ISSUE 15    August 17, 2006



I had one of our weed board officers stop in this morning and was inquiring about spraying Canadian Thistle after a freeze. He was wondering if there is any research that would suggest that this is a good time to spray.


To specifically answer your question, most all perennial weeds respond the same to fall-applied herbicides - if the leaf tissue that intercepts the spray is green and has good integrity and not damaged by freeze then the herbicide will be absorbed. The other major issue that affects activity of the herbicide is the temperature and weather following application. If warm temperatures follow for a few days after application then it is more likely for the herbicide (glyphosate) will be translocated throughout the plant resulting in better kill of underground roots. If cold day time temps follow application then control will be diminished. I believe a warm up is forecast the end of this week and next week so should still be time to spray.

I hope your predecessor kept previous Pest Report issues because there were several article written on fall Canada thistle control. See below the following issues:

2003 issue 12
2002 issue 16
2002 issue 15
2000 issue 15
1998 issue 14
1994 issue 17
1994 issue 15

See also Circular W-799 for more info.



The ND Dept of Ag has reported that leafy spurge has edged out Canada thistle as North Dakotaís worst noxious weed in 2005, but the reported acreage for both weeds was down slightly from 2004. The decrease in reported infestation may be due to effective roadside spraying, cost-share and other programs to limit and reduce the spread of noxious weeds.

The North Dakota Noxious Weeds: 2005 Survey Results report that North Dakota had 993,644 acres infested with leafy spurge in 2005, compared to 1,140,713 acres in 2004. Canada thistle infestation dropped from 1,085,224 acres in 2004 to 956,335 last year. Absinth wormwood, with 452,594 acres reported, was North Dakotaís third most extensive noxious weed in 2005.

Overall noxious weed-infested acreage fell from 3,375,479 reported acres in 2004 to 3,023,631 in 2005. The survey showed that infestations of all, but three, of the stateís 12 noxious weeds declined between 2004 and 2005. Slope County reported the largest area infested with noxious weeds Ė172,858 acres, most of it with field bindweed. Almost all North Dakota counties are now participating in an NDDA program that provides the counties with free global positioning system (GPS) units in return for noxious weed maps and other information. 

North Dakotaís 12 noxious weeds include absinth wormwood, Canada thistle, Dalmatian toadflax, diffuse knapweed, field bindweed, leafy spurge, musk thistle, purple loosestrife, Russian thistle, saltcedar, spotted knapweed and yellow starthistle.



The following is an article written by Chris Boerboom, Ext. Weed Scientist, U of WI. Even though Wisconsin is a fair distance away from ND and cropping and weather system are different, I believe the perception of weed control from glyphosate and issues relating to weed control are similar.

During my winter meetings I asked the audience if they thought weeds were more difficult to control with glyphosate now than 8 years ago. On average, 70% of those responding believed weeds are now more difficult to control with glyphosate. The response was amazingly consistent across the state and ranged from a low of 62% to a high of 79%.

You might ask if this is just a perception or if weeds are truly more difficult to control. I canít prove it, but when 70% of the audience has that belief, I think itís real. There could be several reasons for this change. Less consistent control could be primarily related to application issues (rates, volumes, surfactants, environment, etc.), weed related issues (size, tougher species, resistance), or both. Although the specific factors of application or weed related issues were not described in my question, a majority (62%) of our audience believe both application and weed related factors account for the change.

Why do you think weeds are more difficult to control with glyphosate now than 8 years ago?

One personís comment might capture a significant part of the change. The person wrote, "Producers have been programmed by ads to expect glyphosate to solve all problems. Thus, glyphosate has been expected to do more than it was ever capable of." Part of these expectations might be that glyphosate will control all weeds regardless of their size or without increasing the rate or under most weather conditions. Clearly, that isnít the case. We still need to be good managers to make glyphosate work consistently. We need to understand our target weeds (see previous article), make applications at the correct stage of growth, and use other herbicides, such as preemergence herbicides, to compliment glyphosate.

Richard Zollinger
NDSU Extension Weed Specialist

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