Weed of the Month

Learn about noxious weeds and steps you can take to manage or eradicate.  Each month we will bring you a new profile and helpful resources.

November Weed Profile

Diffuse Knapweed

Centaurea diffusa

Centaurea x psammogena

Diffuse knapweed is one of the most prevalent noxious weeds in western North America. In some places it can be a short-lived perennial but acts as a biennial throughout most of Colorado.

Diffuse knapweed has white to lavender flowers with tan bracts below. The bracts are fringed on the sides and have a terminal spine. They will sometimes have darker spots at the base of the spine but not always.

Diffuse knapweed vegetation contains alkaloids that may act allelopathically to prevent other plants from germinating.

The cells at the base of the plant undergo abscission in the fall which allows the plants to break off and tumble across the landscape, dropping its many seeds as it goes.

It is difficult to distinguish diffuse from the hybrid in the field. In fact, the majority is thought to be the hybrid. The only way to definitively tell whether what you have is diffuse or the hybrid is through DNA analysis, which is expensive and not practical for most weed management programs.

Control is easy if you can access the site at the best growth stage. The optimum stage for removal or herbicide treatment is at the rosette to pre-bud stage. There are also several biological control agents. Check with the Insectary to see if they will work on your site.


Blair, A.C. and Hufbauer, R.A. (2010), Hybridization and invasion: one of North America’s most devastating invasive plants shows evidence for a history of interspecific hybridization. Evolutionary Applications, 3: 40-51

Know Your Knapweeds – NDSU

Items of Interest

Weed of the Month


January Weed Profile – Dalmatian Toadflax

February Weed Profile – Common Mulllein

March Weed Profile – Mediterranean Sage

April Weed Profile – Saltcedar

May Weed Profile – Absinth Wormwood 

June Weed Profile – Eurasian watermilfoil 

July Weed Profile – Yellow starthistle 

August Weed Profile – Hairy willowherb

September Weed Profile – Yellow toadflax

October Weed Profile – Chamomile


January Weed Profile – Cheatgrass

February Weed Profile – Myrtle Spurge

March Weed Profile – Hoary Cress

April Weed Profile – Leafy Spurge

May Weed Profile – Scotch Thistle

June Weed Profile – Dyer’s Woad

July Weed Profile – Orange Hawkweed

August Weed Profile – Purple Loosestrife

September Weed Profile – Teasel

October Weed Profile – Canada Thistle

November Weed Profile – Russian Knapweed

December Weed Profile – Russian Olive