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Rolla, ND | A patch like that was there but probably not that visible. A few plants, stunted by the competition. Then you cut off the tops so the buds in the extensive root system broke dormancy and boom, a big rough circle full of Canada thistle.
You can kill it without chemicals but it takes summerfallow, a year of cultivating it every so often. A guy at NDSU did the research and it's all written up for how often to do it so the plant uses reserves growing and then you cut it and it uses reserves and you cut it. Too often is wasteful, not often enough and you don't kill it.
As the alfalfa establishes and you keep mowing, it will reduce but not be eliminated. Until the next time you do something with the hay stand.
As for you planting it, almost certainly that is how it originally got there, just it wouldn't have been in this planting, it's at least one planting in the past.
Used to drive me crazy that we couldn't have a Canada thistle in a field of alfalfa seed production without trouble but I'd go to Canada and the alfalfa seed fields were full of thistle. Turned out at the time they had a tolerance for Canada thistle seed in alfalfa seed. The year that changed was a real adjustment for seed growers in Canada. That said, there are still weed seeds in alfalfa. You grow the seed clean it and test it, if it comes back with prohibited noxious you clean it again until the small sample tests okay.
This whole area has cleavers from the canola seed.
But the radish seed that is coming in seems much worse than the canola seed. Cleavers has a zero tolerance in Canadian canola seed but here it's just another weed so even certified seed can have a pretty fair amount.
I really think in most cases, seed should be relatively locally produced. Sometimes conditions won't permit that, but I think it's a preferential practice.
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How do you get rid of Canadian thistle without using herbicide
