The Association of South Dakota County Weed and Pest Boards have defined seven plants as noxious weeds. These are Russian Knapweed, Hoary Cress, Perennial Sow Thistle, Leafy Spurge, Canada Thistle, Purple Loosestrife, and Salt Cedar.
All of these noxious weeds are non-native. Hoary Cress is suspected to have been introduced from Eurasia. According to the South Dakota Department of Agriculture Web site, 2005 data shows that Hoary Cress has not yet been reported in Clay County, although it has been confirmed on fewer than 100 acres in Yankton County.
Russian Knapweed was also introduced from Eurasia; speculations are that it arrived as a crop seed contaminant. Russian Knapweed has been spotted in Yankton and Turner counties; however, it has not been found in Clay County.
Perennial Sow Thistle covers five to 10,000 acres of Turner County. It has also been discovered in Lincoln and Union Counties. Clay County had not found any infestations as of 2005. This thistle is also invasive, having been introduced from Europe.
Purple Loosestrife has been found on fewer than 100 acres in Clay County. Often planted as a garden ornamental, this weed invades wet areas. It is known to have originated in Africa and Eurasia.
Saltcedar is South Dakota's newest noxious weed. The South Dakota Department of Agriculture reported that Saltcedar is living on fewer than 100 acres in Union County. As of 2005, it was not reported to be in Clay County. This plant can grow at least 25 feet tall, preferring to root in wet areas along waterways. Saltcedar was introduced from Eurasia for stream bank stabilization. Some have planted it for ornamental reasons.
Canada Thistle and Leafy Spurge are the two noxious weeds we have left on the list.
Canada Thistle originated in Africa and Eurasia; it is suspected that Leafy Spurge came to the western United States as a seed contaminant from Eurasia. Clay County, has reported that both Leafy Spurge and Canada Thistle reside on over 10,000 acres of land each. Clearly, these are the major invasive plant species in Clay County.
Achieving successful control of Canada thistle and Leafy Spurge is not an easy task. South Dakota State University Fact Sheet 525N, "Noxious Weed Control: 2006" is an excellent resource when looking to apply herbicide to these pesky weeds. It is recommended to follow label restrictions, as well as take safety precautions when handling and spraying pesticides.
Canada Thistle and Leafy Spurge have the most options for control in the spring before bud stage. If fall treatment of Leafy Spurge is desired, Plateau or 2, 4, D, Ester are the only recommended herbicides. Canada Thistle may be treated in the fall with 2, 4, D, Dicamba products, Telar, Overdrive, or Tordon 22K. Keep in mind that late fall treatment after a killing frost is not recommended because the plants won't accept the chemicals as readily, thereby reducing the desired effects. More information about noxious weed control can be obtained with a call or visit to the Clay County Extension office.