Dahlia Bishop Of Canterbury Single flowered brilliant purple flowers that can reach 90 cm height. Deep purple beautiful foliage. Dahlias are tender tubers. Plant them before the frosts are over, they may get frosted and die, so it is advises to pot them during late March or early April, in a three litre pot with multi-purpose potting compost. Keep them in a light, frost free place and ensure the compost is kept moist. They can be taken outdoors when the danger of frost has gone . Compost : Dahlias grow and flower best sunny spots and they thrive in a fertile soil, with good moisture and good drainage.