Dahlia Bishop of York

Plant in spring as soon as the last frost has passed. For abundant flowering, plant the tubers in full sun in rich, moist but well-drained soil. Stagnant moisture causes tuber rot. Amend the soil with compost and sand, if necessary. Work the soil deeply and enrich it, for example, with crushed horn or dehydrated blood. Place your tuber and crumble the soil well to fill without air pockets. Cover with about 6cm (2in) of soil. At the end of planting, water once abundantly and then repeat this watering regularly during the first 6 weeks to help with rooting. 

Dahlias are sensitive to cold, so they need to be overwintered. Dig the tubers up when the first frost has blackened the foliage. Carefully remove the tubers. Remove as much soil as possible. Let the foliage dry so that the tubers can replenish their reserves. Then cut the stems to 10cm (4in). Spread your bulbs in a box lined with newspaper. Store them in a frost-free, dry, cool, and dark place, such as a garage or attic. In mild regions without frost, or those close to the coast, it is possible to leave them in place. In this case, simply cover the ground with a carpet of leaves or straw for protection.