See the link -

Here is a link that might be useful: Canada thistle

If you pull it the roots will still be in the ground since they go very deep. But if the question is to pull or not to pull, I say Pull!

You could paint each plant with glyphosate and not get it on desirable plants. As Jean's link says, do it before it frosts.

PS...Stiltgrass my worst enemy

Thanks Jean and Kim for your input! I did use glyco as an experiment on one patch of Canada thistle and stiltgrass....they grow so well together :( ....it "killed" everything but the death was temporary and it all came back w/a vengeance!!! I just can't see myself staying w/chemicals....can't afford it and it doesn't seem effective to me. I used cardboard and mulch to knock back the stiltgrass...extremely labour intensive as it's a fairly large area for one person w/a wheelbarrow but THAT was worth it as it made the SG manageable and now I can just go around and pull here and there. I just can't figure the thistle out. Covering it w/cardboard and mulch makes it easier to pull and I found that if I pulled in early summer there was a second round that came up but the plants were much shorter and they didn't flower....I'm pulling them again now because it makes it easier to pull the stiltgrass if I can see its "stem" which is hidden by the thistles! So am I invigorating the thistle or weakening it by pulling?!

If the greenery of any plant is kept cut off so it cannot take in the sunlight and make nutrients to feed the roots those roots will eventually die. Many of us that will not use plant killers, because of the harm they do to the environment, find keeping these unwanted plants cut back this way is an acceptable method of control.
That way we are not poisoning the world we live in.

Thanks for your input kimmsr...so in your opinion if I got a pr of clippers and cut the thistles nearly to the ground and repeat this as necessary I would be able to wipe out a colony in a season or 2? I just have no idea how often this would need to be repeated or even how long it should take to knock out a particular "colony". I cannot mow in some areas on my property and I do not own a weed whacker but am willing to pick up a pr of clippers if I think something is going to work :)!

Absolutely. All plants need that top growth to pick up light to manufacture nutrients that feed the roots and promote growth. Deprive a plant of the top growth and it will, eventually die as the roots use up the stored nutrients trying to grow more top growth. Keep that top growth cut.

Canada thistle will keep sending up growth. That is one reason it is so feared and called a noxious weed. If you have a colony, you have a problem. That big underground root system is strong. You should search canada thistle and read all about it. The best time to spray it is in the fall when the plant is bringing energy down to its roots. Flowering is hard on plants. If you want to weaken it, pull it when it flowers but don't let it go to seed. Then let it go to flower again and pull it. Repeat!

I saw my neighbors beds get overtaken by thistle even though it was pulled once or twice a year. They eventually had to have it sprayed and they lost all of their good plants. They spent so much money with so called professional landscapers. I was able to control what came in my yard by pulling because I never let it get a good hold.

http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_018027.pdf

Thank-you both for your input. I think I will try cutting and see what happens. I do not have any landscaping or flower beds per se. I live on a mtn in the woods though a lot of the property around this house is disturbed. My goal is really just to try and encourage the forest to regrow and take over as much as it can so I am try to get a handle on the invasives and add plants/trees that will block the worst of the invaders, stiltgrass, Canada thistle etc. until the under and upper stories regrow and block most of the light....100 yrs from now :) I don't really want to use chemicals as there are so many native plants, grasses and sedges intermingled w/the colonies of thistle....so I will take a deep breath, realise that the thistle will take a LONG time to eradicate and get out my clippers!