Diffuse knapweed also exhibits an allelopathic effect on associated species establishing essentially single-species stands. Diffuse knapweed readily colonizes different soils with a wide range of chemical and physical properties. It prefers open habitat, quickly invades disturbed sites, and is well adapted to semiarid areas. Reproduction is by seed, which germinates over a broad range of environmental conditions. Germination occurs in the fall or early spring when environmental conditions are favorable.
Moisture appears to be the limiting factor.
Diffuse knapweed seedlings were not found to emerge from below a soil depth of 3 cm. Seedlings develop into rosettes, and maximal root growth occurs when the plants are rosettes. Plants that overwinter as a rosette bolt in early May producing one or rarely two stems. Flowering occurs from July through September. If moisture is adequate the seeds will germinate and develop into rosettes by fall.