A back garden parasitic plant safari

Parasites in lockdown – a round-up of the parasitic plants I’m growing in our garden this year, so far: Not just mistletoe, but also dodders (two species), broomrape (one species – another due soon) and yellow rattle. And an aspiration for Lousewort and a hope of Toothworts (two).

mistletoe shoots 2020Firstly mistletoe, obviously. There’s lots of that (I wonder why?!). These are young growths, about 4 or 5 years old, planted by Blackcaps on an already mistletoe-laden apple. Looking particularly splendid at the moment with the new lighter-green shoots and leaves.

yellow rattle young shoots 2020A bit more down to earth – Yellow Rattle, Rhinanthus minor, a root parasite of grasses, growing here in a pot amongst greater quaking grass Briza maxima. Yellow Rattle is, like mistletoe, only a hemi-parasite as it has its own green leaves, linking, in this case, to the host grasses via the roots. Pretty flowers in summer – these are only young shoots.

orobanche shoots 2020 aVery down to earth – emerging flowering shoots of Ivy Broomrape, Orobanche hederae, at the base of some ivy in a pot. It’s a full parasite – no chlorophyll of its own – that parasitises ivy roots.  Fairly easy to grow – I’ve been growing this for many years now – but unpredictable year to year – these are the only visible shoots from several previous locations (and ivy-filled pots) round the garden that we have this year. They’ll be 6 inches or more high when they’ve fully grown – unless the slugs and snails get to them first. Which they sometimes do.  Will be trying some more Orobanches later this year…

nettle dodder 2020Up in the air again – this is Greater Dodder, Cuscuta europaea, which parasitises nettle stems by winding round them and linking into the host vascular system. No leaves, so these are fully parasitic, unlike mistletoe. They germinate in soil but become detached from the ground once they’ve reached a host stem. Nettle is the nominal host but they will twirl and link to any plant within reach later on – they seem to really like the Epilobium stem nearby (see pic). This is my second season of growing these – including on the stingless form of stinging nettle, Urtica dioica subsp. galeopsifolia, which is slightly more garden- and human- friendly.

gorse dodder shoots 2020And, a first for us for 2020, another dodder – Cuscuta epithymum – smaller than the nettle one but more familiar to most as the pink stems that festoon and parastise gorse on moorland in the summer. These are, so far, tiny seedlings from seed I (and John Hollier) gathered in north Devon last summer. I’m trying them in pots of gorse – only a few have reached the host stems so far, and where they have done this the tiny white and yellow parasite seedlings are just beginning to grasp the hosts. They’re a long way from those massive pink growths of late summer.

lousewort 2020And, to represent some missing parasites, here’s a pic of some rather dry-looking Lousewort, Pedicularis sylvatica, taken this week not in the garden but up a hill in West Devon (despite the lockdown – we are allowed a bit of exercise!). It normally likes it marshier than this so it isn’t very happy! A similar concept to Yellow Rattle, parasiting roots.

Hoping to grow this next year – and also hoping to get some progress with the Toothworts – common and purple – neither of which I’ve had success with, yet…

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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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…

 

Growing your own, for the common good, in NZ

My recent post about the new mistletoe-eating bird in Borneo reminded me of several other exotic (to us in Britain) mistletoe stories. One particular story from last year came to mind – a project in New Zealand where local residents were being given mistletoe seeds in an effort to re-establish local mistletoe species.

NZ mistletoe berries collected for the project

The project, based in Christchurch, involved the collection of seeds by local ecologists and then the doling out of 20 seeds each to local volunteers, for them to plant onto suitable hosts, especially in gardens.  Participants were asked to monitor their seeds to assess success, or failure.

It’s a curious concept – but one which has echoes over here – I’ve worked with many UK conservation groups trying to get mistletoe established in parks and nature reserves –mainly in the east of England, where our mistletoe is relatively uncommon.  Including some very well-known parks and a few palace gardens.  And of course I’ve been involved in garden mistletoe plantings for many years, both directly and through provision of mistletoe grow-kits online.

The trick, which will probably apply to NZ as well as UK mistletoe, is to remember where (i.e which branch, and whereabouts on the branch) you planted the seeds and to be patient, as initial growth is slow and it may be several years before you get a growth of any size.  I’ve lost count of the number of people who write to me, several years after planting, saying they are astonished to find their mistletoe is growing – they assumed, because they had overlooked the initial small growths, that they had failed, but actually they’d been very successful.

I’m not sure how quickly those NZ species grow – but having looked up the story again I see that 2018 was the second year of the project – I wonder whether that’s a reflection of the same problem, establishment after just one year is difficult to assess, so people try again. Although, on reading the news story published in 2018 it seems that 200 new plants had been recorded as established the year before, so perhaps those NZ species establish more quickly than ours!

The Christchurch project was led by Kristina Macdonald, an ecologist with Christchurch City Council – there’s a video with her explaining the project below.  Note that she stresses the value of the plant for gardens – both as an attractive plant and as a resource for other species.  For info on the original 2017 initiative click here and for the 2018 follow-up click here.  As far as I’m aware this project is now completed.


You don’t have to be in NZ to grow mistletoe in your garden.

Try it in the UK with a Mistletoe Grow-Kit from the English Mistletoe Shop

Details at  https://englishmistletoeshop.co.uk

 

The Spectacled Flowerpecker – a new bird that likes mistletoe

Picture by Chien C Lee

Ten years ago, deep in the Borneo rainforest, a new species of bird was spotted feeding on berries from one of the local mistletoe species.  Small, grey but quite pretty it was given the name Spectacled Flowerpecker – but not, at the time, a formal scientific name because that needs formal examination and description – which means a bird in the hand, not in the bush.

The preliminary announcements about it in 2010 hailed it as a perfect example of how little we still know about the species of our planet and how diverse biodiversity actually is.

The canopy level walkways where the bird can be spotted. Picture by Chien C Lee

Fast-forward 10 years, to 2019, and this little bird still hadn’t been formally described and named.  Primarily because it is so small, just 2 inches tall, and is so difficult to spot, let alone capture.  It feeds largely in the forest canopy up amongst the mistletoe plants whose berries it eats – most sightings have been from the forest walkways erected in the canopy as a new form of ecotourism. Click here for a blog describing some 2017 sightings.  An account on the Audubon website describes the difficulties in documenting it.

This year, 2019, a specimen was finally caught, an incidental capture as part of a wider study with mist nets, and so it was properly documented and given a formal scientific name: Dicaeum dayakorum.  The specific part is in honour of the indigenous Dayak people who live in and help protect the bird’s native forests.  Here’s the header (click to enlarge) from the formal paper describing it – the full paper is at https://www.biotaxa.org/Zootaxa/article/view/zootaxa.4686.4.1

The genus Dicaeum, by the way, includes numerous other species of flowerpecker, many also feeding on mistletoes.  These include the Australian Mistletoe Bird Dicaeum hirundinaceum which featured in one of the BBC TV’s  David Attenborough natural history series some years ago. The broadcast clip showed a bird wiping the semi-digested mistletoe berry, complete with seed, from it’s er, bum, onto a branch. Very efficient. It’s not clear whether the Spectacled Flowerpecker is that good or whether it’s more of a hit’n’miss mistletoe spreader like our own Mistle Thrush (see various older mistletoe diary entries …).

 

Mistletoe seed distribution – why leave it all to the birds?

Try doing it yourself with a Mistletoe Grow-Kit from the English Mistletoe Shop

Details at  https://englishmistletoeshop.co.uk