Dahlia are fantastic plants for late summer colour and sheer exuberance. They may be too colourful for some tastes but the stunning colours and shapes of their flowers simply make me smile. For a really good show, it is important to keep the plants going year on year. There's the snag. In chilly, or downright cold, UK gardens the tubers which produce the flowers are susceptible to frost. So winter protection is important. Depending on where your tubers are growing there are three main ways to do this.
1. Dahlia tubers left in the ground to overwinter
In mild areas and in gardens with well-drained soil tubers can be left in the ground. Only do this if they are in the place where you want them to grow and they did well there this year. If you want them in a different place or to plant them after earlier plants have finished their show, use method 2 or 3.
Cut down to the ground all the leaves and stems once they turn black, usually after the first frost. Then cover the plant with about 8-15cm (3-6 inches) of chipped bark, garden compost, old mushroom compost or leaf mould. Mark where the tubers are and wait for spring, when you should be ready with slug and snail protection for the new shoots which will show in the late spring. In spring the markers will help you check that tubers aren’t overshadowed by rapid spring growth on other plants.
2. Dahlia tubers lifted and overwintered in a frost free shed or greenhouse
In cold areas, in gardens with wet or heavy soils or if you want to plant and grow dahlias in gaps after earlier plants have finished you need to lift the tubers out of the ground. After the foliage has died down push a garden fork gently under the tubers beneath the plant and lift them up out of the soil. Leave the foliage attached to the tubers and shake loose earth off before taking them into a garden shed, garage or porch. Trim the stems to about 10cm long (4 inches) and turn the plant upside down in a cardboard or wooden box or paper bag (not plastic, that will encourage damp). Leave the tubers to dry out – which will take two or three weeks.
Once dry, use secateurs to snip fine roots off the tubers and pack the tubers in dry sand or peat free compost in a shallow and open topped cardboard or wooden box. Be sure to leave the tops of the plants out in the air.
Store the box in a cool, dry, frost free place. If hard frosts are predicted, cover the tubers with newspaper as an extra precaution. During the winter keep an eye on the tubers and throw away any that show signs of mould or rot.
3. Dahlia tubers overwintered in containers
This is midway between the other methods. You will need to lift the tubers – method 2 – but instead of drying and storing them in a cool, dry, frost-free place plant them into large (at least 3 litre size) pots filled with a good garden compost that will not become waterlogged, and carry on as if the tubers were in the ground (method 1). There are a lot of advantages to this approach – you don’t need a frost free storage place, you don’t need to worry about drying and storing the tubers, the tubers can get on with growing until you want to plant them in your garden (or leave them in the pots). The disadvantage is that if the pot becomes very cold and / or wet the tubers may be damaged. Avoid that risk by using a good compost and keeping the pot in a sheltered place.
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