Weed of the Month
December's weed of the month is Bull Thistle (Cirsium Vulgare)
Why is it a Noxious Weed?
Bull thistle may outcompete native plants and desirable wildlife and livestock forage plants. It can invade most any disturbed habitat and grow in dense thickets. The price of hay may decline with the presence of bull thistle.
How would I identify it?
Bull thistle is a biennial herbaceous plant growing between 3 to 7 feet tall with one upright branched stem. It grows a rosette (cluster of radiating leaves at plant base) in its first year and blooms in its second year.
Flower heads are many, 1.5 to 2 inches in diameter. Bracts at base of flowerheads are spine-tipped. Flowers are purple or rarely white, blooming July through September.
Leaves alternate and are coarsely lobed. Each lobe has a spined tip. Leaf bases extend downward from the leaves along prominent ridges of the stem. Upper leaf surface is rough with bristle-like spines while the undersides are covered with white woolly hairs.
Stems are spiny-winged from leaf bases, extending downward producing a winged ridge effect.
Fruit Seed Description
Seeds are less than 0.16 inches (4 mm) long.
Where does it grow?
Bull thistle colonizes primarily in disturbed areas such as pastures, roadsides, and ditch banks as well as in hayfields, disturbed prairies and logged mountain areas.
Click here to see a county level distribution map of bull thistle in Oregon.
In Oregon Noxious weeds fit into classifications;
A weed of known economic importance which occurs in the state in small enough infestations to make eradication or containment possible; or is not known to occur, but its presence in neighboring states make future occurrence in Oregon seem imminent. Recommended action: infestations are subject to eradication or intensive control when and where found.
A listed Weed:
B listed Weed:
A weed of economic importance which is regionally abundant, but which may have limited distribution in some counties. Recommended action: Limited to intensive control at the state, county or regional level as determined on a site specific, case-by-case basis. Where implementation of a fully integrated statewide management plan is not feasible, biological control (when available) shall be the primary control method.
T Designated Weed:
A designated group of weed species that are selected and will be the focus for prevention and control by the Noxious Weed Control Program. Action against these weeds will receive priority. T-designated noxious weeds are determined by the Oregon State Weed Board and directs Oregon Department of Agriculture (ODA) to develop and implement a statewide management plan. T-designated noxious weeds are species selected from either the A or B list.
How does it reproduce?
Bull thistle reproduces by seed and not by vegetative means.
How do I control it?
Hand-pull and dispose of flowering plants in trash to prevent seed spread. Mowing can be effective, but make sure the plants do not flower. If cut too early before flowering, plants may re-sprout and flower again that season. Remove stems from site if plants are cut or pulled with flowers.
Bull thistle seed production is impacted by the seedhead gall fly, Urophora tarlatan. This gall fly's larvae induce and feed on gall tissue in the developing bull thistle seedhead, reducing seed production up to 60%.Urophora stylata may significantly reduce seed production if bull thistle populations are sustained for many years. For more information about the biological control of bull thistle please visit the WSU Extension Integrated Weed Control Project.
Please refer to the PNW Weed Management Handbook, or contact your county noxious weed coordiantor.
Merle Keys, Wasco County Weed Department Supervisor
2705 E. 2nd Street
The Dalles, OR 97058
Phone (541) 506-2653
For more information see the resource pages linked below or contact the Wasco County Weed Department