Early Spring is a busy time if you want to increase the number of Dahlia plants that you have.
Dahlias are normally propagated by either soft cuttings, or by division of stored Dahlia Tuber Clumps.
A heated greenhouse is required, to force the Dahlias into growth during late winter - early spring.
Cuttings are normally taken from over-wintered Dahlia Tuber in early spring and placed in a heated propagator for best results. Alternatively a propagating case built onto the greenhouse staging can be used. It should have gentle bottom heat to ensure both compost and inserted cuttings get the necessary heat to promote root growth.
The timing is quite important when taking cuttings, for it will be found that the cuttings taken earlier will produce a small tuberous root, whereas later cuttings - after end of February usually produce fibrous roots. Both types are suitable for 'home consumption' but if selling rooted cuttings of Dahlias commercially, then it is preferable to have the small tuber attached
The overwintered Dahlia Tubers should be gently brought into growth - either in the storage container, or otherwise potted into a suitable container. Gentle heat and moisture will soon bring side shoots from the base of the attached stem portion. There will be no cuttings growth from the actual tubers.
Small cuttings - around 2-3inches long, and with a pair of leaves at top of cutting are best. They will readily root in heated conditions as outlined above. Be careful not to bruise the stems when taking cuttings of small dahlias in this way.
Once rooted - after 2-3 weeks - gently bring the cuttings into the open from the protection of the propagator case, and acclimatise into normal greenhouse surrounds over a few days. Take care when lifting the cuttings, to ensure that there is no severance of the small tuber that may have formed.
Pot up singly into a 4in pot with a good sterile multi-purpose potting compost. Gradually harden off in greenhouse, and then cold frame before planting out in late May or early June. Keep a watch for aphids at this stage.
The best way to divide dahlia Tubers, is by bringing them into growth as outlined for cuttings. Divide the Dahlia Tubers by slicking down through the old stem - ensuring that there is at least one side-shoot attached to each section of tubers. Dividing dahlias is an easy way to increase plants, but not as productive as taking cuttings
Alternatively, dividing Dahlia tubers can be done in early spring - but whilst the tuber clump is dormant. You will need to ensure that there is a bud swelling on each section of stem attached to your divided Dahlia Tuber clump. Simply dividing the tubers with no stem attached will not produce plants.
The divisions can either be grown on - if taken as growing side shoots - or planted out if taken as dormant growths, but only after all danger of frost is gone.