yes, the heat is an issue for hydrangea.
i checked and raymondville looks to be in
zone 11. which doesn't mean you couldn't successfully grow hydrangea...you'd need to make sure it's in a pretty shaded spot and keep it very well watered. and i'd make sure to get one of the varieties that are recommended for
zone 8 or 9.
anything that is more for
zone 5 or 6 really wouldn't do well where you are (too hot there).
altho, the whole discussion is kind of moot if whoever you spoke with at home depot claimed they are houseplants. that statement basically says it all about whether these are suitable for your area.