Plant of the Week – 10th May 2021 -Bluebell (Hyacinthoides non-scripta)

By Richard Milne

Fig 1. Bluebell woods in full bloom, with wood spurge Euphorbia amygdaloides in foreground.  Photo A.P. Milne

The bluebell (Hyacinthoides non-scripta) is the unofficial national flower of the UK, and in 2002 topped a Plantlife poll to choose the favourite flower of Britain, with the primrose (Primula vulgaris) coming second.   Is not quite as popular in Scotland, perhaps because it is less abundant here, and the poll was topped by the primrose in each of Scotland, Wales and Northern Ireland.  Moreover, H. non-scripta is not even called “Bluebell” in Scotland, because the Scots, never ones to miss any opportunity to do anything differently from the English, call it “wild hyacinth” while giving the name “Bluebell” to the harebell, Campanula rotundifolia instead.  Even more confusingly, the name harebell used to be applied to H. non-scripta, back in the days of Gerarde’s (1597) Herbal.  This sort of confusion around common names is precisely why scientific names were invented!  However, this blog is all about H. non-scripta and we will call it “bluebell” for simplicity.  Like all of the most familiar British flowers, it has amassed quite a few delightful alternative names, including Cra’tae (crow’s toes), Witches Thimbles, Dog Leek (n.b. ‘leek’ not ‘leak’!) and the delightfully strange “Granfer Griggles”.  “Granfer is old dialect for “Grandfather”, and “griggles” might either refer to unwanted apples withering on a tree, or more likely a “grig”, which is a lively lighthearted usually small or young person.  So the name conjures an image of lots of energetic children running around a grandfather’s woods.  Another local name is ‘giggle-sticks’, suggested to refer to the squeaky sound the stems make when they rub together in the breeze.

Fig 2. Detail of flowers, showing the pendulous character and the tubular flower structure (see Table 1). Photo R. Milne

Of course, English is not the only native language in the British Isles, and other languages give us even more names for the plant.  The Irish call it coinnle corra (tapered candles) or méaracán gorm (“Blue Thimble”), whereas in Welsh it is most commonly known as clychau’r gog (“cuckoo bells”), but also clychau glas (“blue bells”) and many other names.  The most common name in Gaelic seems to be brogan na cuthaig or bròg na cuthaige – “the cuckoo’s shoe(s)” The cuckoo theme also turns up in another English name (cuckoo’s boots) and presumably refers to the flowers appearing at the same time as the first cuckoos are heard, as with the cuckoo flower Cardamine pratensis.

Fig 3. A mass of bluebells with wood anemones (Anemone nemorosa) Photo R. Milne.

For all the delightful oddity of these colloquial names, the scientific name Hyacinthoides non-scripta has perhaps the most convoluted origin of the lot.  It refers to the legend of Hyacinthus, the Greek youth whose exceptional beauty attracted the love of the god Apollo.  This ended up costing him his life when he was struck and killed by a discus thrown by Apollo.  In some versions this is an accident, in others the discus was deliberately deflected by a jealous rival.  Either way, a flower is said to have grown from his blood, its petals inscribed with the words “Ai Ai” (alas), written on it by the grieving Apollo.  Apparently this flower was not a hyacinth but possibly an iris, larkspur or fritillary, all considered possible candidates because patterning on their petals could be seen (supposedly) to show these words.  So the flower was almost certainly not the one that now bears his name, Hyacinthus orientalis.  Amidst all this confusion Linnaeus decided, for reasons known only to himself, that the most noteworthy thing about our beloved British bluebell was that it certainly wasn’t the one Apollo had written on, hence ‘non-scripta’.  A sort of botanical version of “These aren’t the droids you’re looking for”.  

Meanwhile the name “Bluebell” has been exported to America and applied to a different species that is native there (Mertensia virginica, Boraginaceae), in much the same was as has happened for familiar bird names like “robin”. There is also the bugloss alpine bluebell Decalepidanthus echioides (again Boraginaceae) in the Indian Himalaya.  There is perhaps something about the name ‘bluebell’ that resonates in the English language.

Fig 4. The rare pure white form occasionally turns up among natural populations of H. non-scripta. Photo R. Milne.

Despite the common name, bluebells are not always blue, and very occasionally one can find white or (even more rarely) pink forms.  If you are reading this and thinking “But I see those all the time”, that is because these colour variants are far more common in the hybrid bluebells commonly grown in gardens, which often escape (discussed in detail below).  In cultivation, rare colour variants are commonly selected for, as happens with numerous other garden plants like crocuses.  However, it is a mistake to assume that non-blue bluebells are always hybrids, because white and pink forms do occur naturally.  To prove it, the white bluebell gets its own separate pictures in Gerarde’s Herbal, indicating it was present in Britain long before the Spanish Bluebell and its hybrids were introduced.  If one looks closely at a native bluebell population, one will also see that the shade of blue also varies.  Some are a rich deep blue-purple, while others are a paler blue.  My guess is that there are two pigments, a blue and a pink; deep blue individuals have both pigments, pale blue or pink individuals have one or the other, while white plants lack both.  I have also seen a pink variant of the native Scilla verna, and pink variants of Scilla forbesii are popular in gardens, suggesting that this double-pigment system applies across many or all of its blue-flowered relatives (see fig 5).

Fig.5. Pink forms of bluebell (left, with blue form), Scilla forbesii (centre) and S. verna (top right, with normal form bottom right).  S forbesii is from cultivar “Pink Giant” but the others were natural forms seen in the wild.  Photos R. Milne. 
Fig 6. A page of Gerarde’s Herbal, showing the autumn squill Scilla autumnalis, the bluebell and its white form.  This Herbal of course predated any concept of “species” and so colour and even size forms are given equal treatment.

Bluebell Culture and Folklore

Bluebells are lovely and abundant, and hence have woven their way into British culture from the distant past right up to the present. They are also very poisonous, explaining why the leaves are able to carpet the floors of forests without being gobbled up by passing deer. This perhaps helps explain the darker side to many of the folklore myths surrounding them, many of which involve fairies. As ever, the fairies in old tales are not the sanitised, friendly beings of recent stories, but dangerous, powerful and mysterious creatures to be avoided or treated with caution.  The flowers were said to be bells rung by fairies to communicate with one another, calling their fellows to balls and important gatherings, but disaster will befall any human unlucky enough to hear them.  Even worse will happen to anyone who picks a flower, for this will anger the fairies and they will exact a terrible revenge.  Should a child do this, he or she will simply be taken by the fairies and never be seen again.  Even the innocent wanderer in a bluebell wood who avoids these hazards may nonetheless become entranced by the magic of the flowers and their fairy associates, becoming “pixie-led”, which means they will wander in circles indefinitely.  There are more real hazards of course: anyone collecting wild garlic must be careful to avoid accidentally picking poisonous bluebell (or snowdrop) leaves as well (this could be the origin of the name dog-leek).  Also, in Scotland it tends to inhabit steep valley sides, and the shiny leaves are rather slippery, because of which I myself rode down a slope on my backside trying to get a photo for this article!

Folklore also tells us that a person wearing a garland of bluebell flowers around their neck is compelled to tell the truth, and looking at Westminster right now I find myself really wishing that this actually worked.  Also, if you can successfully turn a bluebell flower inside out without tearing it (or attracting the wrath of fairies), you are guaranteed to win the heart of your true love.  Bluebell flowers are variously said to symbolise constancy, humility, gratitude, everlasting love, and kindness.  The passion the plants inspire has not diminished today, and some people have asked for their ashes to be scattered in a bluebell wood when their time is over (Goudy, 2018).

Fig 7. The full text of “Bluebell” by Emily Brontë.

Unsurprisingly for a plant so loved and ubiquitous, it turns up in culture and writing all over the place.  Emily Brontë wrote a poem about it (Fig. 7), Keats called them the “Sapphire Queen of the mid-May” and the great nature writer Gerard Manley Hopkins wrote of them many times, for example in 1870 “I do not think that I have ever seen anything more beautiful than the bluebell I have been looking at. I know the beauty of the Lord by it. Its inscape is mixed of strength and grace, like an ash tree.” ‘Inscape’ was his own term for the nature of a thing.  Maverick judge Lord Denning once wrote “It was bluebell time in Kent …” as part of the opening to a judgement about a horrific traffic accident.

There is the famous Bluebell Railway in Kent, and the plant was even co-opted as the name of a Scottish band, The Bluebells, whose song “Young at Heart” made it to Number One in 1993 on the back of its use in an ad campaign, nine years after its original release.  The band showed their sense of humour on Top of The Pops by mocking some of the less lyrical music that was around at the time, inserting phrases like “Techno-techno-techno-techno” and “Shabba!” into their live performances (https://www.youtube.com/watch?v=wjCx6ax8x14). However, the bluebell matches that I am reliably informed were a favourite of young pyromaniacs in days gone by are very much a Scottish thing, and drawings on the boxes (when recognisable, which they are not always) appear to show Campanula rotundifolia.

As perhaps the most popular and familiar flower in Britain, it comes as quite a surprise to learn that the bluebell has been put to practical use, and may yet be again. Remarkably, it has been used as a weapon of war, because a simple glue can be made from the sticky sap of its roots, and this was used to glue feathers onto the shafts of arrows in medieval times. It also played a role in aggressive fashion statements at the time of Elizabeth I, with the same glue being used to stiffen the ruffs of posh people. However, this glue had a more scholarly side, having also been used in book bindings, with the happy side-effect that the toxicity of the sap discouraged certain insects that might have otherwise nibbled the books.

As with many other poisonous plants (e.g. henbane, rhododendron, deadly nightshade, yew) the bluebell attracted the interest of herbalists, and was given by them to prevent nightmares, while 13th century monks used the plant to treat snakebites and leprosy.  Moreover, it might just have a future role in saving and improving lives, because recent research is looking into how extracts form the plant could form a treatment for both cancer and sleeping sickness, by inhibiting respectively cancer cells and the pathogen responsible (Trypanosoma brucei) (Raheem et al., 2019).  These potentially useful properties stem from the fact that it is in fact very poisonous, which explains why the leaves are able to lie unmolested by herbivores on forests floors for much of the year.

Fig 8. Allium ursinum and Hyacinthoides non-scripta competing for space in Dalkeith Country Park. Photo R. Milne

The Bluebell, its ecology, and its relatives.

Part of the great affection for the bluebell in Britain stems from its abundance in the woodlands here, but in fact it has a further claim to Britishness, because it is rare outside the British Isles.  It this it is quite unusual, for most of our native flora the British Isles is merely one part of a much broader range, and many of our common woodland species are found widely across northern Europe.  Yet the bluebell is quite specifically adapted to the wet, oceanic climate of our islands.  It likes the high rainfall and relatively mild extremes of temperature that is brought to our islands by the Gulf stream, and outside the British isles can only be found where mostly similar conditions exist: close to the coast in northern and western France, and the mountains of northern Spain.  But only in the British Isles is it abundant across large areas. 

As an ancient woodland dweller, its typical associates include primrose, wood anemone, dog’s mercury (Mercurialis perennis) and violets (Viola spp.), plus sometimes the early purple orchid (Orchis mascula), though this is sadly quite rarely seen in woods in

Scotland, though I have seen it between Boarhills and Dunino in Fife. In Scottish woods the bluebell jockeys for position with the native wild garlic, Allium ursinum, and it is common to find woodlands with masses of one but little or none of the other. Sometimes the battle for space is more even and they occurr together, as in Dalkeith Country Park. The invasive Allium paradoxum (see https://wordpress.com/post/botsocscot.wordpress.com/3285) might pose a threat to bluebells in the future, as it can already be seen displacing A. ursinum around the Lothian region. However it is less often seen in ancient woodlands so the bluebell may be safe from it, for now.

Fig 9. Hyacinthoides non-scripta near its southern limit, near Cazo in the mountains of Northern Spain.  Photo R. Milne.

Bluebells generally occupy ancient woodlands, the main exception being on the west coast, where it and several other woodland species venture out of the shade to grow in rough clifftop grassland.  In those places, the moist and mild influence of the Gulf Stream is strongest, providing an approximation of the shelter normally offered by trees.  Of course, most grasslands in Britain would have been forest until humans chopped it down, so the bluebell and its fellows probably simply held on after the trees were gone.

Bluebells reproduce mainly by seed, but the seeds have no special mechanism for long dispersal: they have no adaptations for catching the wind, attaching to animals nor entering their bodies via edible flesh.  Hence the plants do not seem very good at colonising new spots, and do not easily find their way into more recently established woodlands.  This is precisely what makes them an excellent indicator species for ancient woodlands – a wood with carpets of bluebells in it has likely had an uninterrupted history as woodland stretching back thousands of years.  However, one must be wary of hybrid bluebells, which can establish happily in relatively young woodlands (see below).

Fig 10. A natural population on a steep slope of a tributary to the river South Esk, Midlothian.  Note the rare pink and white colour forms.  Photo R. Milne.

If this very limited spreading ability seems like a handicap, one should remember that, with very few exceptions, our flora evolved in a human-free world where the pace of change was far slower.  The slow crawl of the bluebell would have been quite enough to keep pace with the equally leisurely northward march of forests when the ice sheets retreated around 11000 years ago.  When the trees moved back into Britain, the bluebell and other woodlands species came with them, except for Hepatica nobilis, which was too slow and was cut off by the refilling English channel.  In an ecological sense, bluebell spread is not that slow, after all! 

Fig 11.  Native geographical range of the genus Hyacinthoides, showing all known species.  After Grundmann et al. (2010).

Going back a little further, our bluebell appears to have its origins in SW Europe, in or around Spain.  Spain is in fact home to about five species of bluebell: our H. non-scripta, the unfairly maligned H. hispanica, and three local endemics (Fig. 10).  These appear to result from an evolutionary radiation event in the area, which as also supplied species to North Africa and Italy, making around 11 in total (one Spanish population may or may not represent a separate species).  Nearly all of these species have corolla-lobes that are free most of the way to the base, making them look far more like a Scilla than a bluebell; in fact our native species is very much the odd-one-out in the genus with its narrow, tubular flowers. 

The similarity to Scilla reflects genuine relationships, and in fact Scilla as previously recognised is not a natural group at all, comprising four or more distinct evolutionary lineages.  The bluebells are in fact much more closely related to the native spring squill (Scilla verna, now more correctly Tractema verna) than either is to the true, somewhat reduced genus Scilla.  Our other native Scilla, S. autumnalis (autumn squill, an uncommon species of cliffs in SW Britain) properly belongs in Prospero, and is closer to the garden hyacinths than either is to bluebells or to the former Scilla bedfellows of the autumn squill.  Hence the bluebells and grape hyacinths (Muscari) are both parts of a large group of Scilla-like flowers.

Fig 12. A greatly simplified phylogeny showing relationships of the bluebell genus Hyacinthoides to other familiar species, most of which used to be placed in Scilla.  Based on cpDNA data from Pfosser and Speta, (1999); about 20 other genera included in the original study are not shown.  Image by R. Milne.

The interloper: hybrid bluebell, Hyacinthoides x massartiana.

A lot of sources, including several conservation organisations, warn that invading “Spanish Bluebells” represent a threat to our native plants. The level of threat is probably exaggerated, as will be discussed below, but let us first clear up a common misconception. The pure Spanish bluebell, H. hispanica, is either very rare or perhaps never occurs in the wild in Britain, and the naturalised plants are probably all H. x massartiana, the hybrid between Spanish and British bluebells. Pure H. hispanica is a delicate plant, with flowers much more open than on hybrid bluebells, and I have been lucky enough to see it once in its native habitat on limestone in southern Spain. A recent paper has examined naturalised bluebells in Britain in considerable detail, finding that they are morphologically distinct from both pure H. non-scripta and pure H. hispanica. The naturalised plants differed from H. hispanica in anther colour, width of leaf, scape and ovary, plant height, and length of scape, inflorescence, tepals, and filaments (Ruhsam et al., 2020). Moreover, the naturalised plants had reduced pollen fertility relative to either parent, which is expected for hybrids. With the classic caution of scientific language, the authors stop short of claiming this as proof that naturalised plants are hybrids, but it is fairly strong evidence. Therefore, if the pure Spanish bluebell occurs at all it is certainly not “frequent” as claimed by Stace (2019), but he is definitely correct to say it is over-recorded! It is very likely that most or all naturalised bluebells are hybrids.

Fig 13. The true Spanish Bluebell, H. hispanicus, can be recognised by the tepals free for most of their length (in other characters it resembles H. x massartiana, see Table 1, but it is much more delicate). The anthers are clearly blue, as in many hybrids.  Photos by author from wild plants in southern Spain

The naturalised plants are probably all H. x massartiana, the hybrid between Spanish and British bluebells.  Hybrid bluebells grow more vigorously in gardens than do either parent, and establish in the wild very easily from discarded bulbs. Indeed they do so well in gardens, spreading readily and elbowing aside more delicate plants, that they are liable to be chucked out of gardens quite often! In particular, there is one form of the hybrid that is triploid (i.e. three sets of chromosomes rather than the usual two), making it exceptionally vigorous but highly sterile. The sterility limits its spread but it can reproduce asexually via bulbs.

Fig 14. A hybrid bluebell (H. x massartiana) is easily recognised if intermediate between the parents, as with this one.  The pollen is blue-tinged in this specimen, rather than fully blue, but the flower shape is a give-away, and hybrids are often a paler blue than pure H. non-scripta. See Table 1.

Telling the hybrid bluebell from the native is slightly tricky, especially because the hybrid itself will cross with native bluebells (see below), with the result that some hybridised plants look very similar to pure native bluebells.  Table 1 below shows a range of ways to distinguish native from hybrid bluebells, of which pollen colour is perhaps the most diagnostic.  However, do note that this table shows characters for typical H. x massartiana that is roughly intermediate between the parent species.  Backcrosses from this to native bluebells can approach the natives in character, and have yellowish pollen.  Therefore, only plants that match native bluebells for all categories can be confidently assigned to that taxon.  The presence of many pink or white forms is an early indication that that the plants are probably hybridised.

Table 1: characters distinguishing pure native bluebells from naturalised hybrids.

 Native H. non-scriptaHybrid H. x massartiana1
HabitatAncient deciduous woodlands, sometimes open habitats where rainfall is high (mainly west coast)Disturbed habitats, urban woodlands, waste places, rarely found a long way from gardens.
OccurrenceUsually forms carpets, thousands of plants together.Often few plants together, usually <<100 together
Colour formsWhite and pink forms very rare, usually <1% of flowers in population, if present at all.  Mostly deep blue-purpleWhite and pink forms common, often >10% of plants present.  More often mid blue than deep blue-purple.
Pollen and anthersAlways pale yellow.Blue or same colour as tepals, except on backcrossed plants approaching H. non-scripta.
Corolla tubeParallel-sided, forming narrow tube.  Tips of tepals strongly curved back.Bell-shaped, width increasing with distance from base; tips not strongly curved back
Flower arrangementAll on one side of stalk, pendulous (hanging down)Pointing in all directions from stalk, erect or pointing sideways.
Flower scentSweetAlmost absent
Leaf widthUnder 2 cmOften > 2cm.
1The characters shown here are from hybrids roughly intermediate between the parents.  Where hybrids have backcrossed with native bluebells, the resultant plants will show some but not all characters of the column on the right, so only populations that match the middle column for most or all characters should be claimed as pure H. non-scripta.
 Fig 15. Close-ups of H. hispanica (left, “H”), hybrid (centre left and centre right, “X”) and pure native (“N”) bluebells.  Note pollen colour, corolla shape and the more divided corollas on the hybrids than H. non-scripta.  These particular hybrids have more divided corollas than some, and may be closer to H. hispanica than many that occur wild (e.g. see photos below). However note the much wider corolla lobes on the hybrids than on pure H. hispanica.  Photos C. Jeffree (all hybrids, plus top right) and R. Milne. 

Conservation and the hybrid.

The hybrid bluebell is not a threat to local plants and wildlife in the way that, say, Rhododendrons and Giant Hogweeds are.  It does not displace other plants and tends to linger, rather than aggressively spread, where it becomes established.  However, it creates concern among conservationists because it is able to backcross (i.e. hybridise again) with native bluebells, making slightly hybridised offspring that is closer to the native bluebell. 

Such repeated backcrossing towards one species is termed “Introgression” and is a way for genes to move naturally from one species into another, with the potential to permanently alter the genetics of the recipient species.  This may be happening for example from introduced Japanese sika deer into Scottish red deer, and there is concern that it may be occurring from hybrid into native bluebells.  However, while this may indeed be an issue locally, the sheer numbers of native bluebells in many sites where they occur suggests that this really isn’t much of a threat to our plants.  Placing one hybrid plant in a wood containing a million natives is very unlikely to lead to a change in the whole population there. 

Fig 16. A mass of native bluebells. Photo R. Milne

A scientific survey of bluebells in Scotland found that native plants outnumbered the hybrids by a factor of 100, but that actually the hybrids are more commonly encountered, because native bluebells exist in huge populations whereas the hybrid bluebells typically exist as small groups.  This, plus the fact that hybrids occur closer to houses and in more accessible places, leads to a perception that the hybrids make up a greater proportion of wild bluebells than they actually do.  The same study also noted that a little under half of all native populations occur within a km of at least some naturalised hybrids, creating the potential for introgression into native plants, but in practice it is highly unlikely that this would happen – a plant surrounded by thousands of flowering individuals of its own kind is unlikely to be pollinated instead by pollen delivered from a distance away.  Only if hybrid offspring have a significant advantage is introgression likely to occur.

Fig 17. A patch of white, pink and blue bluebells from a golf course in Edinburgh.  These approach H. non-scripta in most characters (e.g. note the corolla shape) but the high proportion of pink and white plants suggests they are planted, escapes or at least introgressed with DNA from the hybrids.  Plus it is a small patch, not a large mass.

The threats to our native bluebells are, therefore, much more familiar and universal ones.  Cutting down an ancient woodland will eradicate an entire population, especially if it is done for development (the plants may hold on in the mild wet conditions of the west coast, if a wood is cut down but the ground flora is allowed to grow on).  In the past, massed harvesting of the bulbs to grow in gardens also depleted the populations, but thankfully that is now illegal, and also less profitable due to the ready availability of hybrid bluebells.  In the longer term, as for pretty much everything, climate change is the greatest threat to our bluebells.  As noted above, the species is rare outside the UK, otherwise finding suitable conditions only around the coasts of France and the mountains of northern Spain.  A warming climate could render parts of this range unsuitable to it, either directly or by causing the trees it grows under to die off.  Ironically, introgression from H. x massartiana might end up offering a lifeline to our plants, supplying them with genes to cope with a warming climate.  Whether this is desirable is another matter – some would argue that keeping our plants pure is more important even if their range begins to shrink as a result.

Fig 18. A carpet of bluebells.  Photo Andrew Milne

Footnote

Human activity has not been all bad news for the British bluebell, however.  Not content with exporting the name and attaching it to local species, some British colonists of the past could not bear to live without the plant itself, and brought it with them.  The GBIF database (link- https://www.gbif.org/species/5304283) records our bluebell as being present in the wild in much of central Europe, southern Scandinavia, the western and eastern Seaboards of the USA, New Zealand and southern Australia.  Some of these records may in fact be the hybrid, but it looks like our love of the plant has opened up new ranges to the bluebell, and in these places it may yet have more stories to tell.  

References

Fox N, Jönsson AM (2019) Climate effects on the onset of flowering in the United Kingdom. Environmental Science Europe 31: 89. https://doi.org/10.1186/s12302-019-0271-4

Goudy, R (2018) Bluebells and their Meanings. The Flower writer, acessessed 6th May 2021, https://ruthgoudy.com/bluebells-and-their-meanings/  

Grundmann M, Rumsey FJ, Ansell SW, Russell SJ, Darwin SC, Vogel JC, Spencer M, Squirrell J, Hollingsworth PM, Ortiz S, Schneider H (2010) Phylogeny and taxonomy of HyacinthoidesTaxon 59:68–82 

Kohn DD, Hulme PE, Hollingsworth PM, Butler A. 2009. Are native bluebells (Hyacinthoides non-scripta) at risk from alien congenerics? Evidence from distributions and co-occurrence in Scotland. Biological Conservation 142:61–74. doi:10.1016/j.biocon.2008.09.030.  

Kohn DD, Ruhsam M, Hulme PE, Barrett SC, Hollingsworth PM. 2019. Paternity analysis reveals constraints on hybridization potential between native and introduced bluebells (Hyacinthoides). Conservation Genetics20:571–584. doi:10.1007/ s10592-019-01158-4.  

Pfosser M, Speta F (1999). Phylogenetics of Hyacinthaceae on plastid DNA sequences. Annals of the Missouri Botanical Garden 86: 852–875. 

Raheem DJ, Tawfike AF, Abdelmohsen UR, Edrada-Ebel R, Fitzsimmons-Thoss V. (2019) Application of metabolomics and molecular networking in investigating the chemical profile and antitrypanosomal activity of British bluebells (Hyacinthoides non-scripta). Scientific Reports 9:1–13. doi: 10.1038/s41598-019-38940-w 

Ruhsam M, Kohn D, Jane Squirrell J, Schneider H, Vogel J, Rumsey FJ, Hollingsworth PM (2020) Morphology and pollen fertility of native and non-native bluebells in Great Britain, Plant Ecology & Diversity13: 351-361, DOI: 10.1080/17550874.2020.1765037 

Smith SL, Senn HV, Pérez‐Espona S, Wyman MT, Heap E, Pemberton JM (2018). Introgression of exotic Cervus (nippon and canadensis) into red deer (Cervus elaphus) populations in Scotland and the English Lake District. Ecology and Evolution8: 2122– 2134. 

Stace C (2019).  New Flora of the British Isles, 3rd Edition.  C & M Floristics, UK

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