On a side note, you shouldn't mow too low. That injures the grass and ironically, makes it easier for weeds to move in. As I understand it, most grass should remain a couple of inches long - unless you have a golf course.
I have seen a lot of them in my old place. They creep along the ground and take over the whole area unless pulled out. When the soil is moist they come out quite easily too. But, the problem is they keep coming back.
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