I have a couple shrubs I want to plant on a slanted landscape; nothing severe, maybe 10 degrees. Should I plant perpendicular to the ground, or perpendicular to an imaginary level surface? If it makes a difference, if I plant it perpendicular to the ground, the top of the shrub will be angled directly east towards the sunrise.
Plants will grow up to the light. I recommend planting them perpendicular to the imaginary level surface. Anything else would look odd as the alternative is not seen in nature.
What will help is building a small ring of soil on the downhill side of the shrub and applying a few inches of organic mulch all round.