Ask Mr. Smarty Plants

From a previous answer   about planting under magnolias:

Magnolias are allelopathic to other plants.  This means that the magnolia produces chemicals that inhibit the germination of seeds and growth of plants that are potential competitors for its resources.  The roots and fallen debris (leaves, flowers, etc.) from the magnolia tree contain substances that limit the growth of other competing species that grow underneath it. In the case of magnolias, the substances are sesquiterpene lactones—costunolide and parthenolide—(see Abdelgalel, A. M. and F. Hasinaga.  2007. "Allelopathic potential of two sesquiterpene lactones from Magnolia grandiflora L." Biochemical Systematics and Ecology Vol. 35, no. 11, pp. 737-742.)  A complicating factor is that many plants won't grow in the dense shade created by the magnolia tree—or any other tree, for that matter.

I could find no list of plants that are resistant to the allelopathic effects of magnolias.  So, my recommendations to you are:  1) keep the debris from the magnolia cleaned up from the area where you want the flowers/plants to grow, and 2) choose native plants that are shade-tolerant. There is no guarantee that these will do well, either. Follow each link to the page on that particular plant to learn about propagation, conditions, expected size, etc. Here are a few recommended shade-tolerant plants that are native to Harris County or a nearby Texas county:

Shrubs for Shade in Houston:

Callicarpa americana (American beautyberry)

Hibiscus laevis (halberdleaf rosemallow)

Malvaviscus arboreus var. drummondii (wax mallow)

Symphoricarpos orbiculatus (coralberry)

Herbaceous Blooming Plants for Shade in Houston: 

Coreopsis lanceolata (lanceleaf tickseed)

Lobelia cardinalis (cardinalflower)

Salvia coccinea (blood sage)

Podophyllum peltatum (mayapple)

From our Native Plant Image Gallery: