I have been trying to remove thistle and (common) milkweed roots from my...
I have been trying to remove thistle and (common) milkweed roots from my garden by hand digging. Am I likely to succeed? So far it is taking hours and days as the milkweed roots are up to 6 feet long and 2+ feet deep. I have looked on line and can not find an answer: if I leave the tiny roots (about 1/2 mm thick) will they grow back?
As you are finding, both Canada thistle and common milkweed have extensive root systems and rhizome (underground stems). The roots of Canada thistle have been measured down to 6 ft, with the majority found between 8-16 inches. The roots of common milkweed can reach depths of 4 ft in the soil.
Roots remaining in the soil that are >0.5mm are unlikely to resprout, but you would want to continue monitoring the area. Control via mechanical removal of above and belowground tissues can be effective but may take several attempts to completely deplete these deep root systems.
Spot treatment with a herbicide may be another option to consider if you are interested in speeding up the process. If applied with care glyphosate (Roundup Grass and Weed Killer or other similar products) can be used. The best time to apply glyphosate is in the fall when the plant is moving nutrients towards to root system to overwinter. There are other chemical control options that may work better for thistle control, however some of them can impact what is planted in that location in the future.
When using products containing glyphosate there are a few important points to consider. First, as with any pesticide, remember to read and follow all labeled instructions. Second, glyphosate is a broad-spectrum herbicide, meaning it will injure or kill other plants contacted during application, so care is needed to avoid desired plants. Third, be sure that the product you choose has only the active ingredient glyphosate or glyphosate + pelargonic acid. Products with additional active ingredients may have other unwanted effects and may delay the planting of other plants in the coming season(s).