First records of Mentha cervina (hart’s pennyroyal) in Britain

Baker, A. 2020. Mentha cervina (Lamiaceae), an emergent aquatic alien species naturalising at South Gare, North-East Yorkshire. Vol. 2 No. 1 British & Irish Botany.

https://britishandirishbotany.org/index.php/bib/article/view/37

Mentha cervina in bloom

Published and open access since Feb 26 2020.

Abstract: There is an increasing interest in recording early colonisation of organisms when studying changes in distribution ranges induced by climate change. Here, I describe one population of Mentha cervina L. (Hart’s pennyroyal), naturalising in the wild at South Gare, v.c.62 North-east Yorkshire. Two other populations have been reported in Britain and none are known from Ireland. Of the three populations ever reported from the wild in Britain, two are still extant. It is unclear what vectors disperse M. cervina in Britain and whether the species is becoming increasingly naturalised or not. Diagnostic characters: digitate bracteoles and four calyx teeth, are provided to facilitate the recording of this mint species by field botanists.

Mentha cervina, growing in situ in South Gare

A suspected introgressive population of Ilex aquifolium and Ilex x altaclerensis, Satlburn, Yorkshire, UK

This post introduces to a well-established and complex population of self-sown Ilex x altaclerensis (Highclere holly) observed in Saltburn Valley Gardens, Yorkshire, UK.

Ilex aquifolium - Ilex x altaclerensis hybrid in Saltburn Valley Gardens
Self-sown holly with definite hybrid characteristics, including large, broad and flat leaves.

We are familiar with the native holly, Ilex aquifolium, a small tree whose spiny evergreen foliage and small red berries are associated with end-of-year festivities. Ilex x altaclerensis was born of a marriage only possible as a result of human agency, when it took the Vicorian’s fancy to grow Ilex perado (Madeira holly) in glasshouses and allowed it to cross pollinate with the native I. aquifolium accidentally growing in the wild near that greenhouse. The resulting hybrids remain popular planted hollies.

In most cases, when growing side by side Ilex aquifolium and Ilex x altaclerensis are relatively distinct (See picture above). However, looking more closely among trees in Saltburn Valley Garden, I also found a series of specimens with confusing morphologies suggesting a full-range gradient of intermediates between typical Ilex x altaclerensis and typical Ilex aquifolium (See picture below). This variety in the hybrid population may be a sign of introgressive hybridisation, or introgression.

A range of leaf size and shapes found in Saltburn Valley Gardens: each leaf is a representative leaf for an individual tree.

Introgression is a biological term used to describe the process leading hybrids to re-hybridise with one of their parents. Such population tend to include specimens with the full spectrum of intermediates between typical hybrid and ‘pure’ species, blurring boundary between species.

Introgression is under the spotlight in nature conservation because it is claimed to threaten the integrity of some native species. For example, there are concerns that the charismatic English bluebell could be outcompeted by garden escape hybrids (between the English bluebell and Spanish bluebell). However this threat proved unfounded in recent research, despite the potential for introgresssion.

More controversially, I would happily argue that introgression is an opportunity for native species to become more global-change adapted. For example, a species may assimilate additional genes and characteristics that will render it more resilient to environmental change over time. Following this logic, the introgressive population of hollies from Saltburn Valley Garden may represent novel biodiversity fit to face human-induced environmental change.

Such cases of introgression are interesting case studies to better understand what lays ahead. In fact introgression between natives and non-native is expected to become increasingly common with the rise of the Anthorpocene, a period of earth history where we are seeing a big reshuffle in species distribution as well as changing environmental conditions such as increase CO2 in the atmosphere, climate warming and disruption of nutrient cycles.

In the absence of DNA studies, my suspicion of intr0gression between Ilex aquifolium and Ilex x altaclerensis are based on morphological observation. In Saltburn Valley Gardens, the best vegetative identification criteria between the two taxa can be summarised with the three-choices key as follows:

  • Leaves flat/plane, with small forward-pointing spines more or less adpressed to leaf margin, length:width ratio <2, often dull. __________________________Ilex x altaclerensis
  • Leaves showing one or more of the following characteristics: irregularly undulated, mixture of forward-adpressed and other types of spines, leaves unusually large (>12cm in length), hybrid vigor in terms of yearly growth _________________________ Ilex x altaclerensis
  • Leaves strongly undulated (wavy margin, significantly more three-dimensional than blade thickness, forming folds when pressed flat), spines patent or backward pointing (spines nearer apex often forward pointing but not small and adpressed to leaf margin), length:width ratio >2. ____________________________Ilex aquifolium

Natives and hybrids can often show individual leaves, branches or whole trees with spineless leaves.

Galium cf. murale, Cardiff, UK – new to Glamorgan

I collected this intriguing little Galium with minute cream / pale yellow petals while in Cardiff, Wales, UK, last week for a conference not directly related to field botany. As it was my first time in Cardiff I got up early and went for a random walk – and my eyes got stuck with this miniature plant, growing between pavement cracks at ST19167594.

I first thought of Herniaria glabra – which would have been a surprise –, then looking closer, it reminded me of Sherardia arvensis, yet the pale yellow creamy tiny petals ruled this taxon out. When I got round to look at the specimen collected more closely from the comfort of my home lab, it did not key out well in Stace (2010). Could it be something ‘new’?

As I knew I would not have an opportunity to visit an herbarium in the coming weeks or months, I resorted to the online account of Galium from Flora Iberica (Ortega Olivencia and Devesa, 2007). The specimen collected matches very well Galium murale and no other taxa from Spain or Portugal. However, the plant found in Cardiff could be originally from another region of the World than the Iberic peninsula, so there remains a certain degree of uncertainty regarding its taxonomic identity.

Going back to Stace (2010), a more thorough inspection revealed that G. murale is mentioned as an additional species, but not included in the key. He also suggests that it may be spreading – which might prove right.

A quick internet search suggests that Galium murale has been reported to Britain and Ireland several times in the past, including as a wool alien in the past. In recent years, there is a 2008 report of a large colony in Sussex in a previous issue of the BSBI News (Nicolle, 2008). Looking at the BSBI distributions maps online there are six recent sightings in southern England, Wales and Ireland (excluding the Sussex population – 02/05/2017).

How to spot Galium murale? It looks a bit like Sherardia arvensis, or Galium aparine that shrank dramatically. However, there are important general differences: the general size (G. murale is a very small plant – see picture with one penny coin for scales), the size and colour of the four petals (cream) and the number of leaves by whorl (four). Important finer characters for identification against other Galium: forward pointing bristles on the leaves, cylindric ovary/fruit only partly covered by bristles.

In Stace (2010), it keys out as Galium boreale because of the whorls of 4 leaves but this was obviously not a good match with my plant. Ignoring this and going onto couplet 3, leads one to Galium spurium, however, the description and the size of this species did not match my plant.

Happy hunting everyone – a lovely little plant to be looking for in warmer part of Britain and Ireland. Best time of the year would be early spring/spring time.

References

Ortega Olivencia, A, and Devesa Alvaraz, JA, 2007, Galium, In Devesa Alvaraz, JA (Eds), Flora Iberica, Volumen XV, Rubiaceae-Dipsacaceae, Real Jardín Botánico: Madrid.

Stace, C 2010, New Flora of the British Isles, Cambridge University Press: Cambridge. Third Edition

Nicolle, D 2008. Galium murale – a foothold in Eastbourne? issue 109. BSBI News.

Update on Festuca altissima in the Porter Valley, Sheffield, UK

Very good news from the population of Festuca altissima, Wood Fescue, at Forge Dam, Porter Valley, Sheffield, UK. For the first time since 2013, many plants flowered this year.

The flowering status of this population prior to 2013 is not known, in 2013 and 2014 the counts were zero and in October 2015, I observed 13 flower heads. This is a fantastic increase, likely to be the result of the conservation efforts by the Friends of Porter Valley.

I reported in a previous post how they cleared the site from encroachment by cherry laurel, Prunus laurocerasus. I really hope to report similarly good news in a years time and if the management proves successful in the longer term, we ought to consider reducing even further the cherry laurel on this bank in order to promote Wood Fescue at the site.

Pictures of Festuca altissima, wood fescue, at Forge Dam, Porter Valley, Sheffield, UK, with inflorescences – apologise for the quality of the photography, it was a windy day! Wood fescue is the scruffy looking grass, by the way.

Senecio inaequidens, narrow-leaved ragwort, Kirk Sandall station, South Yorkshire, UK

I got familiar with this species on the continent, where it became very abundant but seeing it near Doncaster on the railway tracks was quite a surprise – I had not seen it for quite a while now: it was like meeting up with a good old friend!

Senecio inaequidens, the narrow-leaved ragwort, is a fast spreading plants from Southern Africa that is mainly found in ruderal situations in Europe. It’s identification is relatively easy because it has the flowers of a ragwort (well, no wonder as it is one…) but it has very narrow leaves without any lobes (see picture below) and it has a stout fibrous stem. Keep your eyes peeled as it is likely to be under-recorded!

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Transport networks such as railway and roads are often vector of spreading species. This is not only because vehicles transport seeds and other propagules but also because the transport network creates continuous novel habitats whose physical and disturbance conditions can strongly differ from the surrounding landscape.

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Cystopteris fragilis, brittle bladder-fern, Multangular Tower, York city centre, UK

Yet another interesting urban plant. I found Cystopteris fragilis, the brittle bladder-fern, thriving on old walls in the city of York. It is possible that improved air quality in the recent decades and a drastic reduction of acid rains is increasingly favouring this species and other alkaline-substrate-loving ferns such as Ceterach officinarum – rustyback – to grow on mortar in urban situations.

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Filago vulgaris in the city of Sheffield

Urban flora fascinates me. One of the reasons is that the exceptional floristic diversity found in urban habitats challenges my ideas about wildlife and conservation. We tend to think of urban development as intrinsically adverse to biodiversity, however, re-colonisation can be quick and novel habitats can be rich and surprisingly biodiverse.

This positive aspect needs of course to be weighed up against irreversible loss of some habitats such as nutrient poor mires that can disappear and are never replaced.

P1030273But I also enjoy urban floras because so many unexpected plants turn up! and because it transforms my weekly trips to the supermarket into genuine botanical expeditions. Above and below are pictures of Filago vulgaris, the common cudweed, in a waste ground in (SK337889) Walkley, Sheffield, UK, near home. I had spotted the hundreds of tiny rosettes early this spring but couldn’t work out whether it would turn out to be a Filago or Gnaphalium uliginosum, the marsh cudweed.

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Malling Toadflax

When I saw the roads signs for West Malling last week I knew I had to make a small detour and look for the largest known population of Chaenorhinum origanifolium in Britain and Ireland. I first encountered this rare alien plant on a wall in Oxford – see my new-to records page. It did not take long to locate the West Malling population on Swan Street (TQ682577). There is a very large number of mature plants, that were in full bloom, and a large number of seedlings:

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Polypogon monspeliensis

There is much written about the spread of Polypodon viridis in Britain and Ireland but its close relative Polypogon monspeliensis may also be spreading. I am posting here a few pictures of P. monspeliensis, observed as a casual in a car park during my recent family holiday in Sussex (plants located at TQ452172). Note the resemblance with P. viridis inflorescences – apart from the long awns!

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Ponds and aquatic plants near Catcliffe and Treeton, East of Sheffield, UK

Last August, I was fortunate to be shown several ponds near Catcliffe and Treeton, East of Sheffield, UK, by Bob Croxton, form Sorby Natural History Society. The area is full of history and is a fascinating example of nature recovering following industrialisation.OLYMPUS DIGITAL CAMERAFor instance, one of the ponds visited was formed in a lost piece of land amongst spoil heaps and three railway line! At this site Persicaria maculata and Nymphoides peltata were in full bloom (see picture) and two Potamogeton species were observed, now confirmed as P. pectinatus and P. pusillus. However only one shore was prospected and other nice aquatic plants are likely to be present in the rest of the pond.

On the whole, too many of the water bodies in the area were absolutely dominated by Elodea nuttallii, an invasive species that colonised the UK from the 1970s. It’s phenomenal spread in Britain and Ireland is summarised in a paper by Simpson that can be read from the BSBI archives. It remains unclear whether this spread has caused arm to aquatic biodiversity or whether it was simply facilitated by degraded habitats.

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Persicaria amphibia (amphibious bistort) in full bloom! East Sheffield, UK