If your grass is green, what does it matter if it's a combination of grass and weeds that look like grass? No harm, no foul – right? Not quite. Weeds compete for air, water, and nutrients, giving the good grass less of a chance to thrive and opening it up to disease and insect infestation. Though it may be just a few weeds to start with, they quickly take over your lawn and kill off your desirable grass before you know it. That's why it's important to know about the different types of weeds that find their way into your yard and disguise themselves as grass.