The stems can dry up in hot, dry weather, which can lead to weak growth, yellowing leaves and leaf drop. I use lots of mulch, water deeply and frequently, and keep them under dappled light as much as possible.

For me, stunted and purple leaves are usually only an issue in winter. I refresh the soil in spring, so by winter a lot of the nutrients have been either used up or washed away over the summer. So if you're getting that in summer, it may be a good time to add some fresh compost to the mix. Alternatively, it could be a sign of stress.

Assuming you have the mint contained in pots, you need to find a good way to keep the soil cool so it does not dry out as fast and then get the plants enough sun but not too much heat. I have my mint in pots on my W-SW facing front porch. It gets lots of morning sun, is shaded for the hottest part of the day, and then gets more sun in the late afternoon and early evening. I also have my plastic pots nestled inside some large decorative concrete urns with the top of the pots an inch or two below the rim so that the sun never hits the sides of the pots and the soil line is somewhat shaded. Apple mint :

Here in northern Florida my chocolate mint seems to be loving the intense afternoon/evening sun we've been having. It's been getting into the 90s. I've never been to Texas so I don't know what the humidity is like there but I think the high humidity we get here is helping. I don't have to water the chocolate mint nearly as much as my other container plants. And I think all the foliage helps keep the soil shaded.

If you have low humidity I would suggest watering often and maybe try to keep it out of direct sunlight during the hottest part of the day.

I keep mine in direct sun planted in a pot with NO HOLES. Fill the container 3/4 with potting soil and a little compost or black cow mixed in. Fill the rest of the way w water to create a bog. I have had all kinds of different mints planted this way. They survive the hot FL summers and light freezes in the winter this way. They really go nuts. Just keep them filled w water. I like to have lots of mint available year around to feed our bunnies.

Aren't you taking a risk by doing that, though? You're describing a stagnant environment with added manure. Perfect for E. coli and other harmful microbes. I grow water mint in standing water, but I make sure that there's no manure in it and I keep it aerated.