Mistletoe in May, and Maths (and pandemics)

May Day! Summer will be here soon.  And mistletoe is doing fine, just keeping on growing.  My first picture here is, literally, mistletoe in May – a small mistletoe growth growing low on a May (Hawthorn) tree in full blossom.  Taken in late April actually – on the banks of the Severn near Framilode earlier this week.  Hawthorn/May is one of mistletoe’s favourite hosts, though the growths are often rather small and mis-formed compared to the spectacular mistletoes of larger trees (the one pictured is certainly rather deformed!).

And how does mistletoe behave in May (the month, not the tree)?  Is it ‘just growing’ as I suggested in my last blog entry  – or can more be said?  What sort of growth is it?

Well… with the current worrying about when Covid-19 lockdown might be relaxed and how the mysterious ‘R’ number has to be satisfactory beforehand I thought a brief comparison with mistletoe might be interesting.  The ‘R’ number is a factor in the rate of disease spread – and is just as relevant to mistletoe as it is to SARS-CoV-2 (the coronavirus that causes Covid-19).

Mistletoe, like SARS-CoV-2, can be considered a pathogen – in this case infecting a host plant – and so the R number concept applies perfectly.  Especially so for our mistletoe (Viscum album) as this mistletoe species does have a very distinctive growth and spread pattern mathematically – in theory at least.

Current media articles (e.g this from the Guardian) are making the point that if R is more than 1 the disease will continue to spread, and spread will be much faster the higher the value. If less than 1 it will decline. The R value is the number of people an infected person will pass the disease on to.  Current efforts seem to have changed R from 3 to about 0.7 in the UK in the last 6 weeks or so.

This has parallels with several aspects of mistletoe growth – though for mistletoe the factor is much more definable and fixed.  And it can be clearly seen now, in May.  Every branch from last year, tipped with a pair of leaves and small flowers (which opened – and closed – in Feb/March) is now producing two new shoots, each one tipped with a new pair of leaves and, between them, a small bud which will be next year’s flowers.  This picture (right) taken this morning shows this clearly.

So, just looking at those leaves to start with, we have a shoot topped with a pair of leaves which is becoming two shoots each topped with a pair of leaves.  Later in the summer last year’s leaf pair will drop off and these two new shoots will become the tip of the plant – the only part with leavesAnd there will twice as many leaves as last year. That is a factor of 2 each year – which suggests an exponential growth rate.  Every shoot (with one pair of leaves) produces two new shoots (each with one pair of leaves) every year.

R is therefore 2 for each pair of leaves.  One pair of leaves becomes two pairs in year 1, 4 pairs in year 2, 8 pairs in year 3, 16 pairs in year 4 and so on.

Such a precise growth pattern is unusual and is one of things that makes our mistletoe, Viscum album, so special.

This is very definitely exponential growth – and it can (of course!) be represented by the formula

y = abn

where a is the original number of shoots/pairs of leaves, b is the factor (the R number) and n is the number of years.  y is the number of pairs of leaves after n years.

So… for year 4, substituting in the values a = 1, b = 2 and n = 4:

y = abn

y = 1 x 24

y = 16
(as already calculated above)

If you know your maths you’ll already be thinking that these numbers could get very large very quickly after a few more years – and you’d be right.  Think about, say, year 10 for example:

y = 1 x 210

y = 1024

That’s 1024 pairs of leaves at 10 years, all from just one pair at the start.

If the exponential formula y =1 x 2n is plotted as a graph you can see the full effect (formula simplified to y = 2n or, on the graphs below, to y = 2x):

 


The left hand graph shows the exponential growth shooting off the scale at about 1400 leaf pairs at about 11 years.  The right hand one is zoomed out showing it going off the scale again, this time at 150000 leaf pairs at just over 17 years.   That’s a lot of leaves.

And that number is extremely important if you think of mistletoe as a pathogen.  Each mistletoe leaf transpires water in a different, more reckless, way than the host tree’s leaves.  The more mistletoe leaves there are the more water-stressed the host becomes.  Doubling each year doubles that impact.  So it can matter quite a lot.  One of the key needs when managing excess mistletoe is simply to cut off the leaves – which, in theory at least, can reduce water stress very effectively.

But how real is all this – this is just a very simple mathematical model – albeit based on observed growth patterns. What happens in reality?  Is it better or worse?

Well it depends – on whether the mistletoe is managed, how high it is the tree and how wind-damaged it gets (branches break off very easily), whether it is cut for Christmas, whether it gets diseased (mistletoe gets diseases too) and whether leaves are grazed or damaged by birds or invertebrates.  So the number is likely to be less in reality.  (Except, of course, for the instances when a shoot produces 4 shoots and 4 pairs of leaves – but that’s another story…)

Still with me?  Good, there’s a little way to go yet….

So far this has been about one mistletoe plant – but the real virus comparison should be with mistletoe spread and new infections…  Either on other branches of the same tree or to adjoining trees.

And for this exactly the same maths applies – since spread is all to do with how many seeds mistletoe produces, and that relates to how many berries develop, which relates to how many flowers and so how many flower buds.  And the flower buds are between the pairs of leaves.  So it is absolutely the same maths – each pair of leaves has one set of flower buds.  And so each pair of leaves has a direct relationship with the number of berries and seeds produced.  The seeds are the mistletoe’s equivalent of the virus.  And, with 3 -5 flowers per bud and so 3-5 berries/seeds per bud, it is a lot of seeds.

So… the number of seeds, in this simple model, increase in the same ways as the leaf pairs.  Numbers way off the scale after only a decade or so.  Which would suggest mistletoe might spread very fast – with all those seeds.

Except that it doesn’t – it spreads very slowly.

Perhaps the model is even less like reality for the seeds?  There are certainly many extra factors to consider – some positive factors some negative.  On the positive side (more seeds) there is the factor of the overlooked flowers – it is a simplification to say flowers are only between the leaf pairs – there are usually many in the stem axils immediately below the leaf pairs too – so the model is under-estimating the potential of flowers/berries/seeds, possibly by quite a big factor.

On the negative side (fewer seeds, or fewer effective seeds) the first, most obvious issue is whether the flowers are pollinated or not –  if not then they won’t form berries and so won’t produce seeds.  And since they rely on insect pollination in February and March – which can be a challenge in cold years – pollination can be variable.

Another obvious factor is considering how many seeds, however perfectly produced, will ever get a chance to germinate on a tree branch – and this very much depends on how well they are spread by birds – and by which birds.  Mistletoe is not liked by most and even those which do eat the berries often only take them in small numbers if other berries are plentiful.  So the seeds may not get spread, or not spread efficiently.   See, for example, these berries, photographed (left) this morning, which are still attached to the parent plant – the seeds in these will never get a chance to grow now.

So, numbers of flower buds doesn’t equal number of berries and number of berries/seeds produced does not equate to numbers of infections.

Nonetheless there is still a massive increase in berry production as a female mistletoe plant matures – and so the older the mistletoe is the more likely it is that there will be spread to adjoining branches or trees.  As regular mistletoe blog readers will know there are also some interesting changes in how mistletoe is spread, larger numbers of overwintering Blackcap birds meaning more efficient spread tree to tree.  Fascinating stuff – and difficult to quantify.

Meanwhile, if you’re growing mistletoe in your garden and are worried by any of the above don’t be (unless you have masses left unmanaged for decades!).  Small to medium mistletoe growths are not a problem and will only become one if left unmanaged, just like most other garden plants.  Look after your mistletoe and, once it’s 10 years old or so, prune it back now and then to adjust that ‘R’ number to a level that’s right for you.

Spring – mistletoe has nothing to do but grow…  

April, in lockdown, and the main mistletoe action season is over – berries ripened (Nov/Dec/Jan), seeds planted (Feb/March), flowers over (Feb/March), pollination done (Feb/March).

From now until next winter mistletoe has nothing to do but grow.  For the seedlings it’s a bit more challenging – they still have to link into their new host’s vascular system, but for a mature mistletoe there really isn’t anything else to do now.  Just grow new shoots and new leaves and, for the female plants, slowly develop the berries over the next 9+months.

This pattern dictates the year for a mistletoe specialist like me – once April comes the talks have finished, the enquiries dry up and we stop sending out grow-kit orders. Time to take stock.  Especially this year as we’re in lockdown because of coronavirus.

This is a time for other plant parasites though, so I may post some news about those soon – dodders, broomrapes, toothworts – this is the start of their time for germination and flowering.

Not forgetting mistletoe though, obviously, so here are a few pictures of the orchard at Standish Court, taken yesterday, showing apple trees festooned with mistletoe.  Too much mistletoe actually – you’ll see in some pics some trees have been blown down, possibly weakened and top heavy due to mistletoe.  The standing tree with most mistletoe is, I’d suggest, doomed unless urgent remedial action is taken to strip off most of the mistletoe.  A few mistletoe growths are fine – indeed I encourage it in moderation – but this many growths mean the tree’s not able to produce enough leaves to keep itself alive (the mistletoe leaves give nothing to the tree) and it will, sadly, die.

Note, by the way, that the trees in blossom are pears, which bloom earlier than apple.  Mistletoe dislikes pears but loves apples, so there’s a marked contrast at this time of year – pears are all in blossom, apples are not – but, in this orchard at least, they are covered in mistletoe.

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Mistletoe on BBC R4 this morning

Happy Christmas! And if you want a mistletoey start to your day have a listen to today’s Farming Today on BBC R4 – which is all about mistletoe, including an interview with me.

Available (for a while at least) on BBC Sounds App or on the website: https://www.bbc.co.uk/sounds/play/m000cmt2

Mistletoe trade – then and now

1864 Herefordshire Newspaper Cutting

Many of my mistletoe talks this season have had a history theme, looking back at mistletoe in days gone by – both ancient (myth, legend etc) and modern, describing how Christmas demand for mistletoe from the 19th century onwards made it a saleable product, not just a curious tree parasite.

1890 Newspaper Cutting

The trade in mistletoe grew and grew – built on the growing popularity of the kissing custom in the Victorian era and a desire, requirement even, for every home to have some mistletoe at Christmas.  This demand led to a massive trade in harvested mistletoe, mostly from apple orchards where it is easily cut.  And most of those orchards were, some still are, in the south west midlands of England and, of course, abroad in France where mistletoe grows abundantly.

1932 Newspaper Cutting

Stories of this trade are fascinating – it was very significant, with huge amounts being shipped by train around the country, across the channel from France and even, before the advent of air travel, shipped from Britain out to Australia, South Africa etc.  The ‘colonies’ wanted proper mistletoe, even if it was a few weeks in transit and a bit shrivelled on arrival.

These days the quantities traded are much smaller – though most is still cut from apple trees in SW English midland and French orchards.  Firm trade figures are almost impossible to obtain as so much is traded informally now – there are no tonnages for ships or railways, it’s just cut and freighted in lorries, vans and trailers with no documentation required.  The only regular source for trade figures is Tenbury Mistletoe Auction, but even this only give a small snapshot of the overall trade as only a fraction of the trade, and certainly none of the imported mistletoe (which is probably the majority), passes though here.  So, data from here shouldn’t be used in scientific analyses of the trade (you know what I’m talking about Jeff!).

But if there are no overall trade figures how can I say quantities have decreased? Well, technically I can’t, obviously.  But several factors suggest major change – not least the amount available to harvest is much less as there are far fewer suitable orchards here or abroad.  Another major factor is the much more laidback approach to kissing we have these days – mistletoe is no longer needed by many people for a quick smooch with a stranger!  And then there’s plastic mistletoe – a trend that’s grown alongside artificial Christmas Trees – why worry about buying the real thing when you can use the plastic imitation you hang every year, kept in the loft the rest of the time with the lights and baubles?

Stats are very hard to come by though – the National Trust recently announced that in a survey of 240 members, when asked what Christmas traditions they no longer took part in, 31% said they no longer hang mistletoe. A small sample but, if it is reflecting the wider population, that’s a third not using mistletoe at all.  And it’s not clear whether the other two-thirds use the real thing or plastic.

But that’s only a small sample, and just one survey.  I recall a survey back in 2007, apparently of 3000 people, which said 9% actually pick their own mistletoe – which sounds great and suggests a thriving tradition until you realise that most of Britain has hardly any mistletoe to pick, so it’s very unlikely 9% of people even have an opportunity to pick their own. There must have been something wrong with the wording of the question, or the interpreting of the results – unless all the respondents lived in the south-west midlands.  Statistics need to be treated with some caution!

And, talking of stats to be treated with caution here are the stats, so far, for the Tenbury Auctions this year, with corresponding stats from the same week in 2017 and 2018.  The most recent auction was yesterday but stats for that aren’t available yet.

  Mistletoe 1st Quality £/kg Mistletoe 2nd Quality £/kg
Tuesday 26th November 2019 1.50 to average 1.00 0.50 to average 0.30
Tuesday 27th November 2018 3.00 to average 1.75 1.00 to average 0.50
Tuesday 28th November 2017 2.50 to average 1.25 0.75 per kg to average 0.25

Make of that what you will – I would caution against any serious analysis – these are just indicators of prices at one venue.  The good stuff (1st quality) is the material with good ripe white berries and deep green leaves, the other (2nd quality) had, mostly, just as many berries but in that week some were underripe and not fully white and some had the leggy-ness or the yellower leaves that always reduce the value. The only major difference to last year’s mistletoe is, from appearances, slightly smaller berries overall and perhaps slightly later ripening (arguably causing those smaller berries).


The best way to ensure a good supply each year is to grow your own!
For Mistletoe Grow-Kits, Books and Cards
 visit the English Mistletoe Shop website at englishmistletoeshop.co.uk

More general mistletoe Information visit the Mistletoe Pages website.

Mistletoe Auction#1 2019 – lively bidding for lovely mistletoe

A good turn out of both people and mistletoe lots for today’s mistletoe auction at Tenbury Wells.  Mistletoe looking good, lots of berries, again, though perhaps not quite as plump as some previous years. A few lots had yellower leaves – which won’t fetch as good a price as the greener stuff. But there were masses of nice green stuff too.

Some lively bidding for lovely mistletoe!

Only two auctions this year – the second and last is next week, Tuesday 3rd December.

Slide show of scenes before and during below:

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More Mistletoe Information: for general mistletoe info visit the Mistletoe Pages website.

Mistletoe Grow-Kits, Books and Cards:  visit the English Mistletoe Shop website at englishmistletoeshop.co.uk

 

 

Not a mistletoe#1: not even a parasite

Viscum album – mistletoe – showing branching pattern

(the first of some blogs about mistletoe-themed plants)

The distinctive geometric branching of Viscum album, the classic mistletoe of legend, is one of its most distinguishing features. Each branch bifurcates once a year, creating an intricate pattern. Not all mistletoes have this property – for example the Phoradendron species used at Christmas in the US don’t – they look really quite ordinary, not like the European plant at all.

Mistletoe Cactus – Rhipsalis baccifera

But a few other plants do have a similar pattern – though they aren’t mistletoe. The best-known of these are, perhaps, species of Rhipsalis the so-called ‘Mistletoe Cacti’.

Mistletoe Cactus with fruit

There are about 35 species of Rhipsalis, all true cacti with leaves reduced to spines and thick photosynthetic stems, some flattened but some cylindrical.  It is the cylindrical-stemmed ones that are known as the mistletoe cacti as these stems, when they divide, seem to echo the growth of our mistletoe Viscum album.  Their fruits even look vaguely like mistletoe berries.

These are ‘jungle cacti’ from Central and South America, with several forms and species popular as house plants.  Some are quite ‘hairy’ with fine spines coating the stems but others are virtually spine free. One of the commonest seen is Rhipsalis baccifera which, in its baldest, most spine/hairless form, is quite distinctive.

Some other Rhipsalis species

They are easily grown from stem fragments, though these take a while to root.  Here (below) is my current specimen, grown from three tiny stem fragments I picked up from the ground underneath a neglected garden centre specimen some months ago (I know a bargain when I spot one!)

No flowers yet, so no berries, but it’s now growing rapidly so I hope for some soon next year…