Canada thistle and perennial sowthistle are tough hombres and it sometimes requires a "scorched earth policy" to eliminate them. Every June too many customers inquire about how to control sowthistle or canada thistle in their corn and beans. There is no good answer to this question. Gyphosate tolerant crops have lulled us into thinking we can control every weed in the spring. When it comes to thistles, this approach is a bust.
Summer is time to hit back and hit hard at these two. A one litre pre-harvest glyphosate application to this wheat field will eliminate more thistles than any other time of year. If the wheat is underseeded, sacrifice the clover and spray. The weeds are flowering and setting seed. Sugars and carbohydrates are moving back into the root systems to get ready for next year which makes the plants extremely vulnerable to glyphosate. Most fields have tramp lines in them from fungicide application so the sprayer just has to follow the tracks. Make sure the wheat is at 30% moisture or less. Pressing your fingernail into a 30% wheat kernel will leave an imprint on the kernel.
To really complete the job, spray again in early September. When next June comes around you won't be asking what to do with thistles. You will be too busy admiring your clean field.