Can We Eradicate Canada Thistle?

Jerry Doll

Canada thistle originated in Europe and is now widespread throughout the upper Midwest and Canada. While it has been on the Wisconsin noxious weed law for many years, it has only become more abundant over time. It seems that it is increasing at a rapid rate in recent years. This can be attributed to several factors, not the least of which are the following:

growth in soybean acreage (we have had few effective treatments in this crop)

the reduction in tillage that often favors perennial weed growth and spread

the use of tined tillage tools as a replacement for the moldboard plow (this drags root pieces to new areas)

the great reduction in the acres treated with and the application rates of atrazine

the lack of mowing and infrequent use of herbicides along roadsides infested with Canada thistles.

Eradication is an ambitious goal for any weed species and if it were easily achieved, we would have solved most weed problems by now. Perennial weeds are perhaps easier to eradicate because we can probably destroy the vegetative means of propagating easier than we can eliminate the seed bank of all viable seeds of an annual weed species. Several producers have commented that they "used to have" yellow nutsedge on their farms and I am sure the same is true of quackgrass on many farms. Can we expect eradication of Canada thistle? I believe it is feasible to achieve this because we have several excellent chemical and biological weapons that can be integrated with the best cultural and mechanical practices to take us well down the road toward eradication.

In Corn

We have tested clopyralid for Canada thistle control for several years in corn and more recently along roadsides. Not surprisingly the best results have occurred in a cropped setting as there we can bring tillage and crop competition into the picture. The data in Table 1 are from the Cropping Systems Trials at the University of Wisconsin Arlington Research Station and the Lakeland Agricultural Complex near Elkhorn, Wisconsin. We applied clopyralid to infested areas of the large plots in corn and soybean production systems.

At Arlington 86% of the plots that had thistles were in a corn soybean rotation. At the Lakeland Complex, most (65%) of the thistle infested plots were in the oat/alfalfa/ corn rotation or the corn/soybean/wheat/red clover system. This observation suggests that thistles are less common in the cropping system that include two or more years of harvested alfalfa followed by corn with tillage (only 3% of the plots with thistles had this system) and that continuous corn (only 18% of the plots with thistles were in continuous corn).

In Soybean

Until the advent of glyphosate, Canada thistle management options in soybeans have not provided the opportunity to approach eradication of this weed. Even when glyphosate became available in the 1970s, few producers made spot applications or preharvest treatments in soybeans because in the first case the crop in the treated areas was killed and in the latter, it is not easy to spray in a field of ripe soybeans, especially when planted in narrow rows. The other options for Canada thistle management in soybeans suppressed the weed sufficiently to give the crop a chance to gain a competitive advantage but long term impacts on the population were not common nor expected.

Now we can use glyphosate selectively in Roundup Ready soybeans and this gives us the possibility of approaching Canada thistle eradication. We tested this alternative in 1997 and the results are very encouraging (Table 2). Glyphosate gave excellent control of Canada thistle at nearly all rates and times of application, including the early treatment time at 3 and 4 pt/a. However, equally effective was 1.5 pt/a applied on June 27 when 10% of the thistles were in the bud stage and beans were 7 to 10 inches tall with four trifoliate leaves. The check plots were intentionally placed in low density areas and thus the low initial thistle pressure. Thistle density doubled in the checks from June to late September. There was no advantage to making split applications of glyphosate and the split application of Basagran also gave excellent thistle control. We will make observations on Canada thistle reinfestation in 1998 as this is essential to determine if we indeed achieved eradication.

In Roadsides

Canada thistle populations have exploded along many of our roadsides in recent years. In 1995, we began a long-term project funded by the Wisconsin Department of Transportation to compare various strategies for their long-term ability to suppress Canada thistle. This research is very relevant to pastures and CRP sites because they are managed in essentially the same way as roadsides: little if  any disturbance other than an occasional mowing.

Clopyralid is available for non-crop use under the trade name Transline. This is the same formulation and concentration of clopyralid as in Stinger. The primary object of the study was to determine how frequently Transline should be applied to obtain long-term control of Canada thistle. The secondary objective was to see if prior mowing or tank mixing with Telar (chlorsulfuron) enhances control. To meet these objectives we established a 25-treatment trial at Avoca and Oregon in southern Wisconsin. Each trial was laid out with three replications and the first treatments were applied in 1995. Observations were made each summer and will continue for several more seasons.

We compared single and multiple applications of Transline at 1 pt/acre (the recommended rate for non-disturbed sites such as roadsides) alone or in combination with Telar at 0.25 oz/acre. Applications were made in the early summer or early fall and mowings were done in late June when thistles were in the bud to early flower growth stage. All treatments were applied in 37 to 40 gal/acre of water with 0.50% non-ionic surfactant. Visual ratings were taken and are reported in Table 3. Because results were similar at both locations, averages are presented.

Mowing alone had little effect on the Canada thistle population. Surprisingly, mowing ahead of a herbicide treatment did not enhance the performance. We expected a summer mowing followed by an early fall treatment would be better than a fall treatment alone. A single Transline application gave 45 to 62% thistle control in June of 1997. The control following a single Transline application in the summer of 1995 declined an average of 1.3% per month and 2.3% for the early fall timing. Including Telar with Transline did not improve the control of single or multiple applications.

Applying Transline twice in the same year gave only 57% control in June 1997, as compared to 81 to 86% control when the two applications were made in different years. Three Transline applications gave excellent Canada thistle control with ratings of 97 to 99% in 1997. Time will tell if these control ratings hold up. The ratings for two applications looked very promising in October 1996, but declined significantly the next year. Perhaps the same will happen following three applications in the future. We will continue monitoring these treatment for two or more years.

The results show that it will take at least two Transline or Stinger applications to obtain reasonable long-term Canada thistle control in non-disturbed sites. The use of GPS technology could certainly be used advantageously in these sites as the thistles are not very visible when the second application would be made. The regrowth of thistles after a single or repeated Transline application is in stark contrast to the long-term control we see with the same active ingredient in corn fields (even when applied at half the rate used in non-disturbed sites) (Table 1). This reflects the great combined impact of crop competition and tillage, in conjunction with an effective herbicide, on Canada thistle.

Presented at the 1998 Fertilizer, AgLime and Pest Management Conf. Vol. 37: 230-234.