Diffuse knapweed is a biennial species that can sometimes live longer than 2 years and turn into a short-lived perennial. Spotted knapweed (C. stoebe ssp. micranthos) is very similar but with purple flowers. The two species hybridize producing plants with white and purple flowers on the same plant. Both are prolific seeders.
Growth Habit – During the first year of growth, the plant appears as a low growing rosette of leaves. During the second year, Diffuse Knapweed grows into a 1 to 3 feet tall plant with many branches, blooming from June to September. This species is often accompanied by Spotted Knapweed and the two often hybridize.
Flower – Small white flowers are found at the end of the branches and prickly spines surround each flower. Spotted knapweed has purple flowers and hybrids have both cream and purple flowers.
Leaves – Greyish-green, alternating, basal leaves whorled and covered with fine hair. Upper leaves are much reduced.
Impact – It has been discovered that the roots of this plant release a chemical which depresses growth of any other plant in its vicinity leading to an invasive monoculture of Diffuse Knapweed.
Reproduction – It only reproduces by producing seeds.
Management – Control is through the cutting of the plant before the seeds are dispersed, or the rosettes can be dug up or sprayed with herbicide before they develop and produce seeds the second year. Control takes many years of persistence to succeed.
- Pull, dig or spray first year rosettes
- Monitor for new rosettes
- Pull, dig, mow or spray plants that are getting ready to bolt but not yet flowering
- Pull, clip or dig mature plants that have set seed however handle with great care. Place them into bags gently and dispose