Integrated Pest Management of Canada Thistle
by Glen Bupp, Penn State Extension Educator, Horticulture
By understating the biology of Canada thistle and implementing a multi-faceted approach, control can be achieved in one season (or two, let's be practical).
As a perennial, Canada thistle quickly develops a perennating root system. Plants with only four true leaves can have roots over 6 inches long with multiple vegetative buds. Mature plants can have roots over 15 feet deep and produce over 200 vegetative buds annually. Cultivation simply distributes these buds and breaks their dormancy, resulting in spreading the plant.
While the plant can be effectively managed through season-long applications of systemic herbicides, this strategy is not always practical when crops or specimen plants are present. Furthermore, the use of multiple strategies targeting the biology of Canada thistle can provide better control than the use of herbicides alone and reduce the amount of chemical applications needed.
Effective control of perennial weeds is focused on the root system. Depletion of root energy will make for more effective chemical applications, especially when these applications are timed appropriately. Energy stored in roots changes through the season and is at nearly the lowest levels early in spring, after dormancy is broken. When Canada thistle has broken dormancy and put on roughly five true leaves, energy reserves are very low and photosynthesis has not yet replaced these reserves. This point is called the compensation point. The compensation point will not last long before energy reserves are replaced and roots put on more mass. Cultivation is best done during the compensation point. Cutting the root below ground is more effective than cuts made to vegetative tissue above ground. The use of contact herbicides is also effective at this time, many of which are OMRI certified (e.g., Ammonium Nonanoate, Caprylic and Capric acids, Horticultural Vinegar). Mechanical or chemical retreatment will need to be done every 500-600 growing degree days to keep Canada thistle at the compensation point.
Failure to keep Canada thistle at the compensation point means roots will quickly establish reserves. Many growers make single applications of systemic herbicides such as glyphosate early in the season only to find Canada thistle abound later. While some control is achieved, the massive root reserves and general flow of nutrients up from the roots mean that systemic chemicals are not as effectively translocated into the root system.
Growers interested in using cover crops can implement very effective competition from summer cover, especially those with large amounts of biomass. Sorghum sudangrass hybrids (Sorghum bicolor x S. bicolor var. sudanese) are well suited to this by shading and choking out Canada thistle. Combinations of early season cultivation and dense Sorghum x sudan cover can reduce Canada thistle by up to 95%.
In late summer, as days begin to grow shorter, Canada thistle will begin to switch the flow of energy from up to down. The plant is preparing for dormancy and building energy reserves for the next season. This is the most effective time to apply systemic herbicides because the plant will carry the chemical down into the roots. When combined with the strategies outlined above, the smaller root system means more effective chemical distribution. Typically, group 4 (e.g., Clopyralid and 2-4-D) and Group 9 (e.g., Glyphosate) herbicides are used for late-season treatments. In studies, both demonstrated effective and similar control in the following season, with glyphosate being slightly more effective.
Plants that are actively growing need to translocate chemicals through the roots effectively. Applications made very late in the season may not effectively move into the roots. Similarly, do not make applications to weeds that have been recently cultivated. These weeds must recover first and begin growing again for applications to work. Do not cultivate for at least 7 days after an application. This allows time for the chemical to translocate. Ideally, leave the treated plants alone to translocate as much chemical as possible into the roots.
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