Herbs as Groundcover in Zone 8 to 9

I live in Houston, Texas, USA and have a plot approximately 15' by 30'. I am considering planting a herb, e. g., thyme, pennyroyal, chamomile or other as ground cover. I want the least maintenance possible. The plot has a thin layer of topsoil of 2" to 3" over clay and drains well. Please advise of your recommendation, plants needed and approximate cost thereof.

Houston, Texas, is on the border between Zone 8 and 9. If you live north of Houston, or if the spot is shaded around noon to 3 PM your best bet for least maintainance is probably creeping thyme (Richters Catalogue number P6463) or for a higher mat, wild thyme (S or P 6470, seed or plants respectively). If you are further south I would recommend orange spice thyme (P6465-800). Other lovely if not as tough choices of Zone 9 thymes can be found in our catalogue.
In chamomiles you should probably stick to Roman chamomile, since it can take more heat. Pennyroyal needs damp, sandy, acid soil.
Plants should be spaced on 6 inch (15 centimetre) centres with plug trays being the most economical solution. You will need 30 x 60 = 1800 plants. The least expensive plugs are wild thyme which are priced at $33 (U.S. funds) for a tray of 120 plugs when you order 10 or more trays; this works out to $500 for 1800 plants that you need. Other plug varieties are priced higher.
Seeding is far more economical, but the choice of thymes is limited to wild thyme, and the only mat-forming chamomile available in seeds is roman chamomile.
Please check our web-site since we have answered similar questions before and posted the answers on our web-site www.richters.com under the "Q&A" heading in the "News & Info" section of the site. Click on "Search Question & Answers" and type in "ground cover", to find the topics that sound as if they would answer your question. Seeding rates can be found in the commercial section under ProGrowers info.