As for the seeds of acacia and the giant sequoiadendron, these will germinate far better if they are scalded with hot water before sowing for in the wild these plants are often exposed to the heat of forest fires and this is one of their forms of adaptation.
After sowing the seeds moisten them from below by standing the seed tray in water and leaving it till moisture appears on the surface of the compost. Then cover the seed tray with a sheet of glass to ensure uniform atmospheric moisture. The glass must be turned over daily so that the drops of water which condense on the underside do not fall on the seeds or young seedlings. As soon as the seedlings are large enough to handle, they should be pricked out.
Vegetative propagation is more widely used to increase plants in the home. The simplest method is division, which can be done when transferring the plants to new pots. Often it is not even necessary to detach a young plant from the parent clump with a knife, for it falls off readily by itself. Offsets of bulbs and tubers are detached in the same way. When using a knife to separate them always dust the cut surfaces, preferably with charcoal powder, though cigarette ash will serve the purpose just as well. The detached plants should be planted immediately, taking care not to damage the roots which are capable of immediate growth.
Taking cuttings is another method of vegetative propagation. These can be rooted in a seed tray or flower pot, or small propagators with heating elements can be bought at garden centres. Moist coarse sand makes a good rooting medium. Alternatively a mixture of peat and sand can be used.
Cuttings are usually made of stems or leaves; only occasionally are plants multiplied by root cuttings (generally only members of the genus Cordyline). Stem cuttings should be about 10 cm (4 in) long and the leaves on the section to be inserted in the compost should be removed. To promote more rapid rooting, dust the cut end with hormone rooting powder. Some stem cuttings (dieffenbachia, dracaena and aglaonema) are placed flat on the surface of the compost and about half their length is pressed into the rooting medium. Leaf cuttings are taken either with the stalk (saintpaulia) or without; the leaf may also be cut into several sections which are inserted in the(sansevieria) or else merely laid on the surface (begonia). In the latter case take care that the section of leaf blade has at least some veining in the shape of a Y.
Cuttings like a warm, moist atmosphere in which to root. This may be achieved by covering them with a sheet of glass, or inverting a jam jar over them, or, if the cuttings are in a flower pot, by placing both pot and cuttings in a polythene bag and sealing the top. The separation of tubers and rhizomes is another method of vegetative propagation and is used for caladiums, aroids, members of the ginger family and marantas. The cut surfaces should be dusted with charcoal powder and left to dry before insertion in the compost.
Some plants, such as ficus and dieffenbachia tend to shed leaves from the base leaving the stem bare and unattractive. Such plants can be propagated by air-. At the point on the stem where you want the plant to form new roots, make an upward cut at a strong angle about half-way across the stem, insert a matchstick so the cut will not close, wrap damp green sphagnum moss round the cut and round that, a piece of polythene, binding it at the top and bottom. To prevent the plant top from breaking off tie it to a stake above and below the wad of moss. When well-developed roots appear, which takes several weeks, remove the polythene, cut off the top and pot it up, together with the moss packing. The remaining, lower part of the stem should be kept, as it will often put out new shoots.
Grafting of succulents, particularly cacti, on reliably and readily growing rootstock is another method of vegetative propagation. By this means it is possible to cultivate rare and delicate species that would otherwise grow slowly and would not flower, as well as colour mutations (red and yellow) which lack chlorophyl, and would be totally incapable of growing on their own. The rootstock should be cut at right angles to the axis, likewise the cactus that is to be grafted on. Then the section is placed on the stock so that the vascular bundles coincide with one another as far as possible, and fastened securely by means of rubber bands. The two unite within a week, after which the rubber bands may be removed.