Mixing slow release fertiliser with the soil when first planting up the container should help keep the plants well fed. But summer containers in particular benefit from additional regular feeding with a high potassium liquid food whilst the plants are in flower.
Regular deep watering of your summer container is much better for plants. It encourages plant roots to grow to the bottom of the container. Whereas quick, short bursts of watering produce shallow rooted plants that will dry out quickly compared to the deeper rooted ones. It’s also better to water first thing in the morning. As evening watering creates a warm, dark, moist atmosphere overnight much beloved of slugs and snails!
Here are 3 different look summer containers to take you through to autumn and beyond: