Considering that diffuse knapweed reproduces solely by seeds, it should be removed before it bears flowers and fruits. Patches of this plant are quite noticeable in the field, so it is easy to locate its colonies. Hand-pull the plants or dig them out of the ground to remove them. In case of large-scale infestation herbicides can be used. Consult an agricultural expert before picking the right herbicide. Spot treatments can be performed in the field, as diffuse knapweed plants are usually clustered together. Spray on a windless day to avoid drift and negative effects on other species. Diffuse knapweed tends to invade empty patches of soil, where there are no competing species. To prevent its establishment, make sure that your field is covered with vegetation evenly.