In the final article in her current series, Anna Pavord gives advice on propagating new young plants from past favourites
Propagating plants is a benign kind of disease. It can be kept reasonably under control by a surfeit of children, but it advances unchecked when the number of mouths round the kitchen table starts to dwindle.

The disease is made more dangerous by the fact that most plants, being infinitely more subtle than people, offer more than one way of perpetuating themselves. The exceptions are the annuals, which germinate, grow to their full potential, seed themselves and die within the space of a single season. The only way of propagating them is by sowing seed.

But with other plants, you can choose whether you sow seed, divide plants up, take cuttings, or make layers - which is a lazy kind of cutting. The method you choose depends on the end result that you are looking for.

The point about cuttings is that each one will grow into a perfect replica of its parent - if that is what you want. Nurserymen depend on this sameness. But each seed in a seed pod may turn out to be a subtle variation of the parent, since the progeny sometimes skip back, as children do, to pick up a trait that has been suppressed for several generations.

The variation is a safety device. A flower that has a slightly different shape or colour from that of its parent may be more successful at attracting pollinating insects. A leaf that grows narrow, or develops a woolly texture, may survive drought more easily. Plants subscribe to the harsh doctrine of survival of the fittest. But gardeners intervene, selecting sickly seedlings to grow on, for the sake of a rare mutation in the flower, or a variegation on a leaf, that has nothing to do with survival.

The seed-sowing season gets into its stride about now, though you should never be in too much of a hurry to sow seed of annuals or tender bedding plants. Many trials have shown that seed sown in April catches up fast with seed sown in March.

Use small pots for initial sowings, two-thirds filled with compost, topped up with vermiculite. Water the pots from above with a fine rose before sowing. Soaking pots in water can mean that the compost gets waterlogged. Scatter seed over the surface of the vermiculite. Very fine seed will not need covering. Larger seeds can be gently stirred into the surface of the vermiculite.

Cover the pots with glass or cling film, and then with newspaper to exclude light - though some seeds, including ageratum, antirrhinum, begonia, cineraria, impatiens, lobelia, mimulus, nicotiana, petunia and salvia, germinate best in light and should not be covered. As soon as the seedlings emerge, remove the covers and keep the pots well watered.

Prick off the seedlings into large seed trays as soon as the first real leaves develop. Very small seedlings such as alyssum and lobelia can be pricked out in small clumps. Harden off the plants gradually before planting them out in permanent positions. In balmy coastal areas, this may be in mid-April. In central Scotland, it is unlikely to be a good idea to do it before June.

Cuttings of plants can be taken at three different stages. Softwood cuttings are those taken from young shoots between March and June. In some ways they are the most difficult to look after, as they often need mist and warmth before they will root, and they need to root fast before they exhaust their own food supplies. The exceptions are geranium and fuchsia, both of which root very easily from softwood cuttings. This system also works with cotinus, lilac, lavender and potentilla.

Fuchsia cuttings taken now will themselves be flowering plants by late summer. Take shoots with three pairs of leaves, cutting just below the last pair of leaves. Set them in a pot filled with fast-draining compost. Cover with a polythene bag and keep at a temperature between 50F and 60F. Move the cuttings into separate pots when their own growth shows that they have rooted.

Cut old, overwintered geranium plants hard back in early spring. Water and feed them to encourage new shoots, which will provide softwood cuttings. Any healthy shoot, about 3-4 in long, will make a cutting. Snap it off just below a leaf joint. Take off all the leaves except the very young ones at the tip of the cutting. Set the cuttings round the edge of a 5- in pot of compost. Do not cover them. Pot them on when they are rooted.

Semi-ripe cuttings are the ones you take when the current season's shoots are just beginning to harden, but are still pliable (generally between mid-June and August). The shoots must be healthy and vigorous. Simple stem cuttings can be snipped in 4-in or 6-in lengths from any likely-looking section of stem. Internodal cuttings are made by cutting half-way between leaf joints on a stem.

Nodal cuttings are made through the bump immediately below a leaf joint. You then whip off the bottom leaves attached to the lump before putting the cutting into its pot.

Hibiscus roots well from stem cuttings, taken at the end of July or August. Take 6-in sections of stem and line them out in sandy soil in a cold frame, where they have some winter protection. Try the technique with hydrangeas, too, by taking 4-in sections of semi-ripe wood and sticking them round the side of a pot of compost. Cover the pot with a plastic bag until the cuttings have found their feet.

A heel cutting is what you get when you take hold of a side shoot (not one that is flowering) and give it a sharp tug downwards. It comes away with a bit of the old stem attached. That is the "heel". Both buddleia and chaenomeles root from heel cuttings, taken in late July or August. Choose plump lateral shoots 4in or 5in long, and pull them off with a heel. Trim off the growing top and the bottom leaves and put the cuttings into a cold frame, pushing them into the ground to about half their length.

A basal cutting is one made with a clean cut through the slight swelling that usually occurs where side shoots join the main stem. This is all that distinguishes it from a heel cutting.

Basal cuttings of choisya taken in late July or August will root in pots covered with a plastic bag or (in mild districts) lined out direct in the ground. A propagating frame set at about 60 F will hurry up the rooting process of shrubs such as choisya and ceanothus, but is not essential.

The older the wood you take for cuttings, the longer the cutting itself is, and the longer it takes to root, so hardwood cuttings, taken from the ripened wood of a shrub or tree in autumn and early winter, sometimes take a year to root. Many common shrubs such as berberis, dogwood, cotoneaster, escallonia, privet and ribes root from hardwood cuttings.

A layer is a kind of hardwood cutting, with the added advantage that if it doesn't take, the evidence isn't so obvious. Shrubs with naturally low-growing branches are the easiest targets. Rhododendrons and azaleas propagate particularly well by this method, and I have also had 100 per cent success with Hydrangea villosa.

When you notice a likely branch for layering, snick the underside of it about a foot back from the growing tip. Scrape out a hollow in the ground underneath this point and peg the stem down into it. Cover it with earth and put a flat stone on top of it to stop it springing free. A year later, the stem should have rooted. To free it, simply cut the stem behind the layer.