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Post-harvest Perennial Thistle Control

By Clark Brenzil PAg, Provincial Specialist, Weed Control

October 2019

With a prolonged harvest in many areas of Saskatchewan this year, and concerns over the impact of glyphosate residues on market acceptance, questions about post-harvest perennial thistle control are common. This article will go over some of the basics on post harvest Canada thistle and perennial sow-thistle control.

Application Date

Canada Thistle
Canada Thistle
The combination of day-length and cooler temperatures trigger perennial thistles to transition from the reproductive (flowering) stage to vegetative (or root storage) stage of growth. Research has observed that Canada thistle shoots grown in growth cabinets under varying day lengths found that less than 10 per cent of thistles flowered if the day length was 15 hours or less and the others remained as vegetative rosettes. The same research showed that 100 per cent of thistles flowered at 16 hours of light per day. If we presume that the reverse is also true and similar day lengths trigger the transition from the reproductive stage to a more vegetative stage, this transition would occur roughly during the first week in August at Estevan to mid-August in Meadow Lake. There is also research that shows that root growth is preferred over shoot growth when daytime high temperatures are in the mid-teens C versus mid 20 to 30 C research has been done on perennial sow-thistle specifically but general life-cycle trends are similar to Canada thistle.

Although it is possible that the lack of rainfall could hamper Canada thistle regrowth in some areas, consider that Canada thistle roots commonly penetrate from two to three metres deep and depths of 6.75 meters has been reported, so inability to access moisture is unlikely. Perennial sow-thistle roots on the other hand have only been recorded to two meters deep, possibly leading lower tolerance for dry conditions.

Fall regrowth of cut thistles will not be very tall since short day lengths mentioned above will keep them vegetative. Following harvest, check thistles closely for either new leaf growth from axial buds or for new rosettes popping up near the old stem. Regrowth nearer to the ground is more common in fall.

Optimizing Glyphosate Performance

Glyphosate is still the most practical and effective herbicide for fall perennial thistle control. The following are some tips to maximize its performance:

  1. Rates - The real key to controlling Canada thistle post-harvest is applying correct rates. The single most common misconception is that the same or similar rates can be used with post-harvest glyphosate applications as with pre-harvest. But, even with the recommended four to six weeks of regrowth following harvest, there is still at best only one third the leaf area to intercept spray, compared to before harvest. As a result, up to two thirds fewer spray droplets will hit a thistle leaf during post-harvest treatment. Therefore, to get the same amount of glyphosate into the plant, the glyphosate concentration in the spray solution (or application rate) needs to be three times greater than before harvest. This means post-harvest rates should be 1100 grams acid equivalent per acre (3 L/acre of a 360 g/L form or 2 L/acre of a 540 g/L form).
  2. Application conditions - Apply on the warmest, sunniest day possible. Minimum overnight temperatures of four or five degrees and daytime highs greater than 10 degrees are absolute minimums for good activity, but even for these temperatures to be effective, conditions need to be full sun for the entire day. Preferably temperatures will be around 20 C and not lower than 10 C overnight, but this can be a pretty tall order this time of year.
  3. Water Quality - Make sure the source of water used as the spray carrier is low in hard water ions (less than 20 grains or 350 ppm of combined calcium and magnesium ions as calcium carbonate equivalent) and iron. If there is any hardness at all, add a little ammonium sulphate to the spray water before adding glyphosate, especially if it is budget glyphosate. For hard water ion content greater than 40 grains of hardness or 700 ppm calcium carbonate equivalent select another water source.
  4. Dust – Dust accumulation on leaves can inhibit glyphosate activity because it binds with the herbicide and prevents it from being taken up by the plant. If there is heavy dust on leaves wait for a light rain shower to wash them off prior to attempting to spray.
  5. Frost/Cold damage – Canada thistle can be quite tolerant of frost but only if night-time temperatures have dropped very gradually progressing into fall. A sudden frost of four C when night-time lows have been well above freezing, can be more damaging to leaf tissue than a 10 C frost if temperatures have declined gradually. The only way to determine whether thistle leaf tissue is receptive to herbicides following a frost is to physically check them in the field. Within a day or two of a frost, damaged leaf tissue will take on a black or dark green appearance, then over time, will dry and turn brown. Monsanto suggests that at least two thirds of the original leaf tissue needs to remain for glyphosate applications to be effective. In some cases, leaves may remain green but still not be appropriate targets for glyphosate. Suitable leaves must be green and pliable. Leaves that crack when bent are not good targets and should be considered damaged.
  6. Dew - Wait until most of the heavy dew is off. The adjuvant in the spray droplets contacting the heavy dew on the leaf surface will cause the dew to run off of the leaf quickly taking the spray with it. The evaporation of the heavy dew is likely to coincide with the temperature being warm enough that day to begin spraying.

Thistles treated in the fall may not show symptoms of herbicide damage following application. Sometimes movement of glyphosate from the leaves to the roots is so efficient that the upper part of the plant remains unaffected. A slow death is a good one as far as glyphosate effectiveness on perennial plants is concerned. There have been cases some years where treated plants have remained green for a couple of months until freeze-up in November but the control in the spring was excellent.

If you are planning to do post-harvest perennial thistle control with glyphosate, be sure to increase your glyphosate rates. Tank mixes or other additives not focused on correcting water quality issues, are not going to help control any more than additional glyphosate in the tank. At current glyphosate prices, it is challenging to find a cheaper addition to glyphosate for perennial control than more glyphosate. If there are weeds that could be prone to the development of glyphosate resistance present in the field (kochia, cleavers, Canada fleabane) it might be prudent to add a dicamba-based product to the tank unless there is a sensitive crop like pulses or oilseeds following next spring.

For more information, contact your nearest Regional Crops Extension Specialists or the Agriculture Knowledge Centre.

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