Depending on the variety, dahlia flowers can reach up to 40 cm across or produce flowers as small as 9 cm in diameter. While the plants are perennial, they cannot tolerate freezing temperatures so the tuberous root system must be dug up and stored indoors each autumn. Planting the bulbs at the right time not only protects them from cold weather, it also ensures you get the longest possible flowering season from the plants.
Spring outdoor planting
Dahlias cannot tolerate even light frost. If you plant the tubers directly in garden beds, wait until one to two weeks after the last expected spring frost. Waiting until after all chance of frost is past also ensures the soil has warmed up sufficiently for the tubers to sprout and grow quickly. Space dahlias around 1 metre apart in the bed to provide enough space so the plants don't become overcrowded once they reach their full size.
Potted tuber planting
Planting the tubers in pots allows you to start the plants earlier in spring and results in earlier flowering. Start the tubers in pots four to six weeks before the last expected spring frost date. Place the pot in a sunny window and keep the soil moist at all times once new growth emerges from the soil. The plants must be kept in a warm area indoors when you start them earlier in spring, as cold can damage or kill the plants.
Moving pots outdoors
Pot-grown dahlias can be left to grow on in their pots, or you can transplant them out to the garden bed. Move the pots outdoors after the last expected spring frost. Bring the pots indoors if cold weather is predicted. If you prefer to transplant the dahlias to the garden bed, wait until two weeks after the last spring frost date. The additional time spent outdoors in the pots helps harden off the plants so they are well adjusted to outdoor conditions before you transplant the flowers.
Dahlias do not grow true from seed. The flowers and size of the plant may be drastically different than that of the mother plant that supplied the seeds. If you choose to start from seed, plant them indoors in late February or early March. Seeds take approximately seven days to germinate. Once the seeds sprout, grow them indoors as you would potted dahlias and move them outdoors only after frost danger is past.