Bluebells: The science behind Britain’s most beloved spring flower

Cambridge scientists and horticulturists explore the science behind the UK’s iconic bluebell, showcasing stunning scanning electron and digital microscopy from Sainsbury Laboratory Cambridge University.

Contributors: Jean Watelet, Juliet Anderson, Sally Petitt, Gareth Evans, Trevor Groves, Edwige Moyroud and Kathy Grube.

More than a spring spectacle

Every April, a tiny woodland plant steps into the national spotlight, making headlines and inspiring a seasonal pilgrimage of families across the UK to witness its brief but spectacular display.

It is the British bluebell (Hyacinthoides non-scripta).

Behind this cultural icon lies a plant with a fascinating biology.

In this article, written to mark International Plant Appreciation Day on 13 April, scientists and horticulturists from Cambridge University Botanic Garden, Department of Plant Sciences, Cambridge University Herbarium and Sainsbury Laboratory Cambridge University come together to explore the science behind the British bluebell.

Is a bluebell really blue?

Bluebells are celebrated for their deep violet-blue flowers.

Their appeal is no surprise: blue is one of the rarest colours in nature, found in fewer than 10% of flowering plants.

This is possibly because blue pigments require plants to use a sophisticated chemistry toolkit to both synthesise and to stabilise their blue colour.

The colour that we perceive as blue in bluebells is produced by flavonoid compounds from the anthocyanin family.

Anthocyanin pigments range from vivid red to deep purple, with their exact hue influenced by several factors, including the pH inside the plant cell’s vacuole (appearing red in acidic conditions and blue in more alkaline environments), the addition of chemical decorations that influence light absorption/reflection and the presence of other flavonoids. Interactions between those pigments and metallic ions can also enhance blue tones, depending on their availability in the soil.

The anthocyanin that gives bluebells their characteristic hue is delphinidin. Delphinidin is the primary pigment also responsible for the blue hues of delphiniums, Himalayan blue poppies and violas/pansies.

Delphinidin has three hydroxyl decorations attached to it, which shifts the colour towards purple and blue.

This is not merely aesthetic. Blue and violet tones are highly visible to pollinators, such as bees and bumblebees, meaning that flowers capable of producing these colours may catch their attention more easily.

In bluebells, colour also changes over time: younger flowers display a richer blue, fading as they age.

This is possibly a signal to pollinators that the flower is past its prime. It could also simply be that pH regulation in the vacuole is less efficient in older flowers.

There are occasional mutations of the enzymes responsible for anthocyanin pigment production and decoration, which can lead to plants with pink or white flowers.

A species under pressure

The British bluebell is native across much of the UK and occurs in calcareous and slightly acidic woodland.

They start growing early in the year while most plants are still dormant. Even in low temperatures, they rapidly produce leaves, complete flowering and produce seeds before the tree canopy closes over and blocks out sunlight. This allows them to capture maximum light to store as energy in the bulb for next year’s flowers.

In England, bluebells can be indicative of ancient woodlands, which have persisted undisturbed since the 1600s, and which have developed unique communities of plants, fungi, lichen, insects, birds and mammals.

However, it faces several pressures, including habitat loss and hybridisation with non-native relatives.

Hybridisation with non-native bluebells, often referred to as the Spanish bluebell (Hyacinthoides hispanica) has been a longstanding concern.

Introduced as a garden plant possibly in the late 1600s to 1700s, the first of the non-native species introduced to the UK actually originated from Portugal and not Spain.

It has interbred with native populations, producing hybrids (Hyacinthoides × massartiana) that are now very common in gardens and parks around the UK.

However, recent genetic studies by the Royal Botanic Garden Edinburgh provide some reassurance.

While hybrids are now widespread, there is currently no strong evidence of large-scale genetic swamping in natural populations, finding only 2% of natural native populations showed evidence of hybridisation.

In fact, native British bluebells may have higher fertility than their hybrid counterparts, suggesting they retain a competitive edge in suitable habitats.

How to identify a true British bluebell

Distinguishing between native, Spanish and hybrid bluebells is not always straightforward.

British bluebells typically have a drooping stem with flowers arranged on one side, narrow tubular flowers with strongly recurved tips, and cream-coloured anthers and pollen. They have a sweet perfume.

In contrast, Spanish bluebells are more upright, with flowers arranged around the stem and have blue-grey anthers. Usually no perfume.

Hybrids often display a mix of characteristics from both British and Spanish bluebells.

British bluebells can be found in Cambridge University Botanic Garden dotted along the boundaries and long grass areas. There are not dense plantings and are variable due to hybridisation.

British Bluebells (Hyacinthoides non-scripta)

Common names: bluebell, English bluebell, British bluebell, English harebell, wild hyacinth, cuckoo’s boots, granfer griggles, witches’ thimbles, lady’s nightcap, fairy flower, cra’tae (crow’s toes)

Family: Asparagaceae

Distribution: Across most of Britain (also northern Spain to Netherlands)

Flowering season: mid-April to May

Habitat: Woodland, along edges of hedgerows, with bracken and also meadows in uplands (mainly in Scotland and Wales)

How to identify:

• Flowers are drooping or nodding to one side
• One-sided inflorescence (raceme)
• Narrow, tubular, blue flowers
• Revolute or turned-back tips on open flowers
• Cream pollen and anthers
• Narrow leaves and slender flower stem

Spanish bluebell (Hyacinthoides hispanica)

Grow upright, with flowers all around the stem. Anthers are blue.

Hybrid bluebell (Hyacinthoides x massartiana)

Mix of British and Spanish bluebells. It is often very similar in appearance to our native bluebell and so can be very hard to distinguish. Studies have shown that hybrids are very common in the UK in parks and gardens.

What’s in a name?

Like all plants, the British Bluebell also has its taxonomic binomial name, Hyacinthoides non-scripta, however it has been known by many different names over time.

Historically taxonomists would describe new scientific species based off key morphological characteristics of the plant, however with the advent of genetic techniques it has often been found that many plants which were historically thought to be different species, are all the same. These species are then bought together under one ‘correct’ name (in this case Hyacinthoides non-scripta), and the other names become known as synonyms.

Many of these name changes are captured by herbarium specimens, dried plants that can be hundreds of years old, many of which are still annotated with their historic names.

An unusual example of this is a Cambridge Herbarium specimen CGE00075962, which captures eight of the historic names used for this species on its label, showcasing its complicated taxonomic history. Another nice example is specimen CGE00075950, which was originally collected in 1832, and is annotated with three different names, the most recent being the correct Hyacinthoides non-scripta.

Herbarium specimens provide a valuable resource that record the locations and flowering time of bluebells and other plants over long periods of time, providing historic records for tracking plant phenology and response to climate change.

Facts you (probably) never knew about bluebells

• Cold-ready chemistry: Store energy in their bulbs as fructans (not starch), to fuel early spring growth below 10°C.
• Slow starters: Can take 5-7 years from seed germinating to first flower.
• Play tricks: Look like they have 6 violet-blue petals but actually only have 3! The 3 outer sepals (normally green and leaf-like in most flowers) are identical to the petals, producing a bluebell flower with 6 ‘apparent’ petals. Petals and sepals that looks identical are known as ‘tepals’.
• Hidden seeds: Mainly reproduce by seed, each plant producing up to 100 seeds, but the seed capsules and seeds are rarely noticed. These often-unseen fruits have been preserved in some of the specimens kept in the University Herbarium, an example of which can be seen in the Herbarium specimens pictured.
• Self-burial system: Contractile roots pull bulbs down deeper into the soil, protecting the bulb from frost or heat.
• Not just a British icon: Found from northwest Spain to the Netherlands, but it is estimated that around half the world’s population grows in the UK.
• Invasive weed: Naturalised in other parts of Europe and parts of North America, where it has become invasive.
• Toxic plant: Harmful to livestock, animals and humans. Sap can cause dermatitis. Despite this, seeds are eaten by mice and voles.
• Fit for a queen: A vital early nectar source for bumblebee queens, although some cheat by ‘robbing’ nectar by making holes at the base of the flower and bypassing pollination.
• Historic uses (despite toxicity): Limited records, but used as a styptic, to treat snake bites and leprosy, and bulb extracts supposedly once made glue for bookbinding
• Unexpected chemistry: Being studied for potential cancer and parasite treatments and extraction of oil from the seeds.
• No such thing as ‘real bluebell’ perfume: Bluebell perfumes are synthetic recreations or natural fragrances mixed to mimic the scent of a bluebell woodland.
• Portuguese bluebell: Genetic analysis of non-native bluebells in the UK indicate that the first non-natives introduced to the UK originated from Portugal, not Spain.