I am sorry for this delayed reply to your gardening question as we are transitioning to a new Q&A system. I hope this reply is not too late to be of help.
Verbena can suffer in the heat of a Texas summer but with adequate soil moisture and a little fertilizer will stay in decent shape. Your plants probably bloomed themselves into a weakened state. Shear them back by half and then fertilize and water to try and reinvigorate them.
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