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SCHMIDT: Fall is a good time to manage noxious weeds

By Staff | Aug 17, 2018

While it does not seem like we should be far enough into the year to be talking about fall, it is right around the corner. Fall is a good time to control noxious weeds.

A noxious weed is a plant species that has been designated by county, state, and/or federal agricultural authority as one that replaces native plants and disrupts natural ecosystems. Noxious weeds are typically non-native introduced plants that are capable of rapid growth and multiplication. Due to their aggressive growth habits, infestations of noxious weeds occurring in pastures, rangelands, and other natural areas readily spread onto adjacent land reducing crop yield and quality.

North Dakota’s noxious weed list includes:

– Absinth wormwood

– Canada thistle

– Diffuse knapweed

– Leafy spurge

– Musk thistle

– Purple loosestrife

– Russian knapweed

– Spotted knapweed

– Yellow toadflax

– Dalmatian toadflax

– Salt cedar

Noxious weed control is everyone’s responsibility and their control is to everyone’s benefit. It is the duty of each person who owns or controls land in North Dakota to effectively control noxious weeds on such land. County weed boards are the local entity responsible for administration of noxious weed control at the county level. Currently, there are weed boards in all of North Dakota’s 53 counties, and in 7 cities (Bismarck, Devils Lake, Dickinson, Fargo, Grand Forks, Wahpeton and Mandan). To learn more about specific noxious weed control regulations, check out “North Dakota’s Noxious Weed Law and Regulations” publication at “http://ndweeds.homestead.com/15_NoxiousWeeds_web.pdf”>ndweeds.homestead.com/15_NoxiousWeeds_web.pdf .

Prevention is the most powerful form of weed management. One of the best ways to prevent weeds from spreading is to control existing infestations. Fall is an excellent time to treat Canada thistle, knapweeds, leafy spurge, and other noxious weeds around the farm and home. For many of these weeds, the growing season is not finished. For the best results, treatments should be applied to new vegetative growth, which is actively growing from underground plant parts. Treating old leaves or stems will be less effective as herbicide will not be translocated throughout the plant. Plan to spray when the growth is 6 to 8 inches. The more new growth that is present at the time of herbicide application the better the chances for seeing good results.

If a frost occurs before an herbicide application is made, it is advised to wait 24 hours to evaluate the foliage. A killing frost is classified as one that occurs when temperatures fall to below 26-28 degrees Fahrenheit and remain there for at least 4 hours or more. After a killing frost, leaves will be wilty and turning black if damaged. If herbicides are applied after this occurs, success will be reduced. If applying herbicides after a frost it is recommended to try to apply the herbicide at a time when the expected high for the day will be over 60 degrees Fahrenheit.

Success of a fall weed control herbicide program can be enhanced by mowing or tilling areas two to four weeks prior to anticipated herbicide application. This sets the stage for the re-growth to develop and opens up the area for the herbicide to be applied.

Yellow Toadflax Presence Increasing

Since being added to the state’s noxious weed list in recent years, toadflax infestations are becoming more numerous in North Dakota counties, including Pierce County. There are two species of toadflax considered invasive in North Dakota and both are on the state’s locally noxious weed list. Yellow toadflax and Dalmatian toadflax are members of the figwort family, which is more commonly known as the snapdragon family. Like most noxious weeds, these two species of toadflax, were introduced to North America as ornamental plants that eventually escaped cultivation.

Landowners should be on the lookout for this noxious weed and waste no time in their efforts to control and contain this invasive noxious weed as it can spread up to 10 feet per year. The toadflax species are aggressive and will displace forage in pasture and rangelands. Yellow toadflax can be mildly poisonous to livestock that graze it. Although the toadflaxes may be slow to establish, once plants take root, control is very difficult since most herbicides are ineffective. Both species also have an extensive rhizomatous root system that spreads like leafy spurge.

Yellow and Dalmatian toadflax have distinct snapdragon-like flowers that are yellow with bearded orange throats. Yellow toadflax flowers are about one inch long and can vary from yellow to pale cream in color. Dalmatian toadflax flowers are about .75 to 1.5 inches long and tend to be a brighter yellow. Both species flowers appear as dense terminal elongated clusters. Dalmatian toadflax blooms in June and on into the fall, while yellow toadflax blooms later in August and September. The most distinctive difference between the species is that Dalmatian toadflax has broad heart-shaped leaves that clasp a woody stem, whereas yellow toadflax has narrow linear leaves with a narrow stem. To identify the toadflaxes and other noxious weeds, check out the following publication “A Guide to North Dakota Noxious and Troublesome Weeds” at www.ag.ndsu.edu/pubs/plantsci/weeds/w1691.pdf .

Chemical control of toadflax can be achieved, but as mentioned previously, there are a limited number of effective herbicides available and often requires repeated treatments at high rates. Tordon (picloram), Plateau (imazapic), and Telar (chlorsulfuron) will control Dalmatian toadflax when applied at maximum use rates during flowering or late fall. No herbicide is labeled for Yellow toadflax control, but research at North Dakota State University has found that a combination treatment of Tordon plus Overdrive (dicamba plus diflufenzopyr) applied in late fall will reduce yellow toadflax infestations for at least two years. Please see the latest edition of the “North Dakota Weed Control Guide”, for application rate and timing recommendations. Weed Control Guides are available in most local county Extension offices. The 2018 Guide can be accessed online at www.ag.ndsu.edu/weeds/weed-control-guides/nd-weed-control-guide-1 .

It is important to note that burning is not an effective control method because soil temperatures do not get high enough to kill the roots. Burning may even have a detrimental effect and cause an increase in the number of toadflax stems due to reduced cover and competition by competitive forage species.

Biological control in the form of stem-boring weevils is available for controlling Dalmatian toadflax. However, these weevils have been unsuccessful in yellow toadflax stands due to the much narrower stem of yellow toadflax interfering with survival of the weevil larvae.

For more information on either of this week’s topics call the NDSU Extension Service Pierce County office at 776-6234 ext. 5.

Schmidt is the NDSU Extension agent for Pierce County.

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