Groundcovers for Zone 3 Half Shade

I am living up a bit north of Parry Sound, Ontario, Canada. Very close to Killbear Provincial Park. I would like a ground cover for my septic system, that will attract birds and butterflies. We are close to the water, but the area is half shade and sun. The property is all natural with oak, beech and pine. My concern is that the roots of the plants should not grow deep enough to clog the system. I have tried to seed wildflowers for half shade. Some flowers grew but I believe with the wind it was too dry and I believe my soil needs some help too. The soil is mostly sand.

Your property appears to be in zone 4, but the zones are so compressed at your area, that I am sure zone 3 shifts down to your location in many winters. You should therefore stick to plants that are hardy to zone 3. I presume your area has acid soil, since you are on the Canadian shield.
To increase the water retaining properties of your soil and get some microbial action going in your soil, you will have to add lots of humus. Applying a four to six-inch (10 to 15 centimeter) mulch after planting will also help.
To prevent interference with the septic tile bed, you will have to stick to shallow rooted plants. Not much comes to mind that meets all of the criteria and attracts butterflies or birds as well. You have to stay away from trees and strongly rooted shrubs. Trees such as maples are a definite no-no!
You could try lowbush blueberries, Faassen's catnip, chives, meadow clary, red clover, columbine, periwinkle, queen Anne's lace, roseroot, uva ursi (Arctostaphylos uva-ursi) and many annuals such as adonis, balsam, borage, calendula, kennikir (yellow cosmos), citrus scented and French marigold, nasturtium, parsley (for swallowtail caterpillars).