Dahlia Blue wish initially opens up from a purple tipped, white petals into a more uniform shade of lilac. And as the flowers are large it creates and impressive effect. They are eye-catching plants for borders or cutting Height 1.2 M. Flowering July-September. 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.