This informatiom is specific to Minnesota and may or may not be applicable to your area.

Permanent vegetation cover is great because it minimizes topsoil loss. Open areas are more susceptible to frost, heaving, and erosion. Plants can trap snow, which acts as mulch and prevents erosion. Here are some guidelines that will help you landscape your septic mound:
1. Always wear gloves to minimize contact with the soil.
2. Topsoil on the mound should be at least 6 inches and no more than 30 inches.
3. Use minimal tilling when planting and establish a cover as quickly as possible to limit erosion.
4. Use plants that like dry soils to prevent roots from interfering with the septic system.
5. Unfortunately, you will need to use nonwoody plants on the mound not trees and shrubs. You can frame it with trees and shrubs at a distance, but the trees need to be at least 20 feet away. Trees that seek water, such as poplar, maple, willow, and elm, should be planted at least 50 feet away.
6. Never plan to irrigate this area.
7. Minimize traffic on the mound to avoid soil compaction.
8. Do not plant edible plants, such as vegetables and herbs, on a mound or drainfield.

Perennial flowers, such as day lilies and peonies, can be grown on mounds. Mulch or plant them close together so soil will not be exposed and erode.

Native wildflowers and grasses seem to be the most successful. Some are:
Prairie onion (Allium stellatum)
Pussytoes (Antennaria neglecta)
Butterflyweed (Asclepias tuberosa)
Prairie clover (Dalea spp.)
Pale purple coneflower (Echinacea angustifolia)
Rattlesnake master (Eryngium yuccifolium)
Prairie smoke (Geum triflorum)
Oxeye (Helianthus helianthoides)
Rough blazing star (Liatris aspera)
Wild bergamot (Monarda fistulosa)
Penstemon (Penstemon spp.)
Heath aster (Aster ericodes)
Bigleaf aster (Aster macrophyllus)*
Pennsylvania sedge (Carex pensylvanica)*
Wild geranium (Geranium maculatum)*
Violets (Viola spp.)*
Sideoats grama (Bouteloua curtipendula)
Blue grama (Bouteloua gracilis)
Little bluestem (Schizachyrium scoparium)
Prairie dropseed (Sporobolus heterolepis)
June grass (Koeleria macrantha)

Your local Extension office has more information on landscaping a septic mound.