The Forgotten Science of Mistletoe

This feature explores the surprising and overlooked history of mistletoe, beyond its holiday symbolism. The article blends botanical concepts, medical history, folklore and modern research and shows that mistletoe is more than a festive decoration.

Mistletoe. Image Credit: Inn from Pixabay

What is Mistletoe?

You can find mistletoe growing high in the branches of trees, especially lime, apple and hawthorn trees. They are often in open areas like orchards, parklands and gardens in Southern England, the West Midlands and on the Welsh border. Mistletoes are generally classified as hemiparasites because they derive some of their nutrition from host plants, but are also capable of producing their own energy. When a mistletoe sprouts, its specialised root-like structures (haustorium) penetrate the host’s bark. This gives the mistletoe direct access to the water and nutrients that are actively being collected by the host plant. The mistletoe does, however, have green leaves containing chlorophyll which is used for photosynthesis, meaning it  can make some food on its own. 

Apart from its partial parasitic nature, mistletoes are also picky eaters. You see, in order to receive a lifetime of food supply from its host, it must first break the bark and connect to the tissues that conduct water through the plant tissues (xylem). To do this successfully, the mistletoe targets relatively softer and thinner barks like the young trees of Malus sylvestris (crab apples), Crataegus monogyna (hawthorn) and Tilia sp. (lime).

Ecological Significance

Mistletoe is a vital component of woodland habitats. Once the mistletoe acquires its host plant, the haustorium pulls nutrients into a localised area and they are transported throughout the growing shoots. Chaos erupts and instead of one branch growing outward, several shoots grow simultaneously and form a dense clump around the host, also known as a witch's broom. 

The witches’ broom. Image Credit: Nelly Vincent from Pixabay 

Witches’ brooms attract a lot of wildlife, for example, they are home to many birds including spotted owls and hawks. The plant is also popular amongst insects; several species of hairstreak butterflies lay their eggs on the leaves of mistletoe and, being one of few species that flowers in winter or early spring, it receives a lot of attention from pollinators. Alongside this, their berries are an important source of food for squirrels and many species of birds throughout  the cold winter months. 

However, mistletoes aren’t the best at recycling their waste. When shedding leaves, most plants gather up the last bit of nutrients the leaves may contain, mainly nitrogen and phosphorus, before letting the leaves fall. But, mistletoe decides to skip this step and shed its leaves while they're still green. While this uncommon technique may not benefit the mistletoe, it can improve the health of the soil below and creates hotspots of fertilised soil near the host plant.

History of Mistletoe 

The Druids:

Long before the evergreen nature of mistletoe was known, the first century Celtic druids considered mistletoe to be sacred. Mistletoe was considered an antidote to all poisons and a boost for animal and human fertility. The druids even had a ritual where a white-robed priest druid would climb an oak tree and cut the mistletoe with a golden sickle. They believed that if the mistletoe touched the ground, it would lose its magic. In England, mistletoe was only regarded this way when it grew on oak trees. 

Norse Mythology:

In Norse Mythology, Baldr, the most beautiful amongst the gods, was prophesied to die. To avoid this, his mother, Frigg, extracted an oath from every living being on earth, except for the mistletoe, which she considered too young and harmless. To celebrate his new found immunity to the prophecy, the gods of Asgard decided to throw things at Baldr. The only one who wasn’t participating was his brother, Hodr, the blind god. Loki, the god of mischief, found out that the only thing that wasn’t sworn on, was mistletoe. He made a dart of the mistletoe and tricked Hodr into throwing it at his brother, inevitably killing him. The white berries of mistletoe are considered Frigg’s tears. Eventually, the plant was considered a token of love and peace, to ensure that it would never be used as a weapon again. 

A drawing of Viscum album. Image Credit: Step, Edward

Early Medicinal Use:

Ancient Greek and Roman physicians used the mistletoe leaves to treat ulcers, several inflammatory conditions and even epilepsy. However, by the 19th and early 20th centuries, medicinal use of mistletoe was largely dismissed because of its diverse nature, which is largely attributed to its hemiparasitic nature. Since it derives its nutrition from the host plant, the nutrients it receives entirely depends on the characteristics of the host. Mistletoes that grow on Locust trees have higher protein content compared to mistletoe that grow on pine trees, because the locust tree is high in nitrogen. The host also has an influence on its morphological characteristics, like the shape and the size of the leaf. If mistletoe  grows on hosts that have easy access to water, the leaves are larger and thinner. Similarly, if the mistletoe grows on hosts that do not have easy access to water, they develop succulent leaves to store water. Therefore, the chemical composition of mistletoe depends on the kind of host it clings to, making it increasingly difficult to narrow down a standardized dose.

Christmas Tradition:

In the 1840s, before the Christmas trees became the centre of attention, English homes featured a kissing bough. This spherical structure was made with wood, wire and covered with evergreens like holly, ivy and mistletoe hanging from the center. Under the bough, the rigid social norms of the 18th century were temporarily suspended and allowed a brief moment of physical contact. Originally, a man could only claim a kiss if he plucked a white berry from the hanging mistletoe. If the berries ran out, kissing was not permitted, symbolizing that the plant’s fertility was gone and with it, its vitality. Writers like Charles Dickens popularized the idea of adding a touch of greenery to the festive home and simplified the kissing bough to sprig of mistletoe we use today. 

Bonne Année, 1917 Image by Xavier Sager

Modern Medicine: 

Extracts of mistletoe, such as Iscador and Helixor have recently been used as complementary cancer treatments in Germany. Recent trials like the Phase I trial at Johns Hopkins University (2023) hint that intravenous mistletoe (directly into the bloodstream) can improve immunity of individuals who have advanced cancers and have undergone treatment several times. Mistletoe improves the cell’s biological response; helping the immune system to recognise and fight the disease. It induces cell death in cancer cells and aids in stimulating natural killer cells. 

While mistletoe is a parasite and induces stress in host trees, its berries are a source of nutrition for birds, squirrels and deer in the winters. Mistletoes are also used as nesting material for birds. In fact, experimental removals of mistletoe in Australia led to a 26.5% decrease in woodland bird species richness. 

Research shows that this hemiparasite is a sensitive bioindicator of climate change and its distribution is being used to map forest vulnerability to increasing atmospheric drought (persisting dry weather). As we are aware, mistletoe leaches water and nutrients from its host, thereby inducing stress. Studying the responses of these hosts under such conditions and continual climate change helps us in forming strategies that are suitable to protect nature. 

Why Is It Relevant Now?

Apart from being the master of adaptation, mistletoe also bridges history with science. What was once considered magic by the druids now simply highlights its evolutionary survival. Back then, its evergreen nature was deemed as a sign of immortality, but science has made it possible for us to deduce that it had little to do with immortality and more to do with it being a hemiparasite. Earlier, the credibility of mistletoe was dismissed for its hemiparasitic nature, but its diverse characteristics have turned to our favour. Mistletoe may be the solution to two of the current world’s biggest challenges: evolution of cancer treatment and nature’s response to climate change.

About the Author:

Harsh Sinha is currently pursuing his Master’s in Biotechnology at Nottingham Trent University. You can find him on LinkedIn at: Harsh Sinha.