MISTLETOE

VISCUM ALBUM L.

by Charles DECLERCK, Belgium
Go to the Update of december 1st 2001
MAIN ARTICLE :  THE MISTLETOE

Among all the plants growing in our country, mistletoe is certainly something special, because its appearance on the trees looks so mysterious. It grows across the bark and forms huge round shrubby organisms, especially visible on the foliage trees ; these trees lose their leaves wheras mistletoe doesn't. It is less visible on spruce-firs and other non-deciduous trees. It strikes me that plants of mistletoe are distributed widely over some areas, and are hardly seen in other parts of the country.

Many people have tried to make mistletoe berries germinate on their trees, but the hardly ever succeeded. Only in rare occasions they were successful. My own unsuccessful trials made me look for the reason of my failures and after many years I finally discovered the "Secret of the Mistletoe Plant". At the same time I found an acceptable explanation why a tree does not suffer from some mistletoe plants but is destroyed by too many of them.

1. Distribution.

The seed of mistletoe, arrived on a tree, must try to survive its long germination process as an epiphyte. It may take a year before it can such the sap of the tree and become a half-parasite. When its site is not protected, it is very likely that the seed dries up during a hot summer or is washed away by a heavy rain-storm.

As for the places where mistletoe can be found, I should divide these places into three areas, based upon the microclimate of each area. If this microclimate is favourable for the seed during its long epiphytic germination process, there will be a favourable evolution. The requirements are : regular rainfall, a high atmospheric humidity, fog and nightly condensation during warm and dry spells in the summer so that the seeds retain enough humidity and do not dry out.

In this kind of microclimate, mistletoe is often found and we see older, younger and very young plants. All the same too dry years can occur there, so that seeds dry out ; a germination trial can fail there. Even there mistletoe cannot be found everywhere sporadically. In this way the word "microclimate" gets its full meaning.

Next there are areas where mistletoe is not so often found and young plants are rarely seen. The older and younger plants have obviously the same age. From time to time there was a favourable microclimate for the germination.

And then there are certain locations in wich mistletoe is never found. Even there it sometimes happens, but very exceptionally, that one or some plants appear, but they will never have descendants. In this case the favourable microclimate was a very exceptional happening.

Another fact proves that my thesis is right : every year masses of seeds are spread by birds ; these seeds land on branches nearly and far away. We may assume that a large part of them will be able to germinate. The major part however will not succeed in surviving epiphytically because they simply dehydrate.

It is the germination process which reduces the spreading of mistletoe in nature and for the same reason the germination is seldorn successful when we transfer seeds to a host tree.

The reader who does not know the problem should know that as far as I know, no one ever succeeded in discovering the secret of the germination process. We see that mistletoe is widely distributed over some areas and doesn't grow at other places, but so far nobody has tried to find out why, and they blindly accept the hypothesis "bound to calcareous areas" without examining whether this thesis is right or wrong.

2. The Germination Process.

If you break the skin of a berry, you see a liquid, transparent viscid content and a milky-white little ball of firm pulp in wich there is one single invisible seed. The whittisch little ball is so glit that you cannot get the seed out of it. If however you rupture the skin of the berries and leave them a few days, you will notice that all the pulp has turned viscid and transparent and now the little seed has become visible. It is green and it is retained by a net of little fibres. When it gets dry, the little seed has the colour of wood. Dry it with a towel and you can watch it : it is flat and oval, but many seeds are much broader and even heart-shaped.

It is very important to know where you can find the germ. The fibrous structure leads to one point, the back side which was connected with the little stem. The germ is always at the opposite side ; it may be recognized by a round spot. The broader seeds always have two or three germs, which so far nobody seems to have noticed. In order to germinate, the seed must not necessarily pass trough a thrush's digestive organs ; it germinates as well when directly placed on a branch.

It is useful to know that, when a trush produces seeds in a viscid pulp of droppings they eventually land on a branch and gravitate to wands the underside of the branch or to wands another sheltered place, especially when the branch is moist.

There the viscid get dry and stick to the branch, protected from heavy rainfall and the scorching sun, and above all with a small amount of fertilizer, which might be more usefull than we think for the absorption of moisture as an epiphyte.

The germination proceeds very slowly : the seed absorbs water and photosynthesizes because it is green. The germ, green as well, appears and twins away from the light, towards the branch. As soon as it touches the branch, its widens and a kind of sucker is formed, wich fixes itself to the surface of the branch. Now a fine little radicle, is developed ; it goes through the bark and the inner bark, but does not penetrate the wood. Afterwards - and this may take two years - the "radicle" develops long cylinder shaped fibres longtwise against the wood. Now new "radicles" are produced, and they penetrate their hosts' tissues. Only now the seedling can suck the mineral sap and it becomes independant from the climatological situation. The little plant is now a half-parasite and will develop its two first leaves.

As this epiphytic germination may take more than a year, the possibility of dehydration is very real. Even a favourable year can have dry spells, so that the seed gets dry. These interruptions in growth may delay the germination (and be the cause of the long germination period ?). On the other hand it seems logical that without a lack of moisture the germination proceeds fluently.

3. The mistletoe plant.

Mistletoe is dioecious, which means that there are male and female plants. So far I don't know yet whether one single seed, showing two or three germs produces only uni sexual or male and female plants, in that case one simple seed, which has been developed somewhere completely separately, can still produce berries and seeds.

Mistletoe plants produce a huge amount of nectar, because the flowers are a meeting-place for bees; this is also proved by a good fructification; the plants are always loaded with berries. For bee-keepers it might be very interesting to sow mistletoe on there trees !

Misteltoe grows on practically all kinds of trees and woody plants, foliage trees as well as conifers. It is never seen on plane-trees (Platanus) and rarely on beeches (Fagus) and oaks (Quercus).

The reason is purely technical. The bark of plane-trees peels off every year and so the seedling is removed before the germination is finished ; as for beeches and oaks : their bark is covered with a layer of wax. The glue, wich initially seems to adhere so well, decayes after some time and so the seedlings are easily removed by the slightest touch. The adhesion happens only by the intimate contact between the fibres of the seed and the bark, but when this adhesion is not strong enough, the seedlings generally fall off. This is the only reason why mistletoe does not grow on some trees, seldom appears on others and is hardly ever to be seen on a third kind of trees.

For the druids the oak was a holy tree, and so the rare mistletoe on oak was very special.

In our country we often see mistletoe on poplar (Populus), not because it likes to grow on these trees, but because after a long flight, the mistle thrushes (Turdus viscivorus) tend to rest on these tree-tops and deposit their faeces there. Consequently these birds involuntarily locate the situation of mistletoe. So mistletoe does not "choose" the tree, but it is right that it grows abundantly on some trees wheras the growing power on other trees is very poor. Mistletoe has a longer, wider or shorter leaf depending on the kind of host tree. Some people say that there are different species of mistletoe.

A long time ago the bark of mistletoe was used to make bird-lime ; people noticed that mistletoe on maple (Acer) and on elm (Ulmus) produced more bird-lime than other trees. Personally I think there is a good reason to believe that these are examples of the so-called "influence of the rootstock" in the cultivation of fruit trees and ornamental grafted plants. The graft reacts in a different way according to its rootstock. Obviously for mistletoe there are also physiological differensces depending on the kind of tree it grows on. Consequently the definition of "sub-species" does not hold in this case. I am convinced that, if seed of mistletoe with short leaves is put on a tree, and it forms long leaves, these seedlings will undoubtedly form long leaves.

Some researchers argue that mistletoe growing on spruce-fir (Abies), does not germinate on pine (Pinus) and vice versa. This cannot be true. This assertions is the result of a failing germination trial. If they had tried a simultaneous sowing test of pine on pine and spruce-fire on spruce-fire, they would have noticed at once the failure of the test. As no one so far understood the problem of germination, no one could sow efficiently.

Some people pretend that mistletoe is an asset to its host, because it remains green in winter and photosynthesizes, wich is in favour of the tree. This seems rather unlikely to me because first of all I do not see how the prepared sap of the mistletoe could penetrate into the bark of the tree and secondly : if this thesis is true, then trees covered with mistletoe would grow better, but we can only notice the opposite. It is also out of the question that the tree is poisoned as some others pretend, why is one tree, covered with mistletoe, destroyed whereas another one, with only a few plants, doesn't suffer ? I think I have a logical answer to this question. The draining of raw sap is only a small loss for the host tree, but foliage trees lose their leaves and their root system is asleep during the dormancy. Mistletoe however continues its transpiration so that a tree, completely covered with mistletoe, is literally drained ; in spring the tree cannot recover anymore ; it has come beyond the "point of no return".

The phenomenon is identical to the so called "freezing" of hardy conifers and laurel-cherry (Prunus laurocerasus). This happens exceptionally when at the same time the soil is frozen and in late winter a raw east wind together with an already hot sunshine make them evaporate so rapidly that they get beyond the point of no return. Weeks later the spruce-firs are completely dehydrated whereas laurel-cherry always buds.

They are frozen, so we say, but as a matter of fact they have dried out, their roots not being able to absorb water to compensate the evaporation.

When mistletoe develops on a thinner branch, we see that the upper most part disappears and the plant stands at the front end of that branch. Here too the explanation is quite simple : by quick evaporation in summer, mistletoe requieres so much raw sap that the upper side of the branch lacks sufficient supply, dries and perishes ; the dead branch will be blown off by the next thunder-storm.

A tree covered with mistletoe may get dehydrated during the summer ; year after year the parasite requires more mineral sap, so that the viability of the tree decreases progressively.

The tree start to wilt, its photosynthesis is limited, so that the roots get less nourishment. In the end the tree may even perish, maybe also after mould infection on its weakened root-system. We also notice that the mistletoe wilts together with the tree by its lack of mineral sap.

4. Sowing Mistletoe.

If you have read the previous lines carefully, you will understand which points are important if you want your sowing to be successful. Store the seed at a light, moist place (no refrigerator or freezer ; they give bad results). When the berries have ripened, you can sow until late spring. If you start, the skin of the berries should be ruptured a few days in advance. Smear the seeds over a site at the under side of a branch, that is averted from the sun. You may make the bark a bit thinner by cutting out small strips, so that the "radicle" can reach the bark more easily.

You can also put them into an incision in the bark but it hardly ever succeeds because the lip which protects the seed so well, dries up and curls, so that the seed gets lost.

In order to protect the little seed from dehydratation you can cover it partly with a strip of polyurethane with open cells which you fix with a bit of string. Of course the little seed may stay wet too long so that it perishes afterwards.

We must make a difference between wet and moist. When something is wet, there is little or no air, whereas moist means that air and water are both present.

Epiphytes are plants adapted to living in the air without suffering from lack of moisture. After a rain-shower the surplus water drains off but the bark (and the air) remain moist for a long time.

Too much water is a problem known only too well both by amateur and professional orchid growers. The substrate should be of an open quality and after watering the plants, they may never remain wet for a long time, only moist.

In our region with the same climate, (the climate in the middle of West-Europe) mistletoe will grow everywhere if only their seeds will be placed favourably and provided they will never be wet during their germination but never be too dry either. This is possible if after a day without rains, the are sprayed with rain-water, to wich you add 1 cc of leaf-nutriënt as the green little seed and the green germ photosynthesize to develop. This thesis makes much more sense than pretending that the little seed has enough reserve to last a whole year, the duration of the germisation. The photosynthese supply the reserve from the seed.

5. About thrushes and the dissemination of the seeds.

Because there are quite a lot of misgivings about the dissemination of seeds by the Turdidae, I want to place this dissemination in the correct context.

The mistle thrush (Turdus viscivorus) spreads the seeds over a long distances. In our country, the return flight of the migrating thrushes takes place in February/March. The birds fly back from the south, where there is mistletoe, to the north, where there isn't.

It is wrongly believed that distances over 2 km cannot be bridged and that the seeds have already been excreted after half an hour. The trush proves otherwise !

When prolonged efforts are produced, most of the blood streams towards the muscles, and the digestive system almost stops completely. It is possible that the observation of captive thrushes shows that the seeds appear after only half an hour, but migrating birds cover huge distances before resting in high trees where they defecate.

"Mistletoe in the Creek District" is a classic example of this. I estimate the nearest source population at a distance of 50 km by air.

Alost brings a similar proof. All mistletoe is found on the first row of the trees, not in the middle, not at the back, but at the south side of the poplars wood. So the thrushes came from one direction and, obviously, they rested on the first row of trees !

When we find mistletoe on an row of poplars in a Brussels cemetery, the same explanation is valid.

The allegation that this mistletoe is a local contamination from the mistletoe branches laid on the graves cannot be substantiated. The mistle thrush won't eat there and then fly to the poplars. The blackbird (Turdus merula) and the song-thrushes (Turdus ericetorum philomelos) can eat these berries, but then they won't fly high up into those big poplars, they will prefer the lower trees and bushes of their own habitat.

My explanation is logical. Some ten years ago, there was an exceptionally humid year in Flanders, favorable for germination. All this mistletoe appeared some years later in the Creek District, Alost, Hekelgem, Brussels and perhaps other places. So they have all the same age and this can easily be established !

During that year, many branches of mistletoe were probably also deposited in many cemeteries, and blackbirds and song thrushes could have disseminated the seeds locally ... but mistletoe was never reported in the churchyard trees ! As you see, it's not as simple as it looks.

Further on, I will explain why the seeds that are deposited onto trees late in the season (when the thrushes migrate back) have the greatest chance of germinating successfully (provided that the summer is favorable, and this is very very exceptional in Flanders). We mustn't forget that the thrushes disseminate the seeds in the same way year after year, and that, every year, these seeds also dry out during the summer. The seeds of the berries in the graveyards, on the contrary, are deposited on the trees during the winter by the local thrush population, and this is much less favorable.

6. How does the seedling evolve ?

When the suction pad has attached itself, the green seedling grows visibly thicker and, at the same time, the seed thins. The reserves are used to develop the plunger, and photosynthesis has a complementary action.

The strong germ will then lift the emptied seed, and this means that the glue is exhausted. Later on, the seed-case drops off and the only thing remaining is a small green stem, a little stump of circa 4-5 mm.

In the autumn, there is enough moisture for the seedling not to dry out, but in winter, frost has a very dehydrating effect. If the plunger has not reached the sapwood, the seedling will dry out. And as the seedling does not dry out, this proves that the plunger has already reached the "source" in autumn. So this meaningless green stump has already become an independent half-parasite in autumn. This slow germination and the associated epiphytic live do not last longer than one year, but (only) from spring to fall. However, it is possible that certain seedlings do not reach the sapwood before the winter. I think that these will dry out and die during the winter.

Naked seeds on a branch can also dry out, but not those that are still inside berries. It is impossible to imagine that they would die during the winter and that the ones that would germinate later on would be disseminated by the birds (or people) when the frost season is over or even later.

When seeds are deposited "normally", the best period is from December till late spring, but in the winter there's an additional risk. Birds are hungry and the blue tit (Parus caeruleus) liks these seeds... In March/April, however, there are already a lot more insects, and then the birds prefer hunting. This is an additional argument for putting out the seeds later in spring.

During this long germination process, the temperature plays an important part.

In early spring, days can be pleasantly warm, but nights stay cold. That is why plants usually start growing when the nights get warmer as well.

Mistletoe reacts in the same way. The germination process starts late and slowly, when the night temperatures rise significantly.

I think that the further development also stops when nights get cooler in the fall. The consequence appears from the fact that most seedlings only grow their first two leaves when the winter is over and when the temperatures are once more favorable for their growth. In the youth stages of mistletoe (germination and seedling), I see many similarities with epiphytes. Whitin certain small limits, they resist a lot wait till the growing circumstances are favorable again. Without these limits, however, they die quickly.

7. How to obtain successful seedlings ?

Until now, all the "accesories" that I devised to help the seeds germinatig were less successful than just gluing the naked seeds with their own glue onto the branches.

The long germination is a consequence of the slow development, which is itself influenced by the prolonged cold (nights) in spring.

In horticulture seeds are already sown out in greenhouses during the winter in order to make them bloom in April/May. If the mistletoe seeds are given warmth, moisture and light early on, the seedlings will develop earlier and in March/April they will then be ready to be glued on a branch.

This really is the only useful technique.

Method :

Harvest ripe berries around the turn of the year and place them next to each other onto a sheet of wet kitchen roll in a dish. Cover the dish with a glass pane and keep moist. Put the dish in a warm (16 à 18 ° C) and light place out of directly sunlight. The seeds will then germinate within the berries.

Around mid-February, remove the paper and make the berries burst. Wait until the pulp has become liquid and translucent.

Carefully take the seeds out and glue them on the bottom of a dark-colored dish, about 1 cm apart. Let the glue set and moisten the seeds slightly with a spray. If you moisten them too much, the seeds will slide to one another.

Also spray the underside of the glass before covering the dish and putting it once more into a warm and light place.

Repeat daily : first let them dry, then moisten them.

When you have taken all the seeds out of the berries, try and collect as much liquid, sticky pulp as you can in a small jar that you keep in the fridge.

First, the shoots develop just straight as they come out of the seeds. After having reached a certain height, they will turn towards the dark side, i.e. the bottom of the dish. As soon as the shoot touches the bottom of the dish, you must put the dish outside on the windowsill out of the sun, there it is cold and clear, and then glue them onto a dry branch with the glue you collected from the berries. Possible night frost won't do any harm.

These geminated seeds will have taken enough head start to grow their first leaves in the coming fall. There is one absolute necessity : they must never dry out during the summer.

In my conviction mistletoe will be found in the future by amateurs everywhere as long as the little seeds will be helped efficiently during their epiphytic germination process.

Do not be afraid : the area will not be damaged by extensive colonies of mistletoe. After all the trouble that has been taken, you will understand that the seeds of your plants, whish will be spread in a natural way by thrushes, will not have a chance to survive in an area where the microenvironment is not favourable for the germination, in an area where generally no mistletoe is being found.

In areas more in the north, mistletoe cannot be found anymore. In my opinion this is due to the fact that there are a lot of short warm periods during the summer in that area. As a consequence the germs have not enough time to reach the sapstream which causes the seeds to dry out during the winter.

By using the method for pre-germination, it may be possible to let the mistletoe grow on trees in the north.

An other possibility consists in letting the mistletoe germinate on trees, in containers placed in a nursery. When the germination is succesful the trees can be then planted in the areas situated in the north. When the germination is a success, I am sure that the little trees will develop anywhere apart from the place where they are planted and, in my opinion they will even grow in the tree frontier.

Next spring, I wish you a lot of success with your sowing trial.

Charles Declerck

Retired Head of the Floriculture Departement

Ecole d'Horticulture de l'Etat de Vilvorde

(Now called : HORTECO Vilvoorde)

Brusselbaan, 45

B-1790 Hekelgem Affligem (Belgium)

Literature : Les plants. Paul Constantin 1896 - 1898 De maretak De zangvogels van België. R. Verheyen 1948 De vele interessante aspecten van de mistel De maretak meer biologisch bekeken Raising mistletoe from seed Les Druides en France Semis de Gui - Ecole de Botanique

Artikel uit 1898, getekend : "onleesbaar"

Le Gui (zeer uitgebreid en ludiek beschreven) Maretak voor versiering en verzorging Maretak in het Krekengebied Pictures

This drawing shows the seed at its normal size and also enlarged to show how the fibres comes together in one point. The germ is always at the opposite side. Broaden seeds have two or three germs.

When the seed is in the berrie, it is green,but when the fibres are dry, they are light brown, there is no doubt that this fibres are hygroscopic.
 

Photo : Guy LAURENT
This macro-picture of my germination trial was taken July 12th, 1985. This seed develops two germs and the little sucker is being formed : the hottest summer spell is still to come and may dry the little seed. We can also distinguish all sorts of lichen and scales, probably brought there by the wind and having established themselves. All this helps to absorb moisture and store it for the seed. Especially for the tangent place seed/bark the water is stored.
 


This is the copy of a photograph on the front page of "Bulletin de la Société Royale Linnéenne et de Flore" (Octobre/Décembre 1977). It shows us a germinating seed of mistletoe but here too,nobody noticed that one single seed developed three germs. Notice the favourable spot where this little seed happened to land. It has every chance to survive : young wood (thin bark), well protected from the powing rain ; let's just hope it will not dry up.