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
2023
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
2022
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
2021
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
2020
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



