If you never lift your dahlias to overwinter them, lift one and carefully remove the majority of the soil. Alternatively use a dry tuber from the garden centre or supermarket. Put the intact tubers on, or very shallowly buried in, a tray of moist compost or garden soil somewhere warm and light.
Caption: Dahlia cuttings root very quickly
When the shoots (which should be darkly coloured and not etiolated) are 7-8cm high, cut them off at the base, keeping a bit of basal plate if you can. Check that the base is not hollow, as hollow shoots do not root.
Caption: Take cuttings when shoots are 7-8cm long
Insert the stems about 2cm deep in small individual pots or fit them around the edge of a larger pot. With larger pots, it’s a good idea to only partly fill the pot with compost. This leaves the side of the pot for support and partly encloses the cuttings, increasing humidity.
Caption: Insert the cuttings into a pot of compost
Water the cuttings and put them anywhere with good light, a moist atmosphere and mild heat. Bottom heat (like you get with heated propagating mats) is useful but not essential. Avoid direct sunlight and don’t use enclosed propagators and plastic bags, because they make plants wet, which causes rot.
Dahlias root, grow on and flower quickly. It may be as little as three weeks before they’ll need potting up.
You can continue to take cuttings over several weeks or pot up the tubers to plant outside later when the danger of frost has passed.