If your shady spot is prone to standing water, you have a dark bog. In some circles we call these a swamp . If you don't have the time, money, energy, or a combination thereof, you may be better off ignoring it. There is a wide variety of plants that flourish in shady spots all over the world. But even these require some light.
Mushrooms have their good points.
Lush foliage isn't one of them. Any area protected from the elements by trees usually stays dryer than surrounding areas accessible to normal rainfall. Unless your climate includes torrential downpours on a regular basis, your shady places are going to require supplemental watering to get things going. After they're established you will find most plants adapted to shaded areas will develop deep roots and won't need much watering. This is true of shade-loving perennials .