It’s bluebell season. The small flower appears in late spring and is associated with ancient woodland. It gives its name to the ‘bluebell woods’ where the flower forms a carpet of blue beneath the trees around May-time. 
(Pictured here is Macclesfield Forest, maintained by United Utilities – 15 May 2016; the headline image is from Speke Hall, Merseyside (c) National Trust/Andrew Butler).
It is an instantly recognisable native flower, brightly-coloured in the dappled shade, at the time of year when the countryside is lush with fresh green growth, the air warmer and the days long. Unsurprising, then, that the bluebell has polled as the national favourite in both England and Scotland .
In bluebell folklore according to Google, a garland of the flowers can force the wearer to tell the truth, and the ‘ringing’ of bluebells can summon fairies and entrap or even kill humans. If a child picks a bluebell or hears it ringing, it will never be seen again – lost forever.
A quick search of the Thompson motif index via Momfer (http://momfer.ml) reveals a Finnish-Swedish story that the first church bell was modelled on the bluebell.
English tradition states that they bloom on St George’s Day (23 April).
Elsewhere, I find that the bulbs can be used to make glue, and in the medieval period this glue was apparently used in bookbinding and the making of arrows. The flowers are also poisonous.
The bluebell’s association with ancient woodland places it within the popular imaginative landscape of ancient Britain. Walking through a forest in springtime it seems no wonder that it has long held a place in national storytelling as the quintessential otherworld within the world, inherently surreal and magical. As a medievalist, I can’t help but think of the forests of romance or the merry ‘grene wode’ of the Robin Hood ballads. 
Like ancient buildings and texts, the ancient natural landscape is material evidence of the world of the past, and (at least where it features in visual art and storytelling) it is an integral part of the material culture of the past. Ancient woodlands are especially evocative perhaps because they give immediate and uninhibited access (to anyone) to this natural heritage, not mediated through the histories of a particular people but something silently felt.
The bluebell today seems to be an emblem of an idealised image of rural England, ‘our favourite’ flower, but how long has it been regarded in this way? How old is the ‘fairy flower’ folklore?
Looking for evidence of bluebell woods in medieval literature proves difficult. The first problem is that the earliest recorded entry of the word ‘bluebell’ in the Oxford English Dictionary is 1578 (‘Blew Belles’). 
In fact, I realised that in most of the likely examples that spring to mind from Middle English poetry, flowers are described only as ‘flowers’ and the species not given.
The earliest mention of the bluebell in surviving medieval literature may be in the fourteenth-century, Latin herbal’ of a Dominican friar, Henry Daniels, who describes them (a bit unsatisfactorily) as woodland flowers, like daffodils but blue. 
Shakespeare mentions the hare-bell (in Cymbeline) but not bluebells, as far as I can find.
Perhaps the romance of the bluebell is a far more recent thing.
Anne Brönte writes a poem entitled ‘The Bluebell’ (1840) in which she explains its personal association for her:
There is a silent eloquence
In every wild bluebell
That fills my softened heart with bliss
That words could never tell.
O, that lone flower recalled to me
My happy childhood’s hours
When bluebells seemed like fairy gifts
A prize among the flowers.
‘Sad wandered, weep those blissful times
That never may return!’
The lovely floweret seemed to say,
And thus it made me mourn.
In fact, Brönte’s sentiment seems typical of the bluebell tradition in its ambiguity about the flower (and the forest, perhaps). The bluebell’s beauty is bittersweet; in particular, it evokes lost childhood. Emily Brönte also wrote a bluebell poem, similarly mournful.
Lawrence Alma-Tadema’s Victorian painting ‘Bluebells’ strikes the same attitude.  Perhaps it’s simply to do with the ‘blueness’ of the flower by comparison with its cheery, upright yellow counterpart – the way the bluebell’s flower-heads hang drooping, thriving in the shadier spots.
In the present day, bluebell woods are a significant seasonal visitor attraction, but the symbolism of the flower seems unusually ambiguous for one readily identified as a favourite and arguably considered a part of national identity.
Apparently, some of the bluebells you see are actually a Spanish variety that can hybridise with the English: http://www.wildlifebcn.org/events/bluebell-watch/bluebell-fact
Corinne Saunders has written a book on the topic: The Forest of Medieval Romance (Cambridge: Boydell & Brewer, 1993)
Read medieval Robin Hood ballads online at http://d.lib.rochester.edu/teams/publication/knight-and-ohlgren-robin-hood-and-other-outlaw-tales
 An argument made effectively in the opening paper of the recent Borderlines XX conference. Peter Casby, ‘Living Trees as Medieval Material Culture’
 OED entry for bluebell (n). The reference is H. Lyte tr. R. Dodoens Niewe Herball
 I came across the following references in another blog: https://victorianlibrarian.wordpress.com/2014/08/05/medieval-floral-iconography/
Celia Fisher, The Medieval Flower Book (British Library, 2007)
George Keiser, ‘Through a Fourteenth Century Gardener’s Eyes: Henry Daniel’s Herbal”, The Chaucer Review 31:1 (1996), 58-75
There is also: Celia Fisher, Flowers in Medieval Manuscripts (University of Toronto Press, 2004)