It’s seeds are dispersed by birds, mistle thrush in particular. The name mistle thrush comes from this association. The seeds are coated with a sticky material called viscin. Some viscin remains on the seed even after being eaten, and when it touches a branch it sticks and attaches the seed firmly to its future host.

A mistletoe seed germinates on the branch of a host tree or shrub. The first stage of the embryonic root grows downwards until it makes contact with bark. Then the root tissue at its tip ‘penetrates’ the bark - a process that may take a year or more. Until it has achieved a connection to the host tree’s water and nutrient transport system the tiny mistletoe plant is dependent on its own photosynthesis. Once the connection is made the mistletoe can steal to meet its needs and can grow faster.

Actually, mistletoe does not grow into the host wood – it simply tricks host bark cells to develop host wood around them. The resulting fused host/parasite woody structure (the haustorium) grows larger each year, eventually dominating the supporting branch.

Despite depending on trees, mistletoe is not generally a woodland or forest plant, preferring its hosts in open situations with plenty of light around the tree. So it is most likely to be seen in gardens, orchards, parkland (traditional and modern), churchyards etc.

The commonest host for mistletoe is cultivated (not wild) apple trees. But it also occurs on many other tree species, with other favourites including limes, poplars, and hawthorns.

Role of birds

Mistletoe is completely dependent on birds for seed dispersal. Surprising then that only a few bird species are attracted to mistletoe berries. This is because of the berries white colour - most birds seek out red berries. Then to make things more difficult the way in which different species eat the berries has an impact on mistletoe success.

Mistle thrush usually swallow the whole berry intact, excreting a mass of partly digested berry pulp and seeds about 30 minutes later. A few of those seeds, still with their viscin coat, may stick to a tree branch where they can germinate. However, inevitably most will not end up in a suitable spot.

Blackcaps, a smaller bird, cannot swallow whole berries. They only swallow the berry skin and pulp, wiping each seed off their beak on to a branch before swallowing which gives the mistletoe more opportunity to spread.

This raises an interesting question. Mistle thrush are resident all year but blackcaps are a summer visitor to the UK; they arrive in April, breed, and leave in autumn – all at a time when mistletoe is not carrying berries. Hence the very strong association between mistle thrush and mistletoe here in UK. In the late 1990’s something changed. While our blackcaps continue to come for the summer, for some reason blackcaps that have bred in Germany have taken to migrating not south but west. They come in increasing numbers to the east of England to spend the winter and can now be regularly reported on garden bird feeders. This could possibly be a factor in the recent increase in mistletoe in the east of England.

To find out much more about all things mistletoe try The Mistletoe Pages – About the original mistletoe of tradition & folklore.