Adding plants under trees presents a unique challenge for gardeners. Trees require major quantities of moisture and nutrients from the surrounding soil. Planting flowers around trees creates a competition for essential nutrients that keep both the flowers and tree healthy. Strike a balance when planting flowers around trees by choosing compatible plants with shallow roots. Enhance the soil regularly to benefit the tree and flowers, and to create a lasting, healthy garden to complement the space beneath the tree.
Evaluate the amount of light available under the canopy of the tree. Prune back any branches to thin the canopy to allow more light to reach the lower levels of the tree. Perform this task before planting flowers around trees to prevent trampling of new plantings.
Select plants that work well for the soil and sunlight conditions. Choose shallow-root, small plants that won't interfere with the tree roots when they reach full maturity. Shade and partial shade annuals will work fine around trees but consider alternating annual plantings every other year to limit disturbance and potential damage to the tree roots.
Use the trowel and spade to remove all weeds and grass from the planting area. Dig carefully to avoid cutting into tree roots, which tend to travel outward at roughly 12 to 36 inches beneath the surface, according to the University of Minnesota.
Place flower pots between areas of roots for easy digging and to protect the tree. This dry run will help with plant placement as well as with decisions in color, balance and design.
Carefully dig individual, shallow holes for each new flower at least 1 foot from the tree trunk. Throw in two handfuls of peat moss and plant each flower, firming the soil completely around each plant. Move to a different location if you encounter a tree root. Hacking through the root subjects the tree to potential parasites and molds that can stunt growth and harm the tree.
Mulch the flowers with a 3- to 4-inch layer of mulch to improve moisture retention and to control soil temperature. Do not pile mulch around the base of the tree.
Water well until the flowers establish, but don't over water. Remember that the tree receives the same amount of water as the flowers.