I have a dozen youngish pine (red and white) trees ranging from 20-40 feet tall growing on the north side of my lot, more or less in a line running east to west. They are situated on the lawn in regular grass. Away from the house on the road side of these trees, there's a buffer of long grass between the trees and the road. The grass there is 3-4 feet tall, all the other grass under the pine trees is normal long length and is regularly mowed. I'm in the process of sheet mulching the regular grass under the trees to get rid of it, an area roughly 150 feet x 30 feet. Ultimately I would like the ground under the trees to be dirt, covered with pine cones and needles like a forest. Hopefully the pine needles will prevent the grass from growing back. However, I'm concerned grass will come back after the sheet mulching decays in areas that don't get fully covered by a layer of pine needles. So my question - would it help to plant an understory of ferns or other plants to prevent the grass from coming back? If so, can you recommend specific plants that would work best in my area (Duluth)? Thank you very much!