Plant of the week – March 1st 2021 – Butterbur (four species of Petasites)

The butterbur was one of the first flowers I ever became excited about seeing, back when I was about 7 and hadn’t seen a poppy yet, let alone an orchid.  The butterbur captivated my imagination, as I knew it through of one of those old flower books where a plant gets a whole page to itself.  Such books are never that good for identifying things, because too few species are featured, but it did a great job of inspiring me to want to see species depicted, and none more so than the butterbur.  I still recall the painting there, of that broad sturdy candle-stick of pink flowers erupting from the ground.  Because the leaves arise after the flowers, the flower spike is basically covered in flowers all over.  As so often with plants I’d never seen, I imagined it far larger than it actually was!

When I first saw the real plant it didn’t disappoint.  It was a roadside stop halfway between London and Devon, and there they were, hundreds of them carpeting the ground under trees, accompanied by Star-of-Bethlehems (Ornithogalum umbellatum).  Over 40 years later, I still recall the joy of that double find!  Plus one thing that the book couldn’t convey, the subtly sweet scent of the flowers, which I have loved ever since.  Whenever you see the plant in flower, it is well worth bending down to have a sniff.

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The butterbur (Petasites hybridus) is one of four Petasites species regularly found in Scotland, and I’ll introduce the other three below; however it is the only one that is native to Britain.  It’s pretty common in the lowlands, usually but not always in wet places by streams.  In Edinburgh, there’s lots of it in Blackford Glen, and especially along the Innocent Railway cycle track between Peffermill and where it reaches the coast at Newhailes.  Those lovely pink candle-sticks arise around early April, always en masse because the plant spreads clonally.  Within a month or so these have vanished without trace (at least in Scotland), and been replaced by massive leaves nearly a metre across, some of the biggest on any native plant.

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The rapid vanishing of the flower spikes occurs because the flowers are all male, and with no seed set, spread in Scotland therefore seems to be entirely clonal.  This in turn explains why it tends to form huge patches where present, and it probably gets around through pieces being washed along watercourses.  But where are all the girls?  Well, they’re in northern England.  What good are they there, to all the sex-starved male butterburs in Scotland?  None at all.  This is the extraordinary mystery that underlies this lovely flower: the males occur over most of mainland Britain, yet the females are only in the middle; hence males in the south are equally deprived of romance.  I know of no other British plant where the two sexes are so dramatically different in distribution.  It begs the obvious question: why?

Perhaps the most likely explanation is that it is only native where female plants occur, making it an alien in Scotland and southern England.  The males and females look completely different, so much so that unless you know otherwise, they appear to be different species.  There are quite a few dioecious (separate male and female plants) species in Britain, including the stinging nettle, holly, white bryony, and quite a few water plants.  Yet the butterbur is exceptional in just how different the males and females look.  The males form short compact pink candlesticks, but the females and much taller and lankier, with narrower capitula branching from the main stalk on narrow stems.  This, presumably, has evolved to give the seeds a better chance of being caught by the wind when the time comes, which is not of course a concern for the males.  So if one was to cultivate the species, only would almost certainly choose the neater male, and as we’ll see, that’s what normally happens for its relatives. 

Therefore it is plausible that these male-only populations originated as escapes from cultivation, but if that is true it would still leave one big question: why does it not occur naturally in other parts of Britain?  Surely it would have had to pass through southern Britain to reach the north, as the islands were recolonised after the most recent glacial maximum, so why did it not hold on there?  For other species currently confined to the north, like Primula farinosa or various boreal woodland species, we can assume it was because conditions in the south don’t suit them, but we know that the butterbur now grows very happily in the south, albeit not as commonly as in lowland Scotland.  Perhaps it is like the Welsh poppy (Papaver cambricum), whose native populations remain shy and delicate, while more robust introduced genotypes have swept the country.  It’s even possible that our male-only populations might all be one extremely vigorous genotype, as is the case with Japanese knotweed.

There remains the remarkable alternative theory, i.e. that this is a natural situation.  If so, then there would appear to be sexual dimorphism not only in form, but also in fitness and ecological requirements, with fussy and delicate females but aggressive and undiscerning males, a strange form of botanical sexism.  Might some pathogen have swept the country that decimated only the female plants?  Whatever happened, it seems extraordinary that selection would not have fixed this issue, because any female that set root outside northern England would have had a massive Darwinian advantage, potentially becoming Eve for all Scottish butterburs.  Overall, the alien-males hypothesis appears the more likely.

Now let us turn to its relatives, which are fully alien.  First, there is P. pyrenaicus (formerly P. fragrans) the winter heliotrope.  It’s not as common in Scotland as it is down south, but it grows along Lade Braes in St Andrews, on a roadside by Mavisbank south of Edinburgh, at North Berwick, and if you walk the Fife coastal path between Kirkcaldy and Kinghorn, you’ll come upon a very fine patch of it.  The name is well deserved because it is truly a winter flowerer, generally beating snowdrops to the punch.  Sometimes you can see it in flower even before the old year is over, providing a very early herald of spring. 

The heliotrope differs from the butterbur in that there are fewer capitula generally towards the top of a slightly longer stalk, and with their own sometimes branched stalks of varying lengths.  The flowers are pink, and sometimes the colour is a little deeper than in the butterbur.  The leaves are smaller but carpet the ground, and are present at the same time as the flowers, giving the plant a quite different appearance overall, though you can tell they are related.  In this species, wild plants are always male, at least in Scotland.  In England a few years back, a botanist noted an unfamiliar Petasites growing wild, and had to get expert help before determining it as female P. pyrenaicus.  It remains the only record of the female of this species in the wild in Britain, and so of course this generally spreads clonally as well.

Our third species is the Japanese butterbur, P. japonicus.  The plants are probably always male here, and the flowers emerge in March, a month ahead of P. hybridus.  It is easily distinguished because the flowers are creamy white and it has large numbers of distinctive wide bracts below the inflorescence.  Also, the flowerheads tend to be as wide as they are tall, taking on a sort of ball shape; indeed, when they first emerge, they look like cauliflowers erupting from the bare soil.  As with P. hybridus, the leaves come only after the flowers, and here they are even larger – you could use one as an umbrella if caught in a sharp shower!  The effect during summer is like a small population of Gunnera plants by the streamside.  This species is much more faithful to the sides of rivers and streams than is P. hybridus, and probably it gets around by parts of rhizome floating downstream.  A good place to see it in Edinburgh is Blackford Glen, both across the stream from the road on the east side, and right in the middle or the reserve itself; it also occurs sporadically further upstream along the Braid Burn, and at Fairmilehead. 

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Finally we have the white butterbur, P. albus, which resembles a pure white P. hybridus, though it generally has fewer capitula.  It flowers in late February, meaning each Petasites almost has a month to itself, and between January and April there’ll always be one of them in flower, if you can find it!  The white butterbur is perhaps the least common of the four, and I’ve seen it wild only in three or four places, all of them Scottish: Dura Den near Cupar, Cambo (only semi-wild, in an estate), and by the Bilston Burn near Loanhead.  If you’re taking the train between Edinburgh and Glasgow, there’s some right by the railway just west of Falkirk High.      

This is probably the only butterbur that has female plants in Scotland, although in this case they are really, really hard to tell apart from the males.  As far as I can tell, the two sexes in this case are fairly similar in stature.  I know that the Bilston plants are female, largely because I saw them in seed last year.  They grow by a bend in the Bilston burn, with one small patch on each side.  To sex them when in flower is devilishly difficult – even knowing that they had to be female it remained quite a challenge.  That is because, very unhelpfully, female capitula contain sterile male flowers, and vice versa, so they look superficially very alike.  The presence of non-fertile anthers makes evolutionary sense, because pretend pollen will attract pollinators that want to eat it.  However there is no obvious function to sterile female flowers on a male plant, so perhaps there are merely vestigial, like the appendix in human beings.  At any rate, I had to dissect the capitula and use a hand lens to identify ovaries at the bottom of the individual flowers, with wispy pappus threads ready to disperse the seeds later on.  There must be male plants somewhere nearby, because as I say, seeds were set.

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An account of this lovely genus would feel incomplete without an honorary mention for their closest British relative, the coltsfoot (Tussilago farfara).  This is one of the most common native spring flowers in Britain and is always a joy to see as spring struggles to arrive.  Superficially, it looks more like a dandelion than a butterbur, but its yellow capitula in fact contain mainly disk florets, whereas the dandelion has only rays.  Also, the coltsfoot capitulum is on a long stem with bracts on it, whereas the dandelion has none.  But how are these solitary yellow flower-heads in any way like a butterbur? Well, they do often emerge from the ground in groups, so you can imagine something like the butterbur stem is lurking down there at ground level, and its capitula instead of being sessile are borne upwards on individual stalks.  The more obvious similarity is in the leaves, once again borne after the flowers and of a similar shape.

So, if you’re a bored botanist waiting for the summer to start, you can do a lot worse than taking on a quest to bag all four of our butterbur species and the “bonus ball” of the coltsfoot.

Richard Milne