Planting a tree on a hill poses a few extra challenges. Before the roots spread, the tree will have a difficult time gathering sufficient water, since it will flow past on the way downhill. The undeveloped root system can also make the tree vulnerable to erosion, which can expose the roots or even make the tree fall over. If you prepare the planting site correctly, however, you can tackle both problems at once.
Use a pick to dig away dirt from the side of the hill and carve out a flat table area where you want to plant the tree. It should be two to three times the size of the rootball of the plant.
Dig a planting hole twice the width of the root ball and deep enough to cover it completely.
Place the plant into the hole. Position the root ball slightly to the downhill side so that the roots are either level with the surface of the ground or slightly above it.
Cover the roots with dirt. Leave a depression on the uphill side of the plant to form a small water basin. Use any leftover soil to build a small hump on the downhill side. This bump or berm will help the plant to gather water flowing downhill and protect it from erosion.
Spread a two to three inch deep ring of shredded redwood bark around the plant, starting about three inches from the trunk and covering the rest of the table and the berm.
Water the tree until the soil is thoroughly wet. Wait for an hour for the water to soak in, then water it again.