I sprayed the lawn area where the small prairie would go earlier this year. I talked to the person on site and was told about half the area has grown back with some kind of low growing plant (the person did not believe it was grass). Should I go back and spray now and then once more in late Oct or just wait until OCt to spray it?

Also I am using the premixed roundup. Is this OK or do I need to mix a stronger solution myself?