Yes!
How about a sign with pictures of the people running the
park, such as community groups, volunteers, friends, staff, with pictures
of folk gardening? Something that shouts: “While you might not see us everyday,
we are here! This park really is run by you, for you! To get even more involved,
ring this number....”
I'd do away with any signs saying “stay off the grass”, as per
park folklaw.