Planting flowers under a tree requires careful consideration for the health of both the tree and any new plantings. The location of roots should be of primary concern when planting flowers around a tree. Another concern is limiting damage to the roots during planting. For this reason, it's best to plant perennials instead of annual flowers to limit soil disruption.
Remove debris and dig up any weeds beneath the tree.
Examine the area beneath the tree, checking for roots close to the surface. Choose spaces for the flowers between tree roots.
Disrupt the soil as little as possible by digging carefully with a trowel for plants in smaller 4- to 6-inch pots. Dig holes roughly 1 1/2 times the size of the pots. Use a larger shovel for larger potted flowers. Be careful to avoid chopping tree roots when digging around a tree.
Remove dirt from the hole and toss in a few handfuls of peat moss. Mix additional peat moss into the soil in your fill-dirt pile.
Remove plants from their pots. Loosen the roots of the plant by working your fingers into the soil around the root ball to encourage outward root growth when planted. If hole size is a concern, shake some of the dirt off larger root balls to limit the size of planting holes.
Place the plant into the hole to test the depth. Position the flower so the top of the root ball lies about 1 inch below the surface.
Add the soil and peat moss mixture around the roots of the plant. Firm the soil and continue adding dirt until the hole reaches level with the surrounding ground.
Add a 3- to 4-inch layer of shredded mulch to the garden surface to help retain moisture and keep soil cool.
Water the flowers and tree deeply using a soaker hose to allow water to seep down into the soil. Remember that the flowers and tree will compete for moisture and soil nutrients so water frequently and fertilize regularly.