Here is a link to a useful Interactive Map showing first and last frost dates for Texas. If you "click" on "Plant Maps" at the top left, you can also find similar maps for other states as well. These maps are great because you can drill down to great detail -- even to your street.
In addition to first and last frost dates, there are also maps for:
Interactive Drought Monitor Map
Interactive Map of Drought Conditions
Interactive Map of Heat Zones
Interactive Map of Ecoregions
Interactive Maps of Native Trees and Plants
Interactive Plantmaps Hardiness Zone Map
According to this map, for Dallas, the average last frost date varies even within the city with some neighborhoods having a last frost date from March 1st through March 10th; whereas, my neighborhood in North Dallas has a last frost date of March 10th to March 20th.