Dahlias will grow in most soil types but are best planted in fertile, well-drained soil. Plant them outside from the end of April after the chance of frost has gone. Ensure they are in a spot that receives full sun and plant around 15cm deep adding well-rotted organic matter.
Provide protection from slugs for your young plants to stop them feasting on the tender, fresh shoots. It is also a good idea to ward off earwigs which are fond of young plants. Simply fill small pots with straw and put on them on top of canes near your dahlias. The earwigs will shelter in the pots during the day and you can clear these little fellows out in the late afternoon.
When the dahlias are around 30 cm high, pinch out the growing tips to encourage the plants to branch out and produce more flowers. Also remove any stems that look weak. For stronger stems and longer flowering, pinch out the buds that develop in the base (axel) of the stem. Larger dahlias will need staking. Always remove the flowers as soon as they show signs of fading in order to promote more flower production.
Keep your dahlias well watered during the summer months and feed every couple of weeks with high potash feed as the flowers appear.
In winter allow the first hard frost to turn the dahlias black. Cut them down to around 15cm then carefully lift them from the ground. Gently shake off any excess soil and turn upside down on paper to allow the water to seep from the tubers. After a few weeks they will be completely dry. At this point, shake loose any remaining soil and dust with sulphur to protect them from mould and mildew. Store in a frost free place in a container filled with dry to moist peat or sand for winter.