In milder areas of the UK and in sheltered, well-drained, parts of the garden, it may be possible to cut back the tops of tender plants, leave them in the ground and cover them with a thick mulch to protect them over winter.
In very cold or exposed areas, even hardy perennials may need the protection of mulch over winter. Tender plants need protection at the first warning of frost, usually between September and November.
Traditionally the time to lift Dahlia tubers is after the foliage has been frosted, but before it gets so cold that the tuber itself is affected. This video discusses the various ways to overwinter dahlias. Dahlias are lifted once the foliage has been blackened by the first frost.
How to do it
- Cut off the old flowering stems 5cm (2in) from the base and trim away any thin roots. These stems can be composted
- Use a fork to prise the plants out of the soil, taking care not to damage, cut or bruise the tubers, as this can lead to rots developing in storage
- Remove the soil from the tubers by hand. Position them upside down in a cool place for a few weeks to dry off
- Label cultivars and bury in trays or wooden boxes filled with dry sand, soil or compost, leaving only the old flower stalks exposed
- Place the trays in a cool but frost free place, such as a garage attached to a house or a shed
If your dahlias are in a warm, sheltered spot in the border and you don’t want to lift them, add a thick layer of mulch for extra insulation in winter.