Magnolias, especially evergreen varieties, can create dry and shady conditions under their branches as they mature. The fact that the roots are shallow and can spread out farther than the tree’s branches adds to the problem. The situation under large, mature magnolias can be improved by the thoughtful removal of a few low branches to allow in more light. This is a skilled job, so hire a qualified arborist.
It can be tempting to bring in more soil and spread it over the area under the tree to create areas that can be planted. The problem is that a deep layer will tend to cut off the air supply to the roots and then the tree’s roots will usually spread upwards into the new soil and
Some recommend siting your garden shed under the magnolia or, if the view is appealing, a summerhouse in the shade may partly solve the problem. There are also a number of ground cover plants that thrive in dry shade including ivies, periwinkles and pachysandra. Plants like ivies that send out creeping shoots along the soil surface can be planted under the edge of the tree’s canopy, where the light is better, and the shoots guided into the shade.