Dian, I like your idea of growing your own fertilizer cover crops over the septic field! I have never had great luck growing
native legumes for cover crops, but pretty native ground cover legumes for Eastern Canada could include: Canada Milkvetch (Astragalus canadensis), Purple Prairie Clover (Dalea purpurea), Showy Tick Trefoil (Desmodium canadense), False Indigo (Amorpha fruticosa), Round-headed Bush Clover (Lespedeza capitata), Wild Indigo (Baptisia tinctoria), Wild Lupine (Lupinus perennis). Remember that some of these legumes can be toxic. For example, Lupines contain alkaloids that are known to be toxic to humans and animals, including possible dogs. Again, I like your idea of growing alfalfa in beds and/or a mix of annual and
perennial clovers over the septic field.