Cirsium arvense is also known as Canadian thistle, creeping thistle, or just thistle. Due to the root system that this perennial develops, it is very difficult to eradicate from your garden.
First off, NEVER NEVER NEVER let the thistle go to flower and seed. Why? Thistle will reseed all over your yard, and the seed is viable for 22 years- That’s A LONG time.
If there is thistle in your garden and you wish to keep all the plants where they are, your best bet is a cocktail of chemical and manual labor. I always cut any green growth off to soil level whenever I see it. This will eventually starve the roots of energy and food which the plant needs to survive and weakens the vigor in which the thistle rejuvenates. After cutting it down, instead of spraying the Round-up herbicide, I prefer to drip a few drops of it into the hollow stem which protects surrounding plants and seems to weaken the thistle more.
When the thistle issue is also creeping into your lawn, I’ll use a broad leaf product with 24-D, such as Weed-B-Gone, in these areas. Even if you completely remove the thistle from your garden, it will creep right back in via underground runners.
When dealing with a large area of thistle, my best method of control is to lay a thick layer of newspaper (6-8 sheets) and cover with a 3-4″ layer of organic material (woodchips, compost, grass clippings, or leaves). Perennials then may be planted directly through the newspaper/organic material layer. If any thistle pops up, I return to the cut down/Round-up dripping process.
When time isn’t an issue, I lay a heavy duty tarp over the entire area (plus another 3-4′ past where the thistle is growing). Then I cover the tarp with 3-4″ of organic matter. It’l will take about 3-4 years for the thistle roots to die off completely before the tarp can be removed and area planted.