Dahlia tubers which have been in storage over winter can with the help of a heated greenhouse be encouraged into early growth. The shoots which spring up can be used to make cuttings and so the gardener is able to propagate a new generation of plants for this year's dahlia garden. Cuttings provide the ideal means of increasing stock. Mid-February to March is the time to engage in this most rewarding aspect of gardening.
Starting Dahlia Tubers into Growth
Overwintered dahlia tubers should be taken out of storage and inspected to check that they are sound and plump. Any which are a brown soggy mess have succumbed to frost and must be discarded. Sound tubers are put through a process to encourage them into growth known as "setting up". This involves:-
- Washing the tubers with hot water, containing a mild disinfectant.
- Covering the bottom of trays or boxes with moist peat or a peat alternative (low nutrient seed compost is good).
- Placing the tubers on top.
- Covering the tubers almost completely with more compost.
- Labelling the tubers with labels big enough to read when surrounded by new shoots.
Provide Conditions for Growth
Warmth is needed to bring the tubers into growth. At a temperature of about 10 degrees C growth starts in two to three weeks. Keep the compost moist but don't over water.
Prepare the Rooting Medium and Fill Some Pots
A suitable rooting medium consists of two parts sand to one part peat (or fine bark) and one part perlite. Mix theses ingredients thoroughly and then fill some plastic cell trays or 12.5cm wide pots with it. Firm the rooting medium in the containers and water with a watering can fitted with a fine rose.
How to Take Stem Cuttings of Dahlias
When the shoots on the tubers reach about 7.5cm in length and have two pairs of leaves they are ready to be removed and made into cuttings:-
- With a sharp knife or razor blade cut the shoots just above the base where they join the crown of the tuber.
- Check that the cut is clean and just below the bottom pair of leaves.
- Carefully remove the bottom pair of leaves and avoid tearing the delicate stem.
- Moisten the end of the cuttings and dip them in hormone rooting powder.
- With a dibber make holes 2.5cm deep in the rooting medium.
- Insert a cutting into each hole and gently firm it in.
- Several cuttings can be put around the outside of a pot, but for ease of identification it is best to stick to one variety per pot.
- Label each container.
- Discard any cuttings which have hollow stems because these are unlikely to root.
Where to Root Cuttings of Dahlia Plants
The buoyant humid atmosphere provided by an electric propagator set at a temperature of 18 degrees C is the ideal place to root these soft wood cuttings. However, it must be out of direct sunlight otherwise the cuttings will shrivel up before they root. Cover the propagator with damp newspaper in bright weather. After two to three weeks roots will have formed and new growth will be evident. The rooted cuttings can then be potted individually where they will grow on to provide a new generation of healthy young plants for the dahlia garden or mixed border.
Raise New Dahlias From Cuttings Even If Last Year's Tubers Weren't Overwintered
The gardener can still get plants for free even without any overwintered tubers. Garden centres and mail order suppliers sell dahlia tubers. A selection of bought tubers can be given the same treatment as described above and for every tuber purchased another four or five dahlia plants can be grown ready for planting out in June.
David Haigh - David Haigh is a 'Master of Horticulture', a qualification awarded by the Royal Horticultural Society. He spent almost 40 years as a ...
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