Dahlias should be divided in late winter or early spring, just before new growth begins. In New Zealand, this is typically between August and September, as the weather starts to warm but before active growth resumes:

• Wait until the foliage has died back after the first frost in autumn before lifting the tubers.
• Store the tubers in a cool, dry place over winter if needed.
• Divide in late winter/early spring when you see small buds (eyes) forming on the tubers.
• Replant after the risk of frost has passed, usually in early to mid-spring.

Dividing them regularly (every 2-3 years) helps keep the plants healthy and encourages more blooms.