Thistles are not a weed that lawn care customers appreciate.
While most lawn care customers don't like any weeds at all, some are more tolerable than others. The reason thistles are disliked is because of the thorns.
"My children can't play in that area." Or "we can't go barefoot because of the thistles."
There are two main thistles that can plaque Ontario lawns - Canada thistle and Scotch thistle (sometimes called Bull thistle)
The Scotch thistle is the lesser of the two evils- at least in some ways. It does have bigger thorns though. Normally a bit darker green in colour, it is possible to have fairly large plants in the lawn.
In the way that it grows however, Scotch thistle is less of a long-term problem in lawn care. It is a biennial. The young plants often germinate in the fall and may be present in the lawn the next year as well. When Scotch thistle is ready to flower, it sends up a flowering stem that is quite tall. In a lawn with regular mowing, the plant is rarely going to flower. So while the seeds can float into your yard much like dandelion seeds can, the Scotch thistle is not likely to flower and give off more seeds. Not unless, of course it is allowed to grow tall in a flower or shrub bed.
Canada thistle on the other hand is a perennial plant. Not only that it can grow from underground root system that sends up numerous shoots, often 6-12 inches away from the original plant. While the flowering stems also need to shoot up tall to flower, Canada thistle can reproduce via its underground roots without flowering. Its seed may have likely floated in like a dandelion, but once it is established it is difficult to eradicate.
It will to a degree tolerate being mowed in a lawn. It will not flower, but it will manage to exist even when regularly mowed. Digging it up will remove the plant that you see, but new plants can emerge from the roots that are left behind.
In the past, the best way to eliminate Canada thistle was to use Roundup (glyphosate) but this is no longer permitted under Ontario's Cosmetic Pesticide Ban.