Brian Jenks

Weed Scientist

NDSU - NCREC

Minot, ND

We saw a significant increase in Canada thistle densities from 1993-2000. This increase was primarily due to high rainfall. Less tillage may also play a role in the higher densities. A recent survey in Manitoba showed that Canada thistle tended to increase under reduced tillage compared to conventional tillage.

Grower experience and research have shown that well-established Canada thistle patches cannot be controlled with a single herbicide application. It may require patience, persistence, and unfortunately, more money over several years with repeated herbicide applications to reduce Canada thistle densities to acceptable levels.

The best tool for dealing with Canada thistle is to not let it persist and spread for several years without keeping it in check. The most effective methods we have found to suppress Canada thistle are included in the following examples:

1. Summer fallow followed by fall treatment with Roundup to Canada thistle rosettes.

Fewer growers now include a summer fallow year in their crop rotation compared to the 80's and 90's. However, if a grower decides to summer fallow a field, he can use that period to also target Canada thistle infestations. Repeated tillage will keep the plants from growing and developing the underground root system. Once the days get shorter (<14-16 hours) in the fall, the Canada thistle plants will be more concerned with getting ready for winter by storing up food reserves and will be less likely to elongate and attempt to flower. A final tillage pass should come around August 1-15. This will give time for new Canada thistle rosettes to develop sufficiently for a mid- to late-September herbicide application. If we happen to have a late fall, we would recommend spraying as late as possible, but before a killing frost. We have found Roundup to be more effective for fall applications than other products.

2. Pre-harvest treatment in wheat

Pre-harvest treatments are effective in reducing Canada thistle densities. In addition to wheat, pre-harvest treatments are now approved for flax. Pre-harvest treatments are probably more consistent than post-harvest treatments because we don=t always see adequate regrowth in the fall following harvest. Roundup applied at 1 qt or more per acre has generally provided the best control. 2,4-D and Clarity are have been less effective than Roundup in long-term research trials.

3. Post-harvest treatment

In north central North Dakota, we normally expect the first frost by the third week of September. For post-harvest treatments to be effective, we need to have crops off early enough to allow the Canada thistle plants time to develop new growth. We also need some rainfall in August and early September to encourage weed growth. Crops harvested by mid-August will provide a better chance for an effective post-harvest treatment than will an early September harvest. We generally need at least three or four weeks of warm, moist conditions following harvest to have sufficient weed regrowth.

Herbicide treatments following a light frost can still be effective. However, we need to wait a day or two for the plants to recover and to ensure that the plants have not been damaged to the point that they become necrotic. If plants remain green and have a healthy appearance following a light frost, the herbicide treatment can still be effective. Herbicide treatments will be more effective if daytime temperatures are at least in the 60's. We don=t recommend spraying if the daytime highs are in the 40's or if a hard frost is expected within the next few days.

4. Curtail treatment in wheat

A competitive canola crop is often a grower=s best weed control tool. Following row closure, canola provides a canopy that helps shade weeds through most of the season. Weeds that emerge before or with the canola will have more of an impact on yield. However, weeds that emerge several days after the canola are much less of a concern because they are likely to be shaded out.

In 1999, a study was conducted in Minot to determine the impact of single and split applications of Roundup on short- and long-term Canada thistle control. Roundup Ready canola (LG3235) was seeded May 19, 1999 in a conventional tillage system. Individual test plots (12 by 30 ft) received a single application or a split application of Roundup at various canola stages (see Table 1). Each treatment was replicated four times. Treatments were applied on June 5 (1-4" weeds), June 10 (2-10" weeds), or June 16 (4-12" weeds). These application timings will be referred to as the early, mid, or late applications. A split application is an early application followed by a late application to the same plot. Canada thistle shoot densities present in each plot were counted June 5.

Roundup at higher rates (1.5 and 2 pt) tended to burn down the thistle plants faster than the lower rate (1 pt). On June 28, Canada thistle control with the mid and split applications was somewhat higher than the late application, primarily due to the earlier spray date or more time to kill the plant. In general, the late and split applications showed better season-long control compared to the mid application. The late and split applications tended to have fewer plants remaining and the plants present were usually much shorter than those in the mid application.

Although we saw better control at the end of the season with the late treatment, delaying the application also allowed the Canada thistle to grow and compete which effectively reduced canola yield. Yields were 100-300 lbs higher with the mid or split applications than the late application. The canola stand that received only the late application was thin and somewhat shorter compared to plots where Canada thistle has been taken out earlier.

Stinger was very effective in controlling Canada thistle. However, because we did not put down a soil-applied product such as Treflan or Sonalan, the heavy lambsquarters population was primarily responsible for reducing the canola yield in the Stinger treatment.

Canada thistle densities were counted again in June 2000 to determine the long-term impact of the Roundup treatments applied in June 1999. All herbicide treatments applied in June 1999 reduced Canada thistle densities compared to the weedy check (see Table 2). Wheat was seeded in the research plot area in 2000. A blanket treatment of Curtail was applied in-crop followed by a post-harvest application of Roundup (1 qt). Canada thistle densities were again counted in June 2001. Canada thistle densities were significantly reduced in 2001 compared to previous years. For example, in the treatment where Roundup was applied at 1 pt/A in 1999 and followed by Curtail in-crop and Roundup post-harvest in 2000, the Canada thistle density was reduced from 323 plants to just 7 plants per plot. The majority of the thistle plants counted in 2001 were plants coming from seed, not from the underground root system.

Canada thistle control can be achieved. However, heavily infested areas will require more years of attention. If possible, don=t wait for the field to become heavily infested before seriously attempting to control Canada thistle. Control measures should start in the wheat crop with a broadleaf herbicide applied in-crop followed by a pre-harvest or post-harvest treatment. If Curtail is used in-crop, there probably won=t be sufficient regrowth for a pre-harvest treatment. Post-harvest treatments are most effective when adequate weed regrowth has occurred along with warm, moist conditions in mid to late September. A light frost may actually enhance Canada thistle control. Canola can be very competitive with Canada thistle and combined with a herbicide program of Roundup or Stinger can help reduce weed densities in the long run.

¹ All Roundup treatments applied with AMS (1%)

² A=coty to 2-leaf canola (Jun 5), B=3 to 4-leaf canola (Jun 10), C=5 to 6-leaf canola (Jun 16)

³ These numbers represent the average Canada thistle density over the four replications

*Low yield due to high population of lambsquarters, not lack of Canada thistle control

¹ All Roundup treatments applied with AMS (1%)

² A=coty to 2-leaf canola (Jun 5), B=3 to 4-leaf canola (Jun 10), C=5 to 6-leaf canola (Jun 16)

³ These numbers represent the average Canada thistle density over the four replications

⁴ Canada thistle density in 2001 following 2000 wheat crop where Curtail was applied in-crop and Roundup post-harvest.