There are many reason you would want to kill grass quickly. Maybe the grass is of an unwanted variety, like crabgrass. You could also want to completely remove unhealthy grass and start all over again. Whatever your reason, killing unwanted grass can be done in short order with non-selective herbicides. The active ingredient in non-selective herbicides is glyphosate. This chemical should kill all grass in a couple days, according to the University of Rhode Island Landscape Horticulture Program.
Wait for the hottest time of the day. The direct sunlight and heat will make the non-selective herbicide more effective.
Spray all the grass that you want to kill with the non-selective herbicide. Spray the herbicide towards the target area from a distance of at least two feet away. This will prevent you from breathing in any harmful chemicals.
Return to the grass after two or three days to see if yellowing or browning has occurred. The grass has died if it has begun to turn these colours.
Reapply the herbicide to any areas with living grass. Most of the grass, if not all the grass, should be killed since non-selective herbicides are powerful and act quickly.
Wait at least two weeks before replanting any new grass or plants. The herbicide will kill any new plants if it is not completely neutralised in the soil. The chemical is neutralised by the soil in as little as two weeks..
A common example of non-selective herbicide is RoundUp.
Non-selective herbicides will kill any plant. Aim carefully so you do not accidentally spray a plant you had no intention of killing.