When to sow dahlias in Casablanca

Why these dates

Plant tubers two weeks after your last frost, once the soil has reached about 13C, 15cm deep with the crown upward. For earlier flowers, start them in pots under cover six weeks before that and plant out as growing plants. Pinch the leading shoot at 40cm to get a bushy plant rather than one tall stem.

Top growth is blackened by the first touch of frost at about -1C, which is what ends the season. The tuber below ground survives a light frost but not a frozen soil, so in zone 7 and colder lift it a week or two after the tops are killed, dry it off and store it somewhere frost-free.

Dahlias are planted once and then left in place, so there is no sowing offset to resolve. Nor is there a dormant season to plant into: this crop keeps its leaves and keeps growing through the winter, which is exactly why cold damages it. The window on this page therefore waits until the last frost in Casablanca has passed and the soil has begun to warm, giving the roots the longest possible run before the cold returns.

Hardiness is not the obstacle here. Casablanca is zone 11a, comfortably inside the range this crop is rated for, so an established plant should come through an average winter without protection.

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