Planting trees and shrubs too deep will cause a slow death for sure.
It is so very important to be sure that you plant your new trees and shrubs at the same depth as they are growing in the container that you received them in. If you are working with bare root plants be sure to plant all roots just under the soil surface and not bury them too deeply. Many times, the planting depth is visible on a bare root plant showing where they were grown at the nursery.
Planting your trees and other plants too deep where there is less oxygen in the soil and can cause root girdling and death of your plants.
Roots like to grow in the warmer soils closer to the surface where they can grow out in all directions to find food and water. Most tree roots can be found in the top eighteen inches of soil and many feet away from the trunk.
Another kiss of death beside planting too deep is the addition of mulch up against the trunk of the trees and shrubs. Mulch is great over the roots of plants, but not against the stems of the plants above ground.