Spring is a great time to dig thistles--before they become as big as lilac bushes. We seem to always be fighting with thistles, and we want to get them under control before they start spreading their seeds, later in the season. When that happens, it's all over.
A quick cut through the root just below the surface of the soil should do it. The Farmer says to make sure that some dirt gets in the cut, so that the bacteria in our healthy soil can start the decomposition process.
I don't claim to understand it all. I just tell myself it's cheaper (and closer) than a gym membership.