Dahlias are back in fashion! Some of the newer ‘novelties’ may perhaps not appeal greatly to the traditional Dahlia aficionado but flower arrangers love them. For many of us the emphasis is on the garden worthy types with relatively small flowers for cutting, or for spicing up a hot or tropical border. Whatever types you grow, or for whatever purpose, the winter care of them and the ability to regenerate stock is crucial and the procedures are the same.
But why not select the ones you want, plant them where you want them and just leave them there to do their own thing? Many people do indeed leave them in the ground over winter and they reappear the following spring – but not always! The present cycle of very cold spells has increased the risk of losses. The alternative is to lift the tubers after the frosts have blackened the top growth, store them in a frost-free place and set them up in pots or boxes to sprout in a warm, light environment. There are a few crucial points to watch if you are to avoid the commonest reasons for failure.
The tubers themselves are merely storage vessels, and the growth points for next year’s plants are tiny buds at the junction of the neck of the tuber with the old stem. Those buds are lost if the stem rots. There is a surprising amount of water in the stems, so it pays to lift the tubers carefully after cutting down the stem, and turn the whole lot upside-down to dry off for a few days. This ensures that there is no residual water in the hollow stem to cause rotting. Any damaged tubers can be cut cleanly and dusted with sulphur. The garden soil around the tubers need not be removed – it is the best packing there is to hold each tuber firmly in place to avoid damage. If the slender neck of a tuber fractures, the tuber will serve little further purpose, so remove it, cutting cleanly above the fracture. This will not be a problem provided there are at least some sound tubers left to give the plants a start next spring.
As soon as the stool of tubers has dried off (not dehydrated!) pack them (labelled, and the right way up!) in boxes with dryish compost and store frost free. Begin watering, ideally in late February if you have a warm enough spot and shoots will begin to appear after about 3 or 4 weeks. Now it is decision time! Do you divide the tubers, leave them as they are, or take cuttings? If you don’t intend to divide, you may need a pretty large pot to set up a stool for sprouting, but you won’t need to start so early (April would be OK). But having gone to the trouble of lifting, why not divide as well? Dividing makes it easier to accommodate each plant and avoids the congestion that inevitably occurs as the plant makes new tubers year on year. Best of all, cuttings quickly make vigorous plants and ensure the stock is kept fresh.
The division of tubers and the taking of cuttings are illustrated in the accompanying pictures. The demonstrations seen on popular TV garden programmes are usually a prescription for failure. When taking cuttings you need the tiniest sliver of the tuber left at the base of the cutting. If you cut off at, or just below soil level and then trim beneath a node, rooting is slower and riskier, even using a hormone rooting preparation. And if you put a polythene bag over the top, sweating and rotting are virtually guaranteed. No! The cuttings need gentle bottom heat similar to that for seed germination, good air circulation and above all that basal sliver that contains the cells that will readily produce roots. The compost is the traditional open mixture of 1/3rd peat or multipurpose compost with 1/3rd silver or sharp sand and 1/3rd perlite. In the right conditions rooting will be 100% and (as in the picture) take about 3 weeks. From then on it is just a case of potting on and hardening off for planting out in early June.
Trust me. It works!