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.

September Weed Profile

Bull Thistle

(Cirsium Vulgare)

Bull thistle is one of Colorado’s List B thistles. Originally from Europe, Asia, and Northern Africa, it is now found in all 50 states. It invades native grasslands and rangelands, reducing the forage that wildlife and livestock depend on.

Its dark green deeply lobbed leaves distinguish it from the other native and invasive thistles. Its flowers are urn-shaped and light purple with spiny bracts and can be seen June-September. The 1.5-inch-wide flowers grow at the end of branches, usually singly but sometimes in small groups.

Because they are biennials, the plants spend their first year as a rosette (seedling). In the second year, they form a central stalk that branches as it matures. Plants can grow to seven feet tall but are usually 3-4 feet tall. Stems are winged and spiny.

Herbicide is most effective in the fall at the rosette stage or in the spring through the early bolt stage. Treating older plants is not effective because once the flowers form, seeds will still be viable. Control this time of year is limited to removal. Flowers need to be cut, bagged, and put in the trash.

Visit Jeffco Invasive Species Management to learn more www.jeffco.us/jcism.

Weed of the Month is a joint effort between Jefferson County Invasive Species Management, CWMA, and Archuleta County.

Items of Interest

Weed of the Month


January Weed Profile – Common Mullein

February Weed Profile – Cypress Spurge

March Weed Profile – Dames Rocket

April Weed Profile – Sulfur Cinquefoil

May Weed Profile – Poison Hemlock

June Weed Profile – Plumeless Thistle

July Weed Profile –  Puncturevine

August Weed Profile – Common Tansy

September Weed Profile – Bull Thistle


January Weed Profile – Knotweed

February Weed Profile – Rush Skeletonweed

March Weed Profile – Black Henbane

April Weed Profile – Chinese clematis

May Weed Profile – Houndstongue

June Weed Profile – Milk Thistle

July Weed Profile – Oxeye Daisy

August Weed Profile – Spotted Knapweed

September Weed Profile – Perennial Pepperweed

October Weed Profile – Yellow Flag Iris

November Weed Profile – Common St. Johns Wart

December Weed Profile – Field Bindweed


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

November Weed Profile – Diffuse knapweed

December Weed Profile – Poison hemlock


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