First of all, we're begging you, don't buy nor plant those desert willows until late Fall or early Winter. That is asking for transplant shock and early death if you plant them in the blazing heat and drought under which all of Texas is suffering. November is about as early as we would think safe and you don't want them standing in a black plastic pot having their roots fried in the sun until then, either. From our webpage on Chilopsis linearis (Desert willow) here are the optimum growing conditions for this small tree.
Water Use: Low
Light Requirement: Sun
Soil Moisture: Dry , Moist
CaCO3 Tolerance: Medium
Drought Tolerance: High
Cold Tolerant: yes
Heat Tolerant: yes
Soil Description: Well-drained limestone soils preferred, but also does well in sands, loams, clays, caliches, granitic, and rocky soils. Minimal organic content the norm.
Conditions Comments: Allow to dry out between waterings, as this will encourage more extensive waves of blooms. Avoid excessive water and fertilizer, as that can lead to overly rapid growth, fewer blooms, and a weaker plant. Prolonged saturation can result in rot. Wont grow as fast or get as large in clay soil but wont suffer there either. Can be drought-deciduous in some regions. Can survive temperatures as low as 10 degrees F."
According to this USDA Plant Profile Map, this tree is reported as growing mostly in far West Texas, and about the only county where it is reported in North Central/North East Texas is Dallas County, so it would appear you would have an appropriate soil. If you have clay soil, and you very likely do, even though Desert Willow will tolerate clay soils, it will NOT tolerate water standing on its roots, as often happens with clay soils. When you get to the cooler season and can plant your little trees, dig a bigger hole than is necessary for the roots and mix the soil from the hole with some sand, degenerated granite or (our favorite) compost. This will loosen up the clay and permit the tiny hairlike rootlets to penetrate the soil for oxygen and nutrients.
Otherwise, we think your plant choice is a good one, as it won't grow tall enough to interfere with power lines and will bloom much better with more sun. We wanted to establish first what the tree needed, as we think that is of prime importance, and then find grasses that can prosper in the same conditions. The grasses will need to be able to tolerate partial shade but not the deep shade they would encounter in a denser or evergreen tree, like live oaks. We will go to our Native Plant Database, scroll down the page to "Combination Search," indicate Texas as the state, "grass or grass like" for Habit, "moist" for Moisture Requirements, and "part shade" for Light Requirements. This will make it possible for the grasses to live in the same environment as the tree, and tolerant of the part shade the tree will cast. We will also check each of our selections on the USDA Plant Profiles to assure that the chosen grasses will do well in Dallas County. You can follow each plant link to our webpage on that plant to find out its growing conditions and soil preferences.
Native ornamental grasses for Dallas, TX:
If you have difficulty in locating these plants native to Texas in local nurseries, go to our National Suppliers Directory, type in your town and state or just your zipcode in the "Enter Search Location" box, click on GO and you will get a list of native plant nurseries, seed suppliers and consultants in your general areal. Each have contact information so you can find out ahead of time if they have what you are looking for.