How long should you wait to use weed killer on new grass?

Wait for the grass to become green and lush and by lush about three inches.

If you stgarted your new lawn from sod, it will be ready to mow within two weeks. If you started you new long from seed, it may require three weeks or slightly longer to achieve at least three inches in height.

The ideal height for most species of grass is approximately 2 /34 - 4 inches in height. (Some species such as Bermuda grass are much shorter.) Never cut more than 1/3 the height of the grass at any one time.