Who is the patron saint of lawns?

There is no patron saint of lawns. There are a number of patron saints of gardeners, however.
Adam the Patriarch
Adelard of Corbie
Agnes of Rome
Ansovinus of Camerino
Christopher
Dorothy of Caesarea
Fiacre
Gertrude of Nivelles
Phocas the Gardener
Rose of Lima
Sebastian
Serenus the Gardener
Tryphon
Urban of Langres
Werenfridus