Answer: I am pleased to provide you with information on controlling thistles.
Most thistles are biennial and/or perennial forbs with very persistent root systems. Any control system must necessarily be targeted at depleting the root reserves and starving the plant over time. In this letter I will address several methods of control that have had varying degrees of success.
Eleven specific insect feeders and over seventy general feeders affect Canada thistle. These include such insects as the stem weevil, the stem gall fly, the defoliating beetle, and the seed head weevil (Harris, 2005). The ATTRA publication on thistle control highlights some of these insects. Biological controls are not enough on their own, but combined with defoliation, grazing management, and crop rotations, can help reduce thistle dominance by weakening the stand over time. Some states have coordinated noxious weed control beneficial insectaries and strategic insect releases. You might try contacting your local Natural Resources Conservation Service (NRCS) office or Cooperative Extension (CES) office to see if insect releases are available in your area. NRCS info can be found in the federal listings in your phone book, and CES can be found in the county listings.
Controlling Thistle in Pastures
In pastures, thistles take advantage of bare spots to get established. Bare spots are prevented by proper grazing management and providing adequate fertility and lime to assure a dense stand of forage. The ATTRA publication Pasture, Rangeland, and Grazing Management, has detailed information on grazing management, and should assist you in developing a grazing system to minimize weeds.
• Consider rotating perennial pastures with annual cover crops such as small grains, field peas, or clovers to break weed cycles.
• The weakest point is seedling stage – harrowing in the late spring when rosettes begin to form can be effective in removing new seedlings. Older plants will not be controlled with a light harrowing.
• Devise methods to control top growth, seed production, and deplete root reserves – Mow or till thistles at bud-break (pre-flowering) if possible to deplete root reserves. Consider row cultivation or rotary hoes. Also consider deep-tined implements that will pull roots to the surface, not merely cut them off in the soil. Cut roots will grow back from root or crown buds. This type of control will likely require at least two years of control.
• For heavily infected fields, tillage every three weeks for an entire growing season will effectively deplete root reserves. Consider following with a winter cover crop, pasture planting, or cereal.
Cultural Control in Row Crops
• Plant row crops with a closer row spacing, or increase the seeding rate. These methods, coupled with early planting, can give crops a competitive advantage to germinating perennial weeds.
• Consider placing alfalfa or sweet clover in the rotation. Alfalfa for three years has been shown to reduce Canada thistle stands in some studies. Alfalfa is also a deep-rooted legume abundant in soil building qualities.
• Utilize a grass-clover or vetch intercrop/living mulch/cover crop between rows. Research has shown that these types of intercropping methods are effective at (1) increasing water use efficiency, (2) buffering soil temperatures, and (3) reducing weeds in row crops. Clovers will fix atmospheric nitrogen and actually make some of the resulting nitrates available for uptake by the principle crop. If the cover crops are mowed periodically, root death and node separation can occur, yielding more nitrogen to the principle crop.
• Lastly, remember to sanitize equipment after bringing it out of an infected field. This includes trucks and four-wheelers. Irrigation water can carry seeds as well. If you rely on ditches or streams for irrigation, ensure no weeds are growing on the ditch banks.
Rinehart, L. 2006. Pasture, Rangeland, and Grazing Management. Butte, MT: ATTRA.
Sullivan, P. 2004. Thistle Control Alternatives. Fayetteville, AR: ATTRA.
Harris, P. 2005. Canada Thistle Cirsium arvense (L.) Scop. Agriculture and Agri-food Canada.
Farm Facts: Canada thistle and its control. 2005. Saskatchewan Agriculture and Food.
Rasmussen,I.A., and M. Askegaard. 2004. Crop rotation limits Canada thistle, but not Couch grass or annual weeds. Danish Institute of Agricultural Sciences.
Sullivan, P. 2003. Principles of Sustainable Weed Management for Croplands. Fayetteville, AR: ATTRA.
Sustainable Agriculture Network. A Whole-Farm Approach to Managing Pests. Sustainable Agriculture Research and Education.
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