The poinsettia is top of the pots

Top of the pots this Christmas is the poinsettia. Sales of about 5 million, almost all in the space of one month, will put it miles ahead of any challenger.

Potted flowering heathers are not jolly enough, winter cherries and azaleas are not quite the right colours and the elegant cyclamen, sadly, can't take the temperatures we like in our sitting-rooms. But the poinsettia, which comes from the tropics, can stand the heat - even of an extended family get together.

With its flamboyant red bracts and smart green leaves, it might have been made for Christmas. That hasn't stopped plant breeders from producing creams, pinks and peaches, which some think are smart and trendy, and traditionalists think are pointless.

There are other variations on the original plant, too, with standards appearing in the shops, as well as groups of miniaturised poinsettias in bowls.

Poinsettias are big business. From the growers' point of view, they dovetail beautifully with summer bedding and container plants.

When the last of these are sold from the nurseries in June and July, rooted poinsettia cuttings are bought in by the thousand. Large numbers of the cuttings and finished plants are produced in this country: poinsettia growing is one area of the pot plant business that has not become dominated by the Netherlands.

The plants are grown on until mid- to late November, with the temperatures being lowered gradually towards December to harden them off ready to be transported to the shops. Then the whole crop is sold in less than six weeks. After Christmas, as we throw away our bits of holly and store our decorations, the growers are bringing on mother poinsettia plants from which cuttings will be taken in the late spring.

Well over a hundred varieties have been bred over the last few years, although buyers are unlikely to find named types when they reach the shops.

As well as slightly different colours, many of the new varieties have been developed to help the professionals with speed of growth, shelf life, and their "breaking habit". This refers to the plant's ability to produce plenty of strong flower-bearing side shoots when the main shoot is pinched out. The yellowish flowers are tiny; it is the surrounding bracts which give the dramatic colour.

One characteristic that the breeders have done away with is the need for long nights. Until recently growers had to provide 14 hours of darkness during the autumn to encourage the plants to produce flower buds. This is why, we used to be told, amateurs should throw out their poinsettias soon after the holly, because they would never get them into flower again. But John Evans, managing director of H Evans and Sons of Sidcup in Kent, explains that this is no longer true.

Mr Evans's firm produces around 400,000 poinsettias under more than 10 acres of glass, and he says the varieties grown now can be persuaded into flower next year without elaborate arrangements for restricting light. However, the problem for the amateur is size. In its native Mexico and in the West Indies Euphorbia pulcherrima grows to 6ft tall and is often found in hedgerows. The plants we buy have usually been treated with a growth retardant so they flower while smaller than they would be in the wild.

Next year the same plant may flower, if it is looked after and fed regularly, but it will have grown to three or four feet high. If you can accommodate the size, it might be worth cutting your Christmas poinsettias back when they have finished flowering and giving them a rest in the garden when the frosts are over. They should be brought into the house and fed regularly in the late summer.

Avoid overwatering. Mr Evans advises waiting until the compost is dry, standing the pot up to its top in water for a few minutes, then draining and returning it to its potholder. The plant's real enemy is waterlogging, and if it is watered in its cache-pot there is a danger it will be left for several days standing in water.

All the poinsettias now in the shops and garden centres have been grown from this year's cuttings, even the standard ones which are selling for very fashionable prices of around pounds 100. These have been fed lavishly to make them shoot upwards, then pinched out at about a metre high to produce a mophead of colour. The tiny ones, called tots in the trade, have been fed with a growth retardant and grouped in bowls for a pretty effect.

As growers experiment continuously with new strains, we can expect more unusual colours, moving through peaches towards orange.

And breeders are also working on strains which will drape themselves in hanging baskets, so we can expect to see festoons of these soon in Christmas displays in restaurants and shopping malls.