Dahlias Just as many perennials are beginning to flag in the heat of summer, dahlias are coming into their own. They revel in the warmth to produce an array of shapes, sizes and colours that will bring life and beauty back to your garden borders. Plant in dry and sunny spots and deadhead to prolong flowering. Decorative Dahlias Large, fully double flowers with soft, rounded petals. They have uniform, regularly sized, often quite formal style flowers.