Ask Mr. Smarty Plants

First, let us refer you to our How-to Article Water Gardening. Don't forget, a water garden under big trees is going to need frequent cleaning of twigs, leaves, acorns, etc.

Then, from a previous Mr. Smarty Plants question, commenting on the allelopathy of both your black walnut and pecan trees.

"Your Carya illinoinensis (Pecan) may be defending its space—that could contribute to why you are having trouble getting any other plants to grow underneath it. With this defense mechanism (called allelopathy) pecans and all members of the Family Juglandaceae (Walnut Family)—walnuts, hickories, butternuts, and pecans—make and release a chemical called juglone that adversely affects many other (but not all) plants growing near them. The Juglans nigra (Black walnut) appears to produce the most, or at least, strongest dose of juglone.  It can be found in all parts of the black walnut tree—in the roots, the leaf litter and fruit on the ground.  The juglone in pecans doesn't seem to be as strong as that of black walnut and juglone doesn't seem to affect grasses or sedges, but it still might be a factor.  One thing you might do to lessen any possible effect is to remove leaf litter from the ground.  I have found several lists for plants that are resistant to juglone from the Northeast but haven't had any luck with lists for Texas or the Southwest.  Here is information for reducing the effects of juglone and a list of plants from Auburn University that are susceptible to and plants that are resistant to juglone."

Members of the Quercus (oak) genus also exhibit allelopathy, trying to eliminate competition from other plants beneath them.

We are not quite clear about your comment that some plants from our site were missing on a previous answer. We couldn't find the previous answer, but we are assuming we made a plant list in that answer for North Georgia. Obviously, we have thousands of plants in our Native Plant Database, all native to North America. However, when we are offering examples, it will be a selection of plants native to the area from which the question came, habit requested (herbaceous blooming plants, trees, grasses, etc.), and appropriate amount of sunlight and water available for the site. Since there can easily be 1000 of more of herbaceous blooming plants native to a specific state, you're right, lots of those will be omitted from our lists.

To help you with the rest of your question, we will demonstrate how to use our Native Plant Database. Since you are looking at several different parts of your property, including shady and sunny sites (we assume), we will not specify in our sample search the characteristics of soil moisture or light requirement. When you make your own custom search, you can tailor your specifications by those or even so far as bloom color and time, mature height, etc. Just remember, the more numerous the custom specifications, the fewer (or none) plants will fit them and be listed. We suggest you make a map of your property-no surveyors tools required-just sketch in areas with already existing plants and structures, and then watch for several days noting the total amount of sunlight each area gets. As you will note when you are making your search, we quantify "sun" as 6 hours of more of sun a day, "part shade" 2 to 6 hours of sun, and "shade" as 2 hours or less. We selected "part shade" for grass/grass-like, ferns and vines, since you obviously will have a lot of shady areas to plant. We specified no light requirement for the rest, which means you get more choices, but you will have to read the webpage each link takes you to in order to know the growing conditions.

Herbs (herbaceeous blooming plants) for South Carolina: (1,145 possibilities) Amaranthus hybridus (Slim amaranth)

Shrubs for South Carolina: (161 possibilities) Amelanchier arborea (Common serviceberry)

Trees for South Carolina:(193 possibilities) Cercis canadensis (Eastern redbud)

Cacti/succulents for South Carolina: (4 possibilities) Yucca filamentosa (Adam's needle)

Grass/grass-like part shade in South Carolina: (107 possibilities) Chasmanthium latifolium (Inland sea oats)

Ferns for part shade in South Carolina: (25 possibilities) Adiantum capillus-veneris (Southern maidenhair fern)

Vines for part shade in South Carolina: (72 possibilities) Ampelopsis cordata (Heartleaf peppervine)