Trees That Can Be Planted on a Roof Garden

Planting a tree in your roof garden creates a focal point. The upward lines of a tree, even a small tree, convey the feelings of a fresh-air room rather than just a work area. Finding the right tree involves considering space, weight and the quality of light or shade your garden has. The beauty a tree adds to your garden makes the effort worthwhile.

If you are converting new space to a roof garden, an engineering inspection can guide your tree choice. Garden soil from 8 inches to 4 feet deep can exert a roof-weight pressure of between 80 and 150 pounds per square foot. Allow 2 cubic feet of soil per square foot of potential tree canopy, advises PBS "For Your Home" host Vicky Payne. Water adds more weight, so keep this in mind.

Although you may create raised or recessed planting beds for your garden, it is important to remember that, especially for large plants like trees, these structures have more in common with containers than with in-ground beds. Nutrition and water are not available from adjacent soil, and rigid barriers may impede root development. Trees bred for containers make smarter choices for roof-garden planting, even if the garden has beds.

Planting a flowering ornamental tree in your roof garden provides two or three seasons of changing visual interest. You can find both fruiting and nonfruiting deciduous varieties that provide spring flowers, summer shade and fall leaf-drop, which lets winter sun warm your space. For example, the seasonal display of multistemmed "Autumn Brilliance" apple serviceberry (Amelanchier x grandiflora "Autumn Brilliance"), which grows in U.S. Department of Agriculture plant hardiness zones 3b through 7, extends to purplish bird-attracting fruits and red-orange fall foliage. A group of small magnolia hybrids (Magnolia liliflora), casually referred to as "the girls," which grow in USDA zones 3 through 8, tolerate container or planter conditions. Creamy flowers and semi-evergreen glossy foliage create long-season interest.

Dwarf Japanese maples (Acer palmatum) derive their beauty from only their foliage. A "Tamukeyama" Japanese maple (Acer palmatum var. dissectum "Tamukeyama") which grows in USDA zones 5 through 8, has feathery leaves that change from dark purple in summer to bright red and yellow in fall. Approximately 7 feet tall and wide, it contributes textures, dappled shade and an irregular shape to a rooftop garden.

Planting an evergreen in your roof garden creates a consistent profile and definition of space. An evergreen will provide year-round shade, and your choice may depend on how dense you want that shade to be. A "Hillside Upright" spruce (Picea abies "Hillside Upright"), which grows in USDA zones 3 through 8, will cast a fairly classic conical evergreen shadow 10 to 12 feet high and 6 feet wide. The shadow of a similarly-sized "Cesarini Blue" flexible pine (Pinus flexilis "Cesarini Blue"), which grows in USDA zones 3 through 7, on the other hand, is far more fluid, open and less dense.

Planting a fruit tree adds another aspect to your roof garden, either as a focal point or part of a larger edible landscape. Providing the required growing conditions may involve creating microclimate options beyond those customary to the standard hardiness zone. In USDA zones 8 through 10, you can choose semitropical fruits like Meyer lemons (Citrus limon "Meyer") and figs (Ficus carica). Dwarf apple varieties (Malus domestica) offer fruit-growing options in USDA zones 3 through 8, although you will need to find a variety that's self-pollinating or plant two trees.