This section is from the book "The Gardener V2", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
The best way to obtain a supply of this plant is to take plants from the flower-garden in the autumn, and pinch or cut their heads in at the same time. They should be potted in some light sandy soil, and placed in a rather warm and dry greenhouse or pit. By the month of February these will have pushed out a lot of stubby shoots, and at that time the strongest of these should be taken off and inserted as cuttings. When the best cuttings have been taken off these stumps, they may be allowed to grow away undisturbed, and in a short time another batch will be strong enough to be taken from them, and so on till enough plants are obtained. Then the old plants may be thrown away, or, better still, allowed to grow into bushy plants, and used in May along with others. Instead of putting the cuttings into pots or pans, as is the custom, a sounder plan is to put one in the centre of each little pot; and then, when it strikes root, which it will quickly do, there is no necessity for mutilating the roots, as is the case when things are put into cutting pots or pans. If convenient, the pots should be plunged in tan, or in a hotbed, as treated thus they will root more readily. The foliage should not be wetted more than is necessary.
After this it is simply necessary to harden the plants off in the usual way, and so fit them for being used in the flower-garden.