Colourful Summer Garden Essentials. It’s hard to choose which Dahlias to plant these days. From the original 30 species, nearly 60 000 cultivars have been developed in 9 categories - the choice can be almost mind-blowing. Dahlias are happy in borders or beds with other flowers, or in a dedicated display. Wonderful cut flowers, any blooms cut will be replaced with new ones. Flowering time is also extended by removing dead heads. Dahlia tubers like full sun, a sandy soil with good drainage, and should be lifted and stored in winter, or at least protected with peat or straw. Dahlia flowers can range from 5 to 25 cm (2 to 10 inch) in diameter and those larger-flowering ones might require support canes.