When the shoots are between 75mm (3in.) and 125mm (4in.) tall, they are ready to use as cuttings. First fill some pots, 75mm (3in.) or 87mm (3½ in.) with a John Innes compost No. 1 or similar potting compost.
Use a sharp knife to cut away the shoots a little above their base, where they join the crown. Take care not to cut the crown itself, which would prevent further shoots forming.
Trim the stem of the cutting just below the joint of the lowest pair of leaves, then carefully remove the leaves too. Make sure you do not damage the joint itself when doing this.
Sometimes these first cuttings have hollow stems, and I have found them difficult to root. Discard them because more shoots will follow, and they are sure to be more suitable for propagation.
Dampen the end of the prepared cutting, then dip it in hormone rooting powder. Use a dibber or pencil to make a hole about 25mm (1in.) deep in the compost; insert the cutting, and firm it in with your fingers.
Several cuttings can be put in the same pot, but it is best to stick to only one variety per pot. Ensure each pot is labelled to prevent confusion.
Put a propagator top over the pots, or put them into a propagating frames. They need a warm postion, out of direct sunlight.
After two or three weeks, roots will have formed, and when new leaves start to grow, the plants can be moved onto individual pots of John Innes No. 2 or multi purpose compost.
Grow the plants on in these pots until the end of May, then move them to a sheltered position, such as a sunny patio. This will help to harden off the plants before they are planted out in the garden.
Once flowers appear on your dahlia plants, feed with a high-potash liquid fertiliser, such as a Tomato Fertiliser, every two weeks until early September. Feeding dahlias will encourage good quality and long lasting flowers. Feeding will also build up strong tubers if they are to be kept over the winter.