We’ve had Petasites on this site before, in the Butterbur article of early March 2021. Then, Richard Milne gave us an excellent introduction to the four species found in our British Islands (see the Index and you can read what Richard said). They are all somewhat unusual members of the Daisy family Asteraceae, and I want to focus on one of them. I still give it the old name Petasites fragrans because it is indeed fragrant, and I don’t much fancy its new name Petasites pyrenaicus. That name doesn’t seem apt for a species that occurs through Europe and some of North Africa. Its English name Winter Heliotrope also confuses us, as the true Heliotropes are in a different family altogether. It would be better to have it called Winter Butterbur. The word Heliotrope is also the official name of a precise colour – a rather special pinkish-purple, known to fashion designers and interior decorators.
Like the other four Butterburs, this one is a herbaceous perennial with an extensive system of rhizomes whereby the plant forms an ever-increasing patch, often by roads or streams. The basal leaves are rather thick and have a characteristic shape (see below). Leaves of P. pyrenaicus are smaller than the other members of the genus – about 10-20 cm across during flowering but expanding somewhat when flowering finishes. The dense colonies formed of rhizomes and foliage choke out other plants. In Ireland it has been called ‘a veritable ecological cancer’ . It isn’t common in Scotland but I have visited four of its sites in the last few days (early January 2022). At three of them it is flowering profusely, the fourth is a woodland site where it isn’t flowering at all.
There are quite many herbaceous species that manage to flower in winter but very few that flower only in winter (snowdrop springs to mind).
I’m going to tell you about: (i) the fragrance; (ii) all the plants in Britain and Europe are male.
The flowers have an unusual perfume. Someone described it as ‘cherry-pie’ and I do agree (not the cherry pie that mummy makes but the ones you buy in posh cafés). I asked two grand-children what they thought the smell was, as both have extremely acute olfactory senses. Marzipan was the reply. Yes; I wonder, do they put marzipan or almonds into cherry pie? The smell is a bit like vanilla. The same day I was working in the kitchen and I reached for Star Anise as I thought my dish needed spicing up. YES, Star Anise smells of P. pyrenaicus. That’s probably what chefs put into their cherry pies, as cherries can be rather tasteless at times.
I looked into the chemistry of this smell. It turns out the be 4-Methoxybenzaldehyde also known as 4-Anisaldehyde. If you have even a slight knowledge of organic chemistry you will recognise the benzene ring made of 6 carbon atoms, and the aldehyde group where carbon forms double-bonds with oxygen. And, surprise, it’s nearly the same as vanillin, used in the food industry and found in vanilla pods. It’s a colourless liquid. Burdock’s Handbook of Flavour Ingredients says: Sweet powdery, spicy creamy, fruity, vanilla and hay-like. `coumarin almond, anisic with berry nuances.
Why would the flower make such a compound? Well, the answer is provided by Pat Willmer and colleagues at St Andrews University. The research group found that the compound repels ants yet attracts pollinators (Pattrick et al 2017). I suspect that this is the same as the stuff they buy in bottles and put into factory-made cherry pies.
The species is alien, introduced as a garden ornamental in 1806 and recorded in the wild by 1835 in Middlesex. It wouldn’t do in a suburban garden but at a grand house it would provide a talking point in winter and good ground cover. The site at Cairnryan is just outside a big house and it looks as if it could have ‘escaped’ under the wall. How did it get to its Tantallon site? Perhaps on the boots of smugglers. Birds’ feet?
Turning now to its sexuality. Stace’s New Flora of the British Isles states that ‘Female plants are unknown in the British Isles’ and other authors tell us we must go to Algeria to see females (Stace 2019). But such statements can be challenged. The much older flora of Clapham et al. (1959) says of the genus Petasites that the plants are ‘more or less dioecious’ (dioecious means male and female flowers are borne on different plants, literally ‘in two houses’). However, in their quite detailed entry for P. pyrenaicus they say ‘Male heads chiefly of tubular florets, female of slender florets with a short broad ligule’. Moreover Blamey et al. (2003) in their illustrated flora say of P. pyrenaicus ‘flowers housing both sexes’.
So, is it really true that we have only male plants? If so, why are they so fragrant – isn’t the production of all that scent wasteful unless the flower needs pollinators? Keen to resolve this uncertainty, I plucked some florets from freshly-gathered flowers, took photographs and discussed with a colleague. He had some close-up photographs of flowers that help to interpret my florets, co-incidentally taken from the same population as mine (Tantallon). These images confirm Clapham’s view. There are two sorts of floret. More investigation is needed – are the flowers fertile, are plants from different populations different in their sexuality, do seeds sometimes form?
Please note that my florets have a ‘pappus of hairs’ the little feathery structures that many members of the Asteraceae possess to aid dispersal of their seeds. Like fragrance, this is surprising if the flowers are entirely male.
As I was idly googling around the topic, I turned up a paper which I should have found earlier. Desjardins et al. (2016) describe a hybrid between P. pyrenaicus and the giant relative Petasites japonicus. One of the authors (Clive Stace, of the New Flora of the British Isles) has been making artificial crosses between Butterburs for some years. The paper refers to a population of tall Petasites growing in Sussex, previously considered as likely females of P. pyrenaicus. The authors provide strong contrary evidence: the tall plants are certainly not P. pyrenaicus but the hybrid between P. pyrenaicus and the giant relative Petasites japonicus.
Is the case closed? I suspect the female parts are non-functional (vestigial) and the male parts produce pollen in vain. It’s something we may return to when further evidence is gathered. I’m just a beginner; I think I may have to contact Stace for help.
References
Blamey M, Fitter RSR, & Fitter A (2003) Wild Flowers of Britain and Ireland. Harper Collins.
Clapham AR, Tutin TG and Warburg EF (1959) Excursion Flora of the British Isles. Cambridge.
Desjardins SD, Hoare AG & Stace CA (2016) A new natural hybrid in the genus Petasites: P. japonicus x P. pyrenaicus (Asteraceae), New Journal of Botany, 6:2-3, 64-70, DOI: 10.1080/20423489.2016.1271383
Stace CA (2019) New Flora of the British Isles. 4th Edition. C&M Floristics.
John Grace
Botany in Scotland 