Season's sustenance

3rd December 2004 at 00:00
The plants we associate with Christmas offer a lot more than mere decoration. They play a vital role in the survival of wildlife through the bleak midwinter. Ben Aldiss reports

Our wildlife can have a tough job withstanding the rigours of a British winter, but there are three particularly hardy plants that seem to cope better than most. Holly, ivy and mistletoe not only provide us with a traditional splash of colour at Christmas, they also offer food and shelter to some of our most attractive mammals and birds.

Apart from their decorative qualities, these plants have little in common and all are very unusual. They can also provide much in the way of teaching material, covering a range of subjects and ages.

Entirely unrelated to each other, holly, ivy and mistletoe are the sole European representatives of three large families of plants, the other members being largely restricted to the tropics. Their berries may appear at the same time, but their flowering seasons vary widely. Mistletoe flowers from February to April, holly from May to August and ivy from September to November.

A Dane called Iversen discovered that the distribution of mistletoe, holly and ivy depends on the mean temperatures of the coldest and hottest months of the year. It can withstand mean temperatures as low as - 8xC, although mistletoe can only thrive in such climates if the temperature in August averages 20xC.

Holly is less tolerant of cold winters, but can cope with cool summers. Ivy is perhaps the least hardy of the three. It is common over much of Europe but comparatively rare in Scandinavia, where it can only be found clinging to south-facing limestone cliffs.

Separation of the sexes

Most plants are hermaphrodite, having both male and female sex organs in each flower, but holly is diOcious, which means it has separate sexes. This explains the frustration some people feel when their holly tree never produces berries - in that case it will be a male. Though the male plant has small, creamy-white flowers like the female's, it only produces pollen.

You need a female plant for berries. Some months after the flowers have faded, the developing berries assume their familiar scarlet.

Red is supposedly a warning signal, so it seems strange that birds are attracted to it. Yet red berries are eaten in their millions and holly, like others, depends on birds - such as those attractive Siberian winter visitors, redwing and fieldfare - for dispersing its seeds. This apparent paradox is easily explained: with their greater mobility, birds can disperse seeds more efficiently than mammals. To make sure that the right organism eats them, most plants make their berries poisonous to would-be thieves or put them out of reach. But holly goes one step further, and arms its lower leaves with spines to thwart browsing animals. Higher up the tree, the leaves are not so prickly. So the berries are both a warning to creatures like us, and attract birds to disperse their seeds.

Dark side of the bloom

Holly has a sinister reputation, too. Its bark yields an adhesive that dries very slowly. The infamous birdlime, this glue can be spread on the twigs of shrubs and trees, in effect trapping any small bird that perches on it. It is used in parts of southern Europe to capture finches for eating or for selling as caged songbirds.

Ivy also attracts birds. This familiar species belongs to the ginseng family and is related to the plant that produces rice-paper and flaked goldfish food. Its leaves can tolerate the deep shade of the woodland floor, though it will only flower where its shoots can reach sunshine. Its black berries have evolved to be eaten by species such as blackbirds and wood pigeons. The nutritious flesh is digested by the birds while the resilient seeds pass through unscathed, to be deposited some distance from the parent plant in the birds' droppings, which act as a handy dose of fertiliser. There is evidence that the digestive juices of the bird may even help to trigger germination by chemically altering the seed coat.

The importance of being ivy

Ivy is very important to Europe's insects, mammals and birds. It provides a rich source of nectar and pollen at a time when most other flowers have faded, as well as nutritious berries in the depths of winter and much-needed nesting sites and shelter among its evergreen leaves.

In particularly bitter winters, wrens will huddle in large numbers in thick ivy to keep warm. Some misguided landowners believe that the roots of ivy clinging to a tree suck the goodness from it, so they cut through the trunk of any ivy they see. But this is a myth: the roots fasten to the bark but do not penetrate it. It is true that ivy climbing a tree will compete with it for nutrients and light, but the tree rarely suffers badly. From a conservation angle, it is much better to allow the ivy to grow because it offers so many benefits to wildlife.

Of this trio of decorative plants, mistletoe is undoubtedly the strangest.

Sprouting from the winter branches of a gnarled old tree, its fountains of livid green leaves and white berries proclaim vigour and vitality in the lifeless winter landscape. Is it any wonder that since time immemorial it has been the subject of folklore and superstition?

The druids of ancient Gaul used to collect mistletoe from oak trees on the sixth day of the moon at the end of the year, believing that it kept the tree alive over winter and that some of its magic might pass to them. The absence of any visible link between the plant and the ground only served to confirm their suspicion that it was a godly plant rather than merely mortal - a belief that has led to the tradition of kissing under the mistletoe at Christmas time.

Mistletoe mysteries

Mistletoe is a hemiparasite, tapping its host for water and minerals, yet capable of making its own food by photosynthesis. Its preferred hosts are apple and poplar, but it will attack many species of tree, especially deciduous ones. Like holly, it is diOcious, so not every plant has berries, and it relies on birds to disperse its seeds. Robins are very partial to mistletoe berries. Mistletoe also has a special relationship with one particular bird species, the mistle thrush - the Latin names giving clues to this association. The scientific name for mistletoe, Viscum album ("gluey white") refers to its sticky berries, and the name of the thrush, Turdus viscivorus ("mistletoe-eating thrush"), highlights the importance of this bird in the life of the plant.

The reason why mistletoe is often tantalisingly out of reach can be explained by the thrush's behaviour. Male mistle thrushes establish their territories soon after Christmas and can be heard singing their far-reaching song from the tree tops as early as January. At this time of year it is often stormy and, as their territories are comparatively large, these birds prefer to shout their song from the topmost branches.

Mistletoe berries are their favourite food, but they hate the sticky seeds and wipe their beaks vigorously on the nearest branch to remove them. Seeds that get accidentally swallowed emerge in the birds' droppings, still covered in glue. Either way, the seeds can easily get stuck to a branch. In March, when the sap of the host tree begins to flow, the seeds germinate and push their roots (known as haustoria) into the living tissue.

The distribution of mistletoe in Europe is a puzzling one. Although it is common on the continent, in Britain it is found only in the south, and not at all in Ireland. In France, it is a very familiar sight, especially in the apple orchards of the south and along the avenues of poplars that grace so many of the routes departementales.

Best of British

There are other, non-native, plants associated with Christmas. These include the familiar Christmas tree and other, more exotic kinds (see right). But holly, mistletoe and ivy are truly British, and they can all make attractive additions to any school garden and provide excellent classroom material.


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