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EVERY December, I buy as many poinsettia houseplants as I have room for in my home. These tropical plants, natives of Central America, give a stunning red display that, for many, is a perfect living Christmas decoration.

Each bloom of a poinsettia (Euphorbia pulcherrima) is technically an inflorescence rather than a flower, and its 鈥減etals鈥� are actually modified leaves called bracts, which turn scarlet in autumn. The real flowers are unimpressively tiny yellow balls at the centre of each cluster.

In its natural state, this shrub is a straggly monster that can reach 3 metres tall. The poinsettias that we see in shops only look the way they do because they are diseased. To make the short, bushy poinsettias that can sit on a windowsill, growers introduce a type of bacteria called a phytoplasma into the plants. This causes them to grow many more side shoots, each terminating in crimson bracts.

The pathogen seems to interfere with the normal process of apical dominance, which is when the tip of a plant鈥檚 main shoot produces hormones that travel down the stem and inhibit side shoots from growing. This process is why pruning most shrubs can make them more bushy and attractive.

To keep your poinsettia looking healthy over the holiday season, don鈥檛 overwater it, says at Bridge Farm Group in Spalding, UK. He recommends waiting until the plant鈥檚 leaves are just starting to droop, then placing it in a bowl of water for about an hour, which will let the compost take up the right amount of water.

Fuller also advises keeping your poinsettia at a fairly stable temperature. A common mistake is to place it on a windowsill and close the curtains in the evening, leaving it trapped in colder air overnight. The plants don鈥檛 need high light levels for the month or two they are on display, so would be fine sited well away from a window, says Fuller.

By late January in the UK, most poinsettias will be losing their leaves, but it is possible to encourage them to regrow in spring, and then to coax the new leaves into turning red before next Christmas.

The bracts colour up when the plant detects that the days have become shorter in autumn. So it needs to get darkness for 12 to 14 hours a day for about two months.

In most homes, that won鈥檛 happen because of the amount of artificial lighting in the evening. Commercial growers such as Fuller achieve the long hours of darkness in their greenhouses by using dark screens or curtains so the plants can be 鈥減ut to bed鈥� every night.

If you happen to have a spare room that is unlit, your plants could be stored there for two months. Another trick is to put your poinsettia inside a cupboard or place a bag over it every evening 鈥� if you have the patience.

Clare Wilson is a staff writer at New Scientist and writes about everything life-science related. Her favourite place is her allotment @ClareWilsonMed