Research from QC: Suppressing Canada thistle and sow thistle

Field experiments led by Anne Weill, P. Ag., PhD of CETAB+ tested the possibility to reduce sow thistle and Canada thistle pressure by using the progression of a spring fallow followed either by buckwheat (for harvest) or a vigorous legume crop (soybeans or forage peas).  The experiments were carried out on two large-scale organic farms.   Observations were also recorded in field areas besides the formal, experimental field plots, where the M0 treatment (tillage only in June after spring fallow) was applied.

Experimental results:

The progression of: spring fallow —> cultivation in June and/or May and June —>immediately followed by planting an aggressively growing green manure or soybean crop were very effective.  Canada thistle and sow thistle pressure, quite dense at the start of the experiment, decreased significantly following the M0 and M1 treatments.  The effect was lasting: thistles were largely absent in the months (and year) following the experiments.   The number and timing of cultivation passes were important– two cultivation passes in June were sufficient for sow thistle suppression; meanwhile, an additional cultivation pass in May was necessary to effectively suppress Canada thistle.   Following on the next year by planting a competitive/densely growing crop that is cultivated (for example, soybeans rather than buckwheat or squash) helps to make the suppression of thistle complete.

Methods tested:

Follow spring fallow with:

M0- June cultivation only


M1- Cultivation in May & June

For both methods, either buckwheat, soybeans or forage peas were sown directly after June tillage.


Left: Corn planted in May 2012 after the spring fallow/cultivation/soybean rotation used in the experiment. Right: Shows the dense thistle presence similar to that which had been present on the left side of this image in May 2011, prior to starting the experiment.  All photos in this post belong to Anne Weill.

Notably, these methods make it possible to reduce thistle pressure effectively without herbicides.

They can be used by organic AND conventional farmers.


Though the perennial weed pressure was similar for both years for one of the farms and higher in 2012 than in 2011 for the other, the spring fallow was more effective in 2012 than in 2011, with a markedly greater reduction in thistle regrowth at the end of the season. Two factors may have been responsible: (1) the planting of more competitive crops following the experimental treatments; and (2) more effective tillage implements.

(1) Planting competitive crops:

Planting competitive crops immediately after the spring fallow and cultivation makes a difference:  After spring fallow, the area to the left of this image was planted to storage-variety squash, and the area to the right was planted to soybeans.  The squash did not compete effectively with thistle; the two field areas show a clear difference in terms of remaining thistle pressure after autumn harvest.

(2) Effective tillage implements:

The Lemken Company’s Kristall chisel, used in 2012 for this experiment, proved to be very effective relative to the others tested (heavy cultivators or a disc harrow).   If configured properly, this tool’s aggressive chisel points can extract large numbers of rhizomes, leaving them to dry out on the soil surface.   As such, this tool seems to offer great potential in the effort to suppress any rhizome-type weed.

Chisel points must be configured properly to lift rhizomes to soil surface.

The Lemken Kristall cultivator in action, extracting thistles.

The goose-foot cultivator, also tested in 2012, also yielded good results.  This implement, fabricated on the farm, was designed specifically to enable shallow tillage while cutting rhizomes.   Rhizomes are cut in heavy soils and are partially extracted in light soils.

This goosefoot cultivator was designed to enable shallow tillage.

Effective rhizome removal with the goosefoot cultivator!

Access the full report in English from CETAB+:

Screen shot 2015-10-27 at 7.09.07 AMThis report was made possible by CETAB+ as part of the QC Government program “Stratégie phytosanitaire québecoise en agriculture.”


To learn more, read Anne Weill’s full report by clicking on the links to the left. Goodbye, thistles!

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This project has been made possible in part by the Government of Canada.

This project has been made possible in part by the Government of Canada.

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