How to Grow Dahlias
Dahlia tubers should be planted in the Spring, when soil temperatures are warm enough that they won’t freeze (late April-May is a good time in Vancouver).
Find a spot to plant them outside. Directly in the garden is preferable, but they can also be grown in large pots (minimum 30 cm diameter).
Dahlias prefer well-draining soil and at least 6-8 hours of direct sun per day.
Plant the tuber horizontally (parallel to the soil surface), with the eye or sprout facing up towards the sky, about 10-15 cm (4-6”) deep.
Dahlias become tall and bushy, so leave about 30-45 cm (12-18”) of space between plants.
Don’t water until a shoot has emerged. Once the shoot has emerged, water deeply about once a week.
When the shoot is about 20-30 cm tall (8-12”), cut off the top 7-10 cm (3-4”) right above a set of leaves. This promotes branching.
Pick all the flowers! The more you cut, the more they bloom.
In the fall…
After a hard frost, the plants will turn black. Cut down the plant, leaving about 15cm (6”) and leave it for two weeks. After that, carefully dig up the tubers (a fork is best) to store over the winter. Loosely shake off the soil and place in crates or cardboard boxes, in a cool but not freezing space. Divide the tubers in the spring!
Alternatively, you could heavily mulch your dahlias and keep them in the ground. You risk losing them if the winter is very cold, and won’t be able to divide them, but it could save a lot of work and they’ll come back bushier!