Answer: The depth of the soil won't be an issue if you choose dwarf shrubs and some of the smaller perennials. They should all do well in about 12" of soil depth. Many common shrubs, both deciduous and evergreen, are available in dwarf varieties. Among them are:
Mops Dwarf Mugo Pine, Pinus mugo 'Mops': A true dwarf mugo that won't outgrow the landscape. It stays very compact and dense without shearing. This hardy green pine forms a low dense mound. It is globed shaped and grows to 3 feet.
Bagatelle Barberry, Berberis thunbergii var atropurpea "Bagatelle': This upright growing shrub has deep red foliage which turns a brighter red in the fall. Grows 1 1/2 feet high and wide.
Blue Shag Dwarf White Pine, Pinus strobus 'Blue Shag': This long needled globed shaped dwarf pine has a soft texture and blue-green foliage. It grows 3-4 feet high and wide.
Dwarf Alberta Spruce, Picea glauca 'Conica': This dwarf conifer is upright and narrow. It grows 3-6 feet tall. Its conical shape is dense with grass green foliage.
Regent Serviceberry, Amelanchier alnifolis 'Regent':Small deciduous flowering shrub which has soft green leaves. White flowers open in spring. Black edible fruit forms in the summer. It has orange-red color in the fall. Grows 4-6 feet high and wide - you can prune to keep it smaller.
Spanish Gold Broom, Cytisus purgans: This shrub is a dense mound of ascending bright green stems that are adorned much of the spring with golden pea blossoms. It grows to 4 feet high and up to a 6 feet wide.
Dwarf Oregon Grape Holly, Mahonia aquifolium 'Compacta': This broadleaf evergreen has yellow flowers in spring followed by edible blue berries. This plant grows slowly to 2-3 feet tall and 2-4 feet wide.
European Cranberry Viburnum Compact, Viburnum opulus 'Compactum': This shrub has a rounded compact form. Dark green leaves turn wine red in fall. Spring flowers are white. Fall fruit is bright red. This plant grows to 4 feet high and up to 4 feet wide.
Alpine Current, Green Mound, Ribes alpinum 'Green Mound': This plant is a dense dwarf with upright branching. Foliage is dark green and fall color is yellow. It grows to 3 feet high and wide.
Blue mist spirea, Caryopteris x clandonensis: A valuable shrub for its soft gray-green foliage color and clusters of blue summer flowers. Grows 4 feet high and wide.
True Dwarf Norway Spruce, Picea abies 'Little gem': This evergreen is a globe shaped shrub with a solid, dense appearance. It grows to 2-3 feet high and wide.
Hope this list is helpful.
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