Mr Smarty Plants thinks it should have been answered already also, but I did a few searches and either got too much return, with nothing particularly useful, or nothing!
We can, of course, take the direct route. In the ‘Recommended Species” lists we have the capability to search the lists for selected attributes. This link is to the “Central Texas” list of recommended species. When I select for “herbs” [wildflowers] with a bloom time of October, November & December, I still had 25 plants that might bloom late. The six of the first of these are:
Chamaecrista fasciculata var. fasciculata (Partridge pea) [May-Oct]
Conoclinium coelestinum (Blue mistflower) [July-Nov]
Coreopsis tinctoria var. tinctoria (Golden tickseed) [Feb-Nov]
Glandularia bipinnatifida var. bipinnatifida (Prairie verbena) [Mar-Dec]
Helianthus maximiliani (Maximilian sunflower) [Aug-Nov]
Hibiscus martianus (Heartleaf rosemallow) [Jan-Dec]
As you can see from the extended bloom times, these may be the wildflowers that you associate with late summer. The trick is extending the bloom time. In the heat and drought of the “normal” summer we’ve had lately they may well bloom once and be done. With some pampering, this bloom time may well be extended into the late fall!
Yes, a groundcover can also keep your area green, and not necessarily be too dense for the wildflowers to penetrate. Dichondra carolinensis (Carolina ponysfoot) is native in the central texas area. Dichondra recurvata (Oakwoods ponysfoot) and Phyla nodiflora (Texas frogfruit) can also serve well. In the late summer to early winter times though, sort of like the wildflowers, the plants will need to be coddled a bit to keep them green and nice.