If it is Canada thistle, the plants are connected to each other by an underground root.

Take a look at this: Canada thistle ID

An as is said here, You must exhaust (starve) the roots to win your battle: Canada thistle - USDA

roundup takes a few days to kill almost anything

that's the way it works

there could be resistant thistles out there, but not likely in your garden yet. At worse you might need 2-3 applications. Of course, I've never let them form huge colonies, which could prove very difficult to kill. This is why it's one of my #1 priority weeds.

If it's Canada thistle, a few days wait is insufficient. Please read the info at the link, above.

Keep in mind that the World Health Organization has label the glyphosate products as potential carcinogens.

kimmq is kimmsr

Thank you, kimmq. Keep in mind also that some birds & insects love thistle. Why poison them for that? And why poison yourself?

Several nations including France and the Netherlands have banned Roundup, and the EU is in process of banning it across Europe. And this from the latest Chemistry World:

California’s Environmental Protection Agency (Cal/EPA) intends to list the herbicide glyphosate – the active ingredient in Monsanto’s RoundUp – as a carcinogenic chemical under the Proposition 65, which requires the state to publish a list of chemicals known to cause cancer, birth defects or other reproductive harm. Theannouncement came on 4 September, following a conclusion by the World Health Organisation’sInternational Agency for Research on Cancer in March that glyphosate is a probable human carcinogen.

Sales of the product are not restricted by the Cal/EPA listing, but adding glysophate to the Prop 65 list would mean that businesses will be required to provide a ‘clear and reasonable’ health warning on Roundup and other glyphosate products. World usage of glyphosate is at an all-time high, with its use increasing more than 20-fold since 1990, according to the Center for Biological Diversity (CBD). The conservation advocacy group says this upsurge is largely due to the widespread adoption of crops, particularly corn and soy, which are genetically engineered to withstand what would otherwise be fatal doses of glyphosate.Recent research also indicates that chronic, low-dose exposure to glyphosate can cause liver and kidney damage, and studies have pointed to glyphosate as a leading cause of the decline in monarch butterflies.