Start dahlia tubers into growth this week. Overwintering dahlias in damp-ish sand under a bench in a frost-free greenhouse or conservatory is a wonderful way to keep your favourite plants growing from one year to the next. If you didn't get around to it last year, your favourite garden centre has new dahlia tubers in stock at the moment: look out for 'Black Narcissus', a deep wine-red cactus type, 'Twynyngs After Eight' with white flowers over deep bronze-purple foliage, or the blowsy pompoms of 'David Howard' in burnt orange.
Whether you save your own, or buy in new tubers, it's a good idea to get them started into growth before planting them outside, as they're less vulnerable to slug damage. You'll have earlier flowers, too.
First check they're nice and firm by giving them a quick squeeze, rejecting any which have rotted in storage. Then fill a large pot with multipurpose compost and plant the tubers so the stems stick out of the soil. Water well, and you should find shoots emerge within a few weeks. Keep the plant frost-free and grow it on in a sunny spot, making sure to keep an eye out for slugs, before planting out in the garden in late May.
