Underplanting trees gives a sophisticated and manicured look to a garden. It also provides an additional layer of visual interest that frames the tree. Southern magnolia, known botanically as the Magnolia grandiflora, has a dense canopy of large leaves year-round with summer blooms. Underplantings must be very tolerant of shade conditions, be placed carefully and be watered and fertilized regularly, because they are competing with the tree's roots to some degree.
Choose shade tolerant perennial plants for under your Southern magnolia such as hosta, trillium, hylomecon, goldenseal, European ginger, hakonechloa, ferns or heuchera. Perennials keep you from having to disturb the soil frequently, as may be required for annuals or bulbs.
Prepare the planting soil starting a foot or two out from the trunk of the tree extending as far as you desire. Clear the soil surface under the tree, removing old plants, lawn grass, leaves or any other debris. Till the top 6 to 10 inches of soil carefully to loosen it. Lay 3 to 4 inches of compost onto the soil along with a dose of slow release fertilizer following the product label directions. Mix the compost and fertilizer into the native soil lightly to distribute.
Plant your chosen perennials in groups or drifts around the tree, spacing the plants in keeping with their individual size and natural spread. Excavate a hole to accommodate the root ball, set the plants in so that the top of the root ball for each is level with the surrounding prepared soil. Back fill soil around the root ball and press down to secure the plants in place.
Water the plants and surrounding soil well until drenched to a depth of at least 6 inches. Do not water over the top of the plants but under them, flooding the surface of the soil. Irrigate the soil deeply once a month to keep the tree roots moist and lightly every seven to 12 days as needed to keep the surface soil moist for the underplantings.