I am a much better hedge and bush gardener than a flower guy, and that is not saying much. That being said let me introduce to you a product which Mrs. T swears by, Soil Moist.
Soil Moist is a granule, about the size of sea salt grain, that you place in containers or in and around plants. The granules absorb water and expand to almost the size of an ice cube, over time they slowly release the water.
It's perfect for those containers you should water daily, but when its hot you may forget for several days.
There is one catch. Do not use too many, or as they say, follow the directions. The expanding granules may actually push the plants out of the soil, if you over do it.
It works, we approve.