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Why do we kiss under the mistletoe?

Queen’s plant-parasite lecturer, Dr Johnathan Dalzell explains the tradition of puckering up under a sprig of mistletoe

From hanging stockings by the fire to decorating Christmas trees, the festive season is full of weird and wonderful customs and rituals – and kissing under the mistletoe definitely falls under the former category.  There is nothing as awkward as finding yourself surreptitiously standing under a sprig of mistletoe, forced into a public display of affection, as overzealous partygoers insist “it’s tradition.”

But what exactly is mistletoe? Why do we only see it at Christmas and what makes us kiss at the sight of it? Dr Johnathan Dalzell, a senior lecturer in plant-parasite interactions at Queen’s is here with a handy explainer.

Where does this tradition come from?

In Norse mythology, Odin’s son, Baldur (younger brother to Thor), had a premonition that he would be killed. His mother (Frigg) tackled this head on, extracting an oath from every object on earth, to not harm Baldur. This was agreeable to all…except mistletoe, which was overlooked! Loki (god of mischief) exploited this loop-hole and used mistletoe to create a toxin-laced dart, giving it to Hod (blind brother of Baldur), who he tricked into killing Baldur.

The mythology scatters at this point, but in some versions of the story, Frigg’s tears become the pearlescent berries of mistletoe, which was hung over doors as a reminder, and mark of respect to Baldur. It is thought that the Baldur incident, and later adoption of mistletoe as a symbol of fertility by Celtic druids (mistletoe can blossom in mid-winter, unlike many other plants) were responsible for the cultural alignment of mistletoe with the modern day Christmas period.

kiss me gift tag

But why do we have to kiss under it?

Whilst the Norse stories diverge after Baldur’s death, some versions recount that the gods were able to negotiate Baldur’s release from the goddess of death (Hel). Frigg, keen to sweep her mistletoe faux pas right under the carpet, declared mistletoe to be a symbol of love, and vowed to kiss everyone that passed underneath! It seems likely that this was infused with the Celtic druid interest in mistletoe as a symbol of fertility, and ultimately, mistletoe found a home in modern society through the act of kissing. In the 1800s, Washington Irving wrote that “young men have the privilege of kissing the girls under (mistletoe), plucking each time a berry from the bush. When the berries are all plucked the privilege ceases”. The symbolism and act of privilege has become more nuanced since then, but the association remains.

What exactly is mistletoe?

Mistletoe is actually a parasite - it steals water and nutrients from trees! Most mistletoe seeds are spread by birds, which eat the berries, and defecate on tree branches, inoculating the parasite onto a new host tree. The seed releases a compound called ‘viscin’, which dries to form a stiff biological cement. Many mistletoes will wait until the following spring before they will germinate. Some species of mistletoe spread seed by wind, and some are even explosively dispersed! Dwarf mistletoe varieties that use explosive seed dispersal can blast seeds up to 16 metres away, at a velocity of 27 metres per second – rapid!

Most plants grow towards the light, and use gravity to inform their growth patterns, but mistletoe does it’s own thing, and grows away from the light (towards the tree), and ignores gravity so that it can infect the tree in any direction. The parasitic mistletoe will develop a special invasive organ called a ‘haustorium’, which it will use to mechanically invade the tree branch; once inside, it will manipulate the host tree, and steal all the resources that it needs to grow, flower, and reproduce. Few parasites enjoy the cultural celebration of mistletoe!

Dog under mistletoe

To what extent is it a curative plant?

Mistletoe is used in some forms of alternative medicine. However, you should never consider self-medication with mistletoe, or any other ‘medicinal plant’! Always seek professional advice from your GP. Mistletoe does contain compounds called ‘lectins’, which are being tested in clinical trials for cancer treatment, but even if the trials are successful, it could be some time before a safe form of therapy is developed. The berries are poisonous so don’t eat them! Great for birds; not so great for humans.

Can mistletoe really increase fertility?

No, mistletoe was used as a symbol of fertility by the druids, almost certainly because it is an evergreen plant, which can blossom in mid-winter. As it lives on deciduous trees, the contrast of its fertility and growth relative to the host tree during mid-winter is quite stark. The shape of European mistletoe is also considered as vaguely analogous to certain aspects of female anatomy!


 Why do we only see mistletoe at Christmas?

Christmas celebrations emerged from a cultural melting pot of competing traditions and world views. The alignment of mistletoe with Christmas appears to be a hangover from ancient mythology, Celtic druid imagery, and modern-day cultural appropriation.


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Photo: Johnathan Dalzell Johnathan Dalzell
Dr Johnathan Dalzell is a senior lecturer in Plant-Parasite Interactions, and Biochemistry Programme Director within the School of Biological Sciences and Institute for Global Food Security.