This is just from watching my mom who has awesome geraniums all over the place. We have very hot and humid summers in Oklahoma as well, it temps in the 90s are the norm here with temps oftentimes going into the 100s, but sadly, rarely into the 80s for highs. She has all her geraniums in pots, I think about 12 inch. She puts them on an area that gets sun from noon until almost sunset. She does water them almost every day, they can get by with every other day as long as you don't do that too often. I think she fertilizes them every other week. She also deadheads them often. In the winter she just takes them into the green house.

This is what she does and she has been able to keep the same geraniums for years and propagate them endlessly(I don't think she has ever bought new geraniums in 10 years).

I don't think shey should be a problem in Houston, being that they do grow very well in Mexico where they are a favorite. My mom used to grow them down there as well.

My plain old red geraniums have done well for 2 yrs. They
get morning sun for 2-3 hrs then light shade for the rest
of the day. Water about every other day and fertilize
with Agroflash every couple of weeks. They are in the light
weight pots made of styrofoam or something similiar. Old
blossoms are carefully removed. I live north of Houston.