Lawns under oak trees are difficult to maintain and may compete with the tree for water and nutrients. A well-planned flower bed solves both problems, and beautifies the area, as well. Small oak trees may produce deep shade directly under the canopy, while the high, spreading canopies of taller trees allow more dappled sunlight to reach the ground. When selecting flowers, consider the amount of sunlight available. Some oak trees, such as white oak or gambel oak have a drooping or shrublike appearance. Flowers won't grow well under these species.
Mark the area you plan to use as a flower bed with stakes and string.
Remove the lawn in the flower bed area. Water the soil the day before so the earth is moist, but not soggy. Dig into the earth with a shovel to remove the lawn in 6-inch clumps. Hit the clumps against your shovel to shake off excess dirt and discard or compost the grass.
Install edging material around the perimeter of the flower bed if you plan to edge it with bricks, metal or stone.
Select plants suitable for your climate, as well as for the shade provided under the oak tree. In general, perennials and bulbs are better than annuals because they don't require yearly digging, which may damage the oak tree's roots. Choose young plants that require small planting holes.
Dig holes for the plants with a trowel, spacing them at least 12 to 18 inches apart. Consider the mature size of the plant when choosing its location. Put large plants close to the tree with small ones on the perimeter. Back-fill the holes with soil and tamp down lightly. Water the new plants thoroughly.
Lay a 2- to 3-inch layer of wood chip mulch around the perennials. The mulch conserves moisture and keeps weed growth down.
Oak trees have roots that extend directly beneath the surface to as much as 70 feet deep in the ground. The roots may stretch past the canopy, as well. Don't dig more than 6 inches deep into the soil to avoid damaging the roots of the oak tree. Spring-blooming bulbs require full sun, but grow well under oak trees because they bloom before the tree's leaves emerge in the spring. Try daffodils, grape hyacinth or tulips. Choose shade tolerant perennials such as bleeding heart, astilbe, bell flower, day lily, or primrose. Flowering ground covers, such as bugleweed and lily of the valley are also good choices, but avoid aggressive plants that will compete with the oak tree and other flowers.
Don't use power tools, such as a rototiller to prepare the flower bed. You'll injure the oak tree's roots.