long, light-brown to black, with a pappus of bristle-like hairs that are scale-like to less than 1 millimeters long.

Life History

Normally a biennial, diffuse knapweed can complete its life cycle in a single year when germination occurs in the early spring. Otherwise, this species behaves as a short-lived perennial, depending on prevailing environmental conditions. Plants generally die after seed production, and re-growth from the root crown after seed production is rare. Seeds germinate in the fall or spring and seedling survival is dependent on seasonal precipitation. Because germination occurs throughout the growing season, seedling emergence is distributed over time. These factors combined with a continuous seed rain and sequential establishment ensures population occupancy of available safe sites with individuals of different age classes (seedlings, rosettes, and flowering adults). Diffuse knapweed populations effectively draw resources from different levels of the soil profile, reducing intra-specific, while increasing inter-specific competition. This near-complete utilization of soil resources effectively results in monocultures of diffuse knapweed in heavy infestations. Under intense intra-specific competition, plants die before flowering or live for several years as rosettes and die after seed production. Maximal root elongation occurs as seedlings develop into rosettes. Plants that overwinter as rosettes will bolt in early May. Flowering occurs from June to September, provided that adequate moisture is available and temperatures are mild. Pollination is usually via insects; however, plants are self-compatible. Individual plants can produce over 700 seeds and population seed production has been recorded as high as 40,000 seeds/meter².

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Last Modified: 06/02/2008