Yes they will continue to produce flowers all season long until it freezes, but in TX that could be a while. Maybe there they'll just poop out in November or so; not sure.

If left on the plant they will stay nice a week or so, mine never get a chance to stay on the plant. If they start to brown on the ends of the petals, time to cut them off and signal the plant to produce more. Heavy rain- laden blooms invite broken plants, I will just caution you. If we're going to get a big storm or wind, I get outside and cut as many as possible to lighten the load.

Cut dahlias in the early morning or late evening into a bucket or vase of fairly hot water (beyond warm and not steaming hot)with floral preservative in. Put in dark, coolish place to rest for at least several hours, then arrange as you want. I did a little experiment last year re-cutting the stems under water or not recutting and it didn't seem to matter one bit. Cut dahlias can last up to 5 days but more often they start to tire after 3-4. I am constantly amazed at what shreds of beauty people still see in a dahlia bouquet I've brought them. After a week, the flowers look half composted to me and they say "oh but look at the bit of purple still in the center etc etc!"