Canada thistle is persistent for three reasons. Seed production, deep roots giving rise to stems, and root pieces that can regenerate plants.
Seed production per plant averages 1,500 seeds per plant but vigorous plants have been known to produce more than 5,000 seeds with viabilities greater than 20 years. So it will take persistence to reduce the seed load in the soil. Lesson 1: never let thistle go to seed. This will not remedy seeds coming in from an adjoining property, and seeds are reported to be able to travel a half mile in the wind.
Photo 3: Two years underground growth of Canada thistle from original one foot of root.
Canada thistle can reliably regenerate from half-inch long cut root pieces. Lesson 2: attempts to dig out the plant or chopping it up will likely not be successful and may only serve to propagate it.
Canada thistle is a perennial with a complex system of deep-seated roots that spread horizontally and give rise to aerial shoots (photo 2). The seedlings grow slowly at first producing a fibrous taproot which thickens and develops lateral roots in 7-9 weeks. Aerial shoots usually develop from buds on the branches of the horizontal system. The root system goes deep (6 - 10 ft.) and wide (> 10 ft. per year) with some 60% of roots existing at depths greater than 2 ft (photos 3 and 4).
Simply removing the aerial shoots can eventually exhaust the root reserves. One study showed that mowing the plants would eliminate the top growth similar to pulling the aerial shoots, but will not deplete the starch reserves unless it is repeated at 7-28 day intervals for up to 4 years. It is more likely that the thistle would win given this strategy.
If the plant can be isolated, it can be smothered with an impenetrable barrier like plastic or a landscape weed cloth. This would require clearing out the bed and dedicated time to starving the root system. The key problem here is isolating a plant with a creeping underground root system that could send up shoots in adjoining areas which would replenish the root system.
Photo 5: Canada thistle in sedum bed. A situation where a bedding plant could be isolated from spray on a Canada thistle.
If the chemical route is chosen, a non-selective herbicide like glyphosate (Roundup®), which has little or no soil residual, would be the chemical of choice. In some situations sensitive plants can be separated from the thistle and protected from spray with a physical barrier like plastic (photo 5). The plastic may be removed as soon as the spray dries.
The goal is to kill the root system by getting as much chemical throughout the plant as possible. Use the lower of label recommendations as higher rates will kill the leaves and not get to the roots making the treatment ineffective. Make sure that the plants are not drought-stressed and that there is plenty of moisture. If the plants are stressed the chemical will not be effectively translocated throughout the root system. Be sure to read and follow all label directions carefully. It is highly likely that multiple applications may be needed to eradicate this weed.
Given the look of the below ground root systems it looks like the most we can hope for with Canada thistle is not elimination but rather a certain level of control.
"Canada Thistle." Midwest Weeds, Missouri State.
"Control Practices for Canada Thistle." Purdue University, Department of Botany and Plant Pathology.