Thats a tuff one .... lets say it's Crabgrass ... then it's an annual BUT it makes lots of seeds .. making chemical control an attractive option. Mowing the grass high .. staying up to date with fertilizer and getting rid of excess soil miosture will help put things in your favor.

Good Day ...

Crabgrass (Digitaria) is an annual but will root from it's nodes. It blooms in late summer with very slim purplish seed heads.
Foxtail (Alopercurus) is perennial which blooms in midsummer with tufted seed heads which resemble a fox's tail. It will root from stolons.
Both like moist soils and are difficult to control.

If it is flat growing, and has a white center, then it is probably goosegrass. That stuff is very aggressive, and will grow out of a block of cement.

I'm assuming you might be in the Toronto area. If so, the crabgrass started germinating about three weeks ago and with all the heat, it is a banner year for it. It is an annual, but it will come back next year unless you get ALL the plants pulled before it goes to seed. The seeds will also wash down the street from neighboring properties if they let their crabgrass go to seed. If you have lots of crabgrass around you or you miss a few plants before they go to seed you may want to apply a crabgrass preventer next spring (Unless you live IN Toronto, then you can't apply anything because of the pesticide bylaw).