A plant-covered septic bed is better than a naked one (plants use up excess nutrients and help keep soil in place). You just have to pick the right species.
Choose plants with shallow roots: creeping ground covers, or low, dense shrubs.
Avoid periwinkle and pachysandra. They’re non-native, and can choke out other vegetation if they grow too enthusiastically. Instead, check out the berry family (wild blueberry, running strawberry bush, northern dewberry, swamp dewberry) or the cinquefoil family (creeping cinquefoil, three-toothed cinquefoil, shrubby cinquefoil).
Want grass? Use native sedges instead—they stand up well to foot traffic.
Want colour? Add herbaceous perennials such as New England asters, brown-eyed Susans, wild ginger, solomon’s seal, or hepatica.
Don’t buy wildflower seed mixes unless you’re positive that they’re all native. Some commercial mixes include invasives. Oops!
Look at what’s growing on your cottage property already, in areas that have the same soil and sun/shade conditions as your septic area.