A grass management system includes a first moisture-obtaining device configured to obtain a first moisture value of grass in a mowing operation performed by a mower, a position-detecting device configured to detect a mowing position of the mower, and a creation supporting part configured to support creation of an operation plan for a working machine based on the first moisture value and the mowing position, the working machine being configured to perform an operation relating to the grass already mowed. The grass management system includes a moisture map creating part configured to create a moisture map of an agricultural field based on the first moisture value and the mowing position. The creation supporting part is configured to display the moisture map that is created by the moisture map creating part in creating the operation plan.
