Perennials that bloom all summer are few in number. These plants are different from annuals 
in that they must put energy into their root systems in order to survive whereas annuals must put all their energy into flower and seed production.
The few that there are include all the different species of Coreopsis, Geraniums, Chrysanthemums and Campanulas.
Perennial geraniums are leafy plants with small pale pink or blue flowers. The frost tender “geraniums” with their big showy flowers are actually pelargoniums.
You could fill a large garden with the many varieties of coreopsis and have flowers from 6in to 4ft high. What else would you need? Coreopsis flowers come in all shades of yellow from almost white to orange. They are extremely vigorous.
Chrysanthemums need no introduction. Some set seed very easily. In my garden the most common weeds are chrysanthemums.
Campanulas are low growing leafy plants with striking blue flowers.
Foxgloves bloom all summer, too. Be careful to hoe out any seedlings though because most garden foxgloves are hybrids and do not breed true.
Almost Perennials
Most gardeners would include a few other woody long-flowering shrubs as long-flowering perennials. Roses (especially Rosa Rugosa, the hedging rose), Honeysuckles and Potentillas fall into this category.
Dahlias flower from July until the frosts and offer some of the brightest colours you will find in a garden. Their citrus yellow and flaming red flowers brighten up any dull summer’s day. I have only started growing dahlias very recently but I am a convert.
Some showy annuals self seed very successfully, making them as good as perennials in terms of low labour requirements. Snapdragons (antirrhinums) and nasturtiums are both stunning annuals that sow their seed very readily and bloom all summer long. Snapdragons have complex spikes of acid yellow and pink flowers, while nasturtiums have large trumpet-shaped flowers that are most commonly orange, but also come in reds and yellows.
