Originally Posted by
cjclemens
10% grass isn't bad, but you might want to make sure you mow the plot when the grass starts to head out. If it goes to seed, you'll have a lot more than 10% grass next season.
^What he said.^
I'd spray it before it spreads. Otherwise, mow it often enough that the grass doesn't have a chance to produce seed. That was the mistake I made with my clover plots.