Ask Mr. Smarty Plants

Frankly, the best thing we can recommend is just another version of what you already have there, which is native grasses. We know there has been more rain north of Dallas than there has here, so maybe they will get enough moisture to survive; obviously, the grasses that are already there have. Whether they will discourage other plants, we can't say. Even when grasses are planted as a landscaping feature in a lawn with water available, weeds have to be pulled and just about any plant needs water when it is first planted. Another possibility is one of the yuccas native to that part of the state. Since they are very tough desert plants, they could probably withstand the neglect better than just about anything, and as they grow, they should shade out the prairie grasses. Since there is a possibility of rain there, if you choose to plant a yucca, you need to make sure the drainage is very good. You almost can't kill a yucca, but if it has water standing on its roots over a period of time, it will die. The disadvantage to the yucca is that it is going to grow taller than your specified 18 inches, and when it blooms (which is pretty spectacular) it will be taller still. That's a decision you will have to make.

We have all heard of the roses planted in cemeteries that survived for many years without any attention. There are native roses, although most of the roses commercially available are highly hybridized plants, largely originating in China. Of those roses native to North America, ("wild roses") only Rosa arkansana var. suffulta (prairie rose) and Rosa carolina (Carolina rose) are native to Texas, and neither of them are shown in the USDA Plant Profiles as growing naturally in the area you are concerned with.

Finally, if none of these is going to work for you, you might consider some curbing and attractive river rocks or small stones. Weeds will still push up through them, but should be fairly easy to keep under control.

Grasses 

Bouteloua curtipendula (sideoats grama) - 2 to 3 ft. tall

Bouteloua dactyloides (buffalograss) - 3 to 12 inches tall, semi-evergreen

Bouteloua gracilis (blue grama) - 12 to 14 inches tall

Poa arachnifera (Texas bluegrass) - 12 to 18 inches tall

Other plants 

Rosa arkansana var. suffulta (prairie rose) - 6 to 18 inches, blooms pink June and July

Rosa carolina (Carolina rose) - 1 to 3 ft. high, blooms pink May and June

Hesperaloe parviflora (redflower false yucca) - not a true yucca, but a member of Century Plant family, 2 to 3 ft. tall, evergreen, flowering stalk rises 5 ft., blooms red, yellow March to May

Yucca rupicola (Texas yucca) - evergreen, under 2 ft. tall, flowering stalks over 5 ft. tall, blooms white, green April to June

Rosa arkansana var. suffulta

Rosa carolina

Hesperaloe parviflora

Yucca rupicola

Bouteloua curtipendula

Bouteloua dactyloides

Bouteloua gracilis

Poa arachnifera