It’s a natural tendency to want to extend your flower garden to your spaces beneath trees to fill in otherwise bare ground. For the most part, plants that grow below tree canopies must be known to tolerate partial to full shade. Fortunately, there are plenty that take those states provided that you give them routine water. You can use other landscaping strategies for even those plants that need brighter light.
Plant Little
Some trees have shallow root systems that you may damage by digging large holes in the soil around them for new plants. You can avoid damaging roots by planting very small plants and being patient while they grow larger. Or plant perennials, like shade-loving plantain lilies (Hosta hybrids) in U.S. Department of Agriculture plant hardiness zones 5a via 10a, rather than annuals that require you to disturb the soil each year.
Know Your Tree
Some trees are more tolerant of root disturbances compared to others. For those that are very sensitive, like cherries and plums that grow in USDA zones 3 through 9 or dogwoods (Cornus spp. and cvs.) That grow in USDA zones 2 through 9, be very cautious when planting. On the flip side, you do not have to worry as much with crabapples (Malus spp. and cvs.) That grow in USDA zones 2 through 9 or silver maples (Quercus alba) in USDA zones 5 through 9.
Bulbs
Many gardeners plant early spring flowering bulbs under deciduous trees prior to the leaves completely form, so there’s a brief period of time when the ground under the tree becomes sunlight. If you select early-blooming varieties of bulbs, then you may give them the full sunlight they need from the spring. These include Tazetta daffodils (Narcissus), which grow in USDA zones 7a via 10b.
Plants for Partial Shade
For big expanses of partial shade in well-drained, acidic soil in USDA zones 7a through 10a, varieties of azaleas and rhododendrons (Rhododendron) grow from 1 foot tall and wide to individuals who can reach 20 feet tall and 10 feet wide. If you would like a more compact plant, then pick coral bells (Heuchera hybrids) that grow with chartreuse, bronze or yellow leaves in USDA zones 5b via 10b.
Plants for Deep Shade
The lenten rose (Helleborus orientalis) grows in either partial or heavy shade in USDA zones 6a through 10b. Native to Greece, Turkey and the Caucasus, the plant blooms in the winter with 2- to 4-inch, bell-shaped flowers. All hydrangeas do well in partial shade, but oakleaf hydrangea (Hydrangea quercifolia), also a big 6-foot-tall and 8-foot-wide shrub, also tolerates deep shade along with the dry conditions that occasionally occur under trees. It grows in USDA zones 6a through 10b.