List B Species Rangeland, pasture, and riparian site recommendations Colorado Dept. of Agriculture, Scotch thistle Conservation Services Division Identification and Management 700 Kipling Street Suite 4000 and people. Scotch thistle invades Lakewood, CO 80215 rangeland, overgrazed pastures, 303-239-4100 roadsides, and irrigation ditches. It also prefers high-moist soil areas adjacent to creeks and rivers. he key to effective control of TScotch thistle is maintaining healthy pastures and rangeland, guarding against disturbance or overuse, and as with most biennials limit seed production. To reduce seed production, plants with buds or flowers should be collected and immediately disposed of or destroyed. Identification and Chemical control is most effective when plants are in rosette stage, spring Impacts or early fall. Mechanical controls can be used to eliminate small patches or cotch thistle (Onopordum plants in a later growth stage. Details Sacanthium or O. tauricum) on the back of this sheet can help to is a non-native biennial forb that create a management plan compatible reproduces solely by seed. A biennial with your site ecology. is a plant that completes its lifecycle within two years. During the first cotch thistle is designated as a “List year of growth, Scotch thistle appears SB” species in the Colorado Noxious as a rosette in spring or fall. Rosettes Weed Act. It is required to be either can be 1 to 2 feet in diameter. During eradicated, contained, or suppressed the second year in mid to late spring depending on the local infestations. the stem bolts, flowers, sets seed, For more information visit www. and the plant dies. A prolific seed colorado.gov/ag/csd and click on the producer, Scotch thistle can produce Noxious Weed Management Program. up to 14,000 seeds per plant. Or call the State Weed Coordinator at the Colorado Department of Scoth thistle cotch thistle can grow up to 12 Agriculture, Conservation Services Sfeet tall. Stems are numerous, Division, 303-239-4100. branched, and have broad, spiny wings. The leaves of species acanthium are large, grayish- green, spiny, and covered with fine dense hair giving the leaf a woolly appearance. The leaves of the species tauricum are similar in size, but are Key ID Points not hairy, smooth and bright green. On both species, the leaves have a 1. Flower heads distinct mid-rib. The flowers are cluster 2-5 and violet to reddish in color, numerous Onopordum acanthium or O. tauricum are purple to (70-100/plant), and are surrounded dark red in by spine-tipped bracts. The plants color. flower from mid-June to September. 2. Leaves are alternate, stalk- less and hairy ue to the robust, spiny nature of underneath. DScotch thistle, this plant can act as a living barbed wire fence, making Photos © Map above: Crystal Andrews, Colorado Department of Agriculture; All other photos: Kelly Updated on: areas impassible for wildlife, livestock, Uhing, Colorado Department of Agriculture. 08/08 Integrated Weed Management recommendations List B Species CULTURAL Establishment of selected grasses can be an effective cultural control of Scotch thistle. Contact your local Natural Resources Conservation Service for seed Integrated Weed mix recommendations. Maintain healthy Management: pastures and prevent bare spots caused by overgrazing. Bareground is prime habitat for weed invasions. Scotch thistle is best controlled in the rosette BIOLOGICAL stage. For small Urophora stylata, a fly predator, is used infestations, to help control this thistle. The female fly Scotch thistle lays eggs in the seed head of the thistle. can be controlled The maggot then consumes the seed in by severing its the flower. This species has overwintered taproot 1-2 in Colorado but the limited numbers will inches below the not allow for general redistribution. For ground. Control more information, contact the Palisade can be enhanced Insectary of the Colorado Department of by a follow-up Agriculture at 970-464-7916. application of herbicides to the surviving rosettes. It is imperative MECHANICAL to prevent seed Any mechanical or physical method that production. Do severs the root below the soil surface will not allow Scotch kill Scotch thistle. Mowing or chopping is thistle flowers to most effective when Scotch thistle plants appear. are at full-bloom. Be sure to properly dispose of the flowering cut plants since seeds can mature and become viable after the plant has been cut down. HERBICIDES Management Recommendations NOTE: The following are recommendations for herbicides that can be applied to range and pasturelands. Rates are approximate and based on equipment with an output of 30 gal/acre. Please read label for exact rates. Always read, understand, and follow the label directions. The herbicide label is the LAW! Scotch thistle HERBICIDE RATE APPLICATION TIMING Picloram (Tordon 22K 1 pint/acre + 0.25- Apply spring or fall in the rosette stage. - *Restricted Use*) 0.5% v/v non-ionic surfactant Aminopyralid 7 fl. oz./acre + 0.25- Apply spring or fall in the rosette stage. (Milestone) 0.5% v/v non-ionic surfactant Metsulfuron 2 oz. product/acre Apply rosette to early bolt stages of growth. (Cimarron X-tra) 0.25-0.5% v/v non- (Spring) ionic surfactant http://www.colorado.gov/ag/csd Photos © Kelly Uhing, Colorado Department of Agriculture..