You don't want grass growing all the way up to the trees. Are the trees in the ground, yet to be planted this spring or to be planted later? It matters a lot. You would like to have organic material in the soil, which you probably already have. Grass is not the best material for preparing the subsoil. I would consider getting some truckloads of free wood chips from tree cutting companies. You many want to put newspaper down in the areas where you are going to plant trees and put chips on top of that.
John S
PDX OR