Posted: 22/03/2015 at 16:44
Sarah, ordinary multi-purpose compost will be fine. Yes, they are perennial, but like all dahlias they are tender perennials, so you should move the pots inside to somewhere frost-free over winter, or lift the tubers (if planted in the ground) and store them in dry compost or wrapped in newspaper etc over the winter. Sometimes dahlias will survive in the ground over winter. All of the ones I've ever grown from seed have developed a sizeable tuber by the time the first frost comes.