Establishing Trees Into Grass
The tree planter often has to deal with challenges brought on by grass. Establishing trees into grass pretty much requires that you do three things: mulch them well, water them deeply once per week in the summer, and put rodent and deer protection around them.
Rodents love the cover provided by grass. Trees can be protected from rodents with a tube or a collar around the base of the tree. Tree tubes should be at least as tall as the deepest snow of the winter, because small rodents can walk on top of the snow. We make our tree tubes out of rolled aluminum flashing that is 10 inches tall and cut into 12 inch lengths, rolled into a tube and tied with a single wrap of tie wire around it. At our local hardware store, a 50 foot roll of aluminum flashing is 30 bucks, so 50 trees can be protected for 60 cents per tree, which is way cheaper than plastic tree tubes. Wire mesh or screen can also be used effectively, however we find that it is fussier to cut and roll into a cylinder, and sometimes lower branches of the tree grow through the mesh, making it difficult to remove.
To protect trees from deer, a 7 or 8 foot fence around the perimeter of the planting is best. Individual trees can be protected in a wide variety of ways, and it doesn't really matter how, so long as the deer can't get to them. One of our favorite methods for deer protection of individual trees is to stand 4 pallets up on edge in a square around the tree, and tie them in the corners. Additionally the pallets shade the ground and block desiccating winter winds. If fencing a large number of individual trees, at some point the length of the combined tree fences will become greater than the length required to fence the perimeter of the planting area, and you should consider cobbling the individual fences together into one big fence.
Grass sucks up a lot of water, so trees planted into grass require more water than those planted in pots, a mulched area, or a garden bed. If possible, several months ahead of time prepare the area by laying down some scraps of wood, rocks, or a pile of brush in the place you want to plant a tree, which will create a small grass-free area that will give the tree an advantage. At the time of planting, a deep mulch of wood chips, leaves, sawdust, or straw/hay deters the grass from re-invading and makes the grass easier to pull up when it grows into the mulch. If you don't have mulch available, scraps of wood or rocks aid down flat on the ground around the tree can also keep the grass at bay, although they don't have the anti-evaporation quality that deep mulch has.
If you keep after it, eventually the trees will grow over the top of the grass and begin to shade it out. In the meantime, just keep mulching, watering, and pulling any invading grass, and your trees will get established just fine.