When is the best time to water flowers when it's hot outside ?

The night before the hot day. Allows time for the plant to take in the water and is then able to more easily withstand the stress of high temperatures.

https://bonnieplants.com/library/hot-summer-watering/

We live in south Texas, so we water at night (after dark). The plants have a chance to "drink" the water and it looks like a jungle around our house.

Never water when sun is shining directly on the plants. Either early morning or after the sun goes down

Water all your beds early in the morning during those hot sultry days. I use soaking hoses that I keep hooked up to my outdoor water faucets. I turn them on for 15 or 20 minutes every morning. Of course, the hose has to be picked up on mowing days. If you have restrictions on watering, always obey them. Best of luck to you.

After the sun goes down. When sun is out makes them dry out quicker.

The best time to water your lawn or flowers/plants is in the morning before it gets too hot. When watering in the morning, it allows time for the object being watered to dry out during the rest of the day. One reason NOT to water in the evening when it's cooler. There's no time for the plants to dry out before the sun goes down. And if you're watering roses, be sure to water only the ground around the bush to prevent black spot.