A young child, drawing trees or bushes on a hill, will angle the trunks so they follow the slope. An adult, on the other hand, sets those same trees and bushes straight up and down, no matter how steep the hill. There is a lesson in this for gardeners: no matter how the ground may slope, shrubs and trees grow best if they are planted as though on the level.
Dig holes for shrubs on a hill in the same dimensions you would for those on a flat piece of ground. In general, holes should be twice the depth and circumference of the shrub's root ball. Making the hole, will however, require removing more soil from the upslope behind the bush to create a flat planting area. The bottom of the hole should be level for straight up-and-down planting. Picture digging a wedge into the slope on which you might place a stair tread.
Rake through all the soil removed from the hole with your flat garden rake. Set aside stones and debris to create drainage and erosion protection for your shrub. You may need to add additional gravel, stones, bricks or wood scraps for erosion protection.
Mix half the soil removed from the hole with an equal amount of gravel/stones/small debris. Line the planting hole with this mixture. This provides good drainage to let roots establish on your new shrub. Without good root structure, your shrub will fail to grow in its new location.
Fill the remainder of the hole with soil, reserving a small amount to build an erosion-control berm in front of your shrub on the downslope of the hill. If your soil is soft or sandy, you may add rocks, bricks or wood scraps to your berm. The goal is to slow the flow of water downhill so your shrub receives as much as it needs to grow.
The berm shields your shrub's roots from runoff that may leave them bare of soil and exposed to the air. Just as you would shore up a stair-tread you were setting into a slope, you shore up the planting hole for your shrub. Your berm can be anything from a few inches of rocks and gravel surrounding the perimeter of the planting hole to a formal terrace of bricks and railroad ties, breaking up the slope of the hill sufficiently for you to install a new privacy hedge.
Use the materials that best suit your landscaping scheme, but remember the basic principles. To grow well on a slope, shrubs need an area that is flat, well-drained, and protected from erosion. The adult artist is right--shrubs and trees grow best up and down, no matter what the slope of their terrain. Just like people, they make good use of steps to climb a slope. Build good steps to grow healthy shrubs.