Ask Mr. Smarty Plants

That is quite a common problem in East Texas, and your trees are a tremendous asset. However, they often are also the problem. Many trees are allelopathic, and develop self defense measures of emitting toxic substances from leaves, roots or stems (or all three) that inhibit growth beneath them. Trees generally don't develop these tendencies until they are somewhat mature, which explains why you can plant under young trees and the understory plants do okay for a few years and then start to dwindle away. And, of course, shade is a problem, too. We consider "full sun" to be 6 hours or more of sun a day, "part shade" to be 2 to 6 hours a day, and "shade" less than 2 hours a day. So, the first thing you have to consider is the light requirements of the plants you are trying to grow. Since you didn't say what trees you have, we don't know to what degree allelopathy may figure in your problem. Some trees, like members of the Juglandaceae family, including Carya texana (black hickory), Carya illinoinensis (pecan), and Juglans nigra (black walnut) are all especially aggressive in defending their turf. Most oaks are also equipped to clearing the ground under them to ensure they get all the moisture and soil nutrients they need. 

Given the choice between having great trees and having ground cover under them, we would vote for the trees every time. And it's necessary to remember that most tree roots occur in the upper 12" of soil, and may extend three times the dripline of the tree. Planting within the dripline can damage the roots of the tree, and the understory plant probably doesn't have a chance of competing anyway. So, we're going to go to our Recommended Species section, click on East Texas on the map, NARROW YOUR SEARCH to first "Herbs" (herbaceous plants) and look for shade ground covers that can grow in sand, and then on "Grasses or grasslike plants" with the same characteristics. When we didn't get enough results from that search to suit us, we went to our Native Plant Database and did a COMBINATION SEARCH with Texas as the state, "Fern" as the Habit, and "Shade" as the light requirement. The Lady Bird Johnson Wildflower Center recommends only plants native to North America and to the area in which they are being grown, as these are adapted by millions of years of experience to the soil, annual rainfall and temperatures of that area. We still can't guarantee that these plants will flourish beneath your trees, and you may decide to simply cover the bare ground with a good quality shredded hardwood mulch. This will have to be replenished from time to time, but as it decomposes, it will add nutrients to the soil, and improve the soil texture, as well as protecting the tree roots from heat and cold. Follow the links to the webpage on each individual plant and learn more about it.

GROUND COVERS FOR EAST TEXAS

Phlox divaricata (wild blue phlox)

Athyrium filix-femina (common ladyfern)

Asplenium platyneuron (ebony spleenwort)

Polystichum acrostichoides (Christmas fern)

Adiantum capillus-veneris (common maidenhair)

GRASSES FOR EAST TEXAS

Bouteloua curtipendula (sideoats grama)

Chasmanthium latifolium (Inland sea oats)

Sorghastrum nutans (Indiangrass)

Phlox divaricata

Athyrium filix-femina

Asplenium platyneuron

Polystichum acrostichoides

Adiantum capillus-veneris

Bouteloua curtipendula

Chasmanthium latifolium

Sorghastrum nutans