Dahlias are essential in any garden, providing masses of blooms for garden display or for cutting with minimal effort. You can grow them from seed very easily and I sow them in early April because they grow so quickly and there is no need to sow earlier. The dwarf kinds for bedding are easy to grow but the big-flowered types are not as satisfactory because the quality of the flowers (shape and doubleness) is not as good as on plants grown from tubers. An exception I would make would be single-flowered types such as ‘Bishop’s Children’ which gives a good range of single flowers on dark foliage.
When you buy tubers, check the packs to make sure the tubers are alive. Sometimes the tubers have dried out and are dead but you can usually tell by the weight. If they feel light as air then the tubers may have dried out and died. The swollen roots vary in size and shape according to variety and you will find that some are narrow and long while others will be short and compact.
There are two ways to deal with your tubers. You can wait until April and plant them direct in the ground or you can start them in a greenhouse or propagator now. I prefer to start them into growth in pots first because I can plant out bigger plants in May (when the last frost has passed) and because I can get extra plants from each tuber.
Each tuber will have a number of fleshy roots radiating from the stump of last year’s plant. These fleshy roots store food but they are not capable of making more plants so if they break off you should throw them away – they will not make new plants even though they will form new roots. The new plant will grow from a series of buds around the top of the big roots at the base of the stem. I pot the tubers roughly so these buds are above the compost level. I place them in the propagator and when these shoots start to grow I have three options.
I can let them grow on and, after a month or so pot the whole thing up again so it is a strong plant for putting out. When the shoots have three pairs of leaves I pinch out the growing tips of the shoots so the plant is bushy. More stems near the base means lots more flowers later.
Alternatively I knock the plant out of the pot and, with a sharp knife, cut the plant into sections, cutting vertically through the old stem, so that each piece has at least one shoot and some roots. I then pot these up and grow them on.
Or, if I am feeling brave, or greedy, I take cuttings. When the shoots are about the size of the tallest shoot on the plant above, I cut the shoot from the tuber, making sure that the base includes a sliver of the tuber. Put in cutting mix and kept in a warm, humid propagator, these root in a few weeks. Usually new shoots will appear from the tuber even if you remove all you can see so you can get at least four or five plants from a single tuber.
When you lift the tubers in autumn to keep them for next season you will find that the tubers are MUCH bigger than when you bought them so you will need big pots or, preferably, trays to start them in.