In the month of February, when most trees are leafless and night frosts are frequent, the appearance of hazel catkins in hedgerows and parkland is a welcome sign that Spring is just around the corner. In childhood, we called the catkins ‘lambs tails’ and on sunny days we took delight in flicking them to release clouds of pollen.
Old illustration of Corylus avellana. Original book source: Prof. Dr. Otto Wilhelm Thomé Flora von Deutschland, Österreich und der Schweiz 1885, Gera, Germany. Public domain from https://commons.wikimedia.org/wiki/File:Illustration_Corylus_avellana0.jpg
We children didn’t even notice the female flowers. You have to look hard for them. They are less numerous, red and tiny. Viewed with a hand lens they appear quite spectacular when fully open. For successful fertilization, the pollen must come from another tree, a feature well-known to commercial hazelnut farmers who plant several cultivars to achieve a high success rate.
Catkins. On the left, the pendulous male catkins and also a few developing females showing their intense red tinge; on the right, detail of male flowers showing the stamens. Each male catkin can produce 8 million pollen grains. Photos: Chris Jeffree.
The yellow male catkins appeal to gardeners, and thus many horticultural cultivars can be found on-line. I’ve noticed a sharp increase in the use of Hazel for amenity planting, not only in gardens but in parks and along stretches of motorways: it provides variation in colour and texture and lifts the spirits. The Edinburgh City ring road is currently a good example. But at home we have a red-coloured cultivar which is especially attractive and admired by neighbours (see below).
Display of male catkins on a horticultural variety of Corylus avellana. In the wild, the catkins are yellow-green, sometimes referred to as ‘golden’. Photo: John Grace.
The catkins attract large insects. Hazel is wind pollinated so does not produce nectar, but bees gather the pollen as food (a good source of protein). Catkins last long time, and can be seen from January to March.
Female flower, about 4mm long, showing the red stigmatic surface which may remain receptive for up to 3 months. A few pollen grains have alighted on this one. Note also the hairs on the young shoots. Many of them are red-tinted and described as ‘glandular’, having a spherical swelling at their tip. Photo: Chris Jeffree.
The fertilised ovule of the female flower then develops into the familiar fruit with a shell that becomes woody and hard to form the nut. Nuts fall to the ground and are gathered by various animals, most famously by squirrels. I’ve read that squirrels don’t carry these nuts into their dens immediately but they bury them in the ground and return to look for them later. Squirrels are unexpectedly forgetful and many nuts remain planted in the soil, to germinate in the following spring.
Other aspects of the basic biology have been recently reviewed by Damien Hicks in his 2022 article here.
Is it a tree or a shrub? There are many definitions of these terms, usually based on height and the presence of a single woody stem for a tree. Stace (2019) describes Hazel as a ‘several-stemmed shrub’ with a typical height of 6 metres and an exceptional height of 12 metres. It lives for 80-90 years and sprouts new stems from a woody underground organ referred to as a ‘stool’. This process is enhanced by the traditional country craft of coppicing, whereby stems are periodically harvested at their base. Shoots emerge and thicken, thus providing a regular supply of flexible poles which can be used in construction (or, in modern times, useful supports for peas and beans).
Hazel nuts, also called cobnuts. Note the leafy bracts that surround the nuts like a cup. These bracts are are about twice as long in the closely related Filbert Corylus maxima and form a tube that is usually somewhat constricted beyond the nut. The nuts are ripe in September but can be eaten fresh, when green and soft, in August. Photo: Chris Jeffree.
Where does it grow? Most people will know it from hedgerows and as a shrub-layer in lowland deciduous woodlands, especially oak and ash. It forms part of the silvicultural system known as ‘coppice-and-standards’. Most of these stands are anthropogenic formations, where the shoots are trimmed (in hedgerows) or entirely cut down (coppiced to provide poles). It occurs in a more natural state as woodland and scrub in the high rainfall areas of the west of the country, often on limestone: it is a major component of a woodland known as temperate rainforest described here. The term ‘Atlantic Hazel’ has been used to emphasise the heritage value of these woodlands, especially in relation to the high diversity of bryophytes and lichens (Coppins & Coppins (2018).
Interior of Hazel woodland, Glen of Clab, County Clare, Ireland. Photo: Chris Jeffree
Hazel forms an extensive scrub on the windswept limestone pavement of the Burren, in County Clare, Ireland and also in the Lake District of England. The Burren is a famously biodiverse botanical area (Cabot & Goodwillie 2018), known for having artic-alpine species, but it has been winter-grazed for many generations by livestock especially cattle, sheep and goats. To achieve a sustainable form of land use where plants, livestock and people are in some sort of equilibrium it is necessary to cut back the scrub, which consists mainly of hazel and blackthorn.
Hazel scrub on coastal limestone pavement, the Burren, Ireland. Photo: Chris Jeffree
Vegetation control is inevitably controversial as it challenges the concept of ‘naturalness’ and puts farmers in conflict with conservationists. But a European project BurrenLIFE aims “to develop a new model for sustainable agriculture in the Burren in order to conserve the priority habitats of the region as designated under the EU Habitats Directive”.
WIndswept Hazel, growing in a grike on limestone pavement, the Burren, Ireland. Photo: Chris Jeffree
On the Burren, Hazelnuts evidently fall into the fissures (‘grikes’) of the limestone pavement and germinate along with several other species; within the grike they are protected from grazing and woodland species can grow.
Where did Hazel come from? The last Glacial Period began 33,000 years ago and ended only 10,000 years ago. Britain was frozen, covered in ice as far south as the English Midlands and trees were restricted to areas in central and southern Europe including the Mediterranean basin. As the ice melted and the climate warmed, trees spread northwards. According to the pollen record, Hazel entered Britain via the western shores, shortly after the ice retreated. Hazel pollen has been found in deep sediments in the west of Scotland, Wales and England as well as southern Ireland. Radiocarbon dating shows how Hazel colonised some of these sites 9,000 years ago. Hazel nuts float, and we presume they were carried by sea currents from the south-western fringes of Europe to Ireland and then to Scotland. Some researchers have argued that Ireland and Scotland were connected at that time by a land bridge, and that nuts could have been transported from Ireland by animals. However it came here, Hazel must have been rather plentiful: large quantities of charred hazelnut shells turn up at archaeological sites. At one site, on the island of Colonsay in the Inner Hebrides of Scotland, the shells are 8,000 years old. It would doubtless have been a useful food source for Mesolithic people.
How was Hazel dispersed? Arrival, establishment and expansion of its range to occupy the rest of Britain was rapid. Expansion rates have been estimated from carbon-14 dating to be between 0.5 and 2 km per year. How can it possibly have been so rapid? Cabot and Goodwillie (2018) provide helpful information based on observations at the Burren. They say that hazelnuts are taken by Wood mouse, Red Squirrel and birds (Hooded crow, Jackdaw and Jay). They also cite a study which says that nuts stored more than 70 metres away “often survive”. I still find it hard to understand how this species (and other trees) could have moved so rapidly and I wonder how far mesolithic humans may have been involved.
Traditional harvesting of hazelnuts. By William Adolphe Bouguereau (1883). Public domain via Wikiart.
What about its relatives? The genus Corylus has over 20 species scattered over Europe, Asia and North America. It is difficult to be exact on this number, as the species status is sometimes disputed. In Britain, Corylus was featured in Gerard’s Herbal of 1597. He describes and illustrates the same three species that we recognise today: the Common Hazel (he called it Wild Hedge Hazelnut, Corylus sylvestris), the Filbert and the Turkish). Of Common Hazel he says “Corylus sylvestris is our hedge nut or Hazelnut tree, which is very well known, and therefore needeth not any description: whereof there are also sundry sorts, some great, some little, some rathe ripe, some later, as also one that is manured in our gardens, which is very great, bigger than any Filbert, and yet a kind of Hedge nut..”. We may infer that hazelnuts were grown and consumed in Elizabethan England, and that there may have already have been distinct local varieties (‘sundry sorts’) in cultivation.
Now there are more than 400 cultivars, including commercial hybrids between the native Corylus avellana and the Filbert, C. maxima. Most of the Hazel growing wild in Britain is Corylus avellana but the cultivated forms known as the Kentish Cobnut look like a hybrid with Filbert, Corylus maxima. However, according to some taxonomists, the Filbert is not a species at all, merely another form of Corylus avellana.
In S.E. England the Kentish Cobnut is still grown commercially on a small scale, with traditional manual harvesting; several nut farms supply a niche market. However, the global demand for Hazelnuts as an ingredient in various confectionaries is huge, and the biggest producers are Turkey, Italy and the United States, where harvesting is mechanised. It is said that 25% of all global production is bought by the Italian company Ferrero, who make Nutella spread and Ferrero Rocher chocolates. I think we all agree that these luxury foods are delicious.
Hazel is a multi-purpose shrub and so it could have an important future as we move (albeit slowly) towards truly sustainable agriculture, or agroforestry. On agricultural soils it grows quite quickly and can provide a harvest of both nuts (not quite as good as imported macadamia nuts) and gardening poles (better than imported bamboo canes); its poles might also be a source of fibre and its nuts have a high oil content and have been suggested for biofuel.
A hazelnut orchard. Attribution: Jennifer C., Creative Commons. 2.0 Generic (CC BY 2.0) Downloaded from https://www.flickr.com/photos/29638108@N06/7715079136/
Hazel is a topic in British folk-lore, much of which is recounted on several pages by Mabey (1996). He points out the advantages of Hazel as a raw material in earlier times. Poles can be split, twisted and bent, hence made into a lattice work called wattle, and used for fencing or combined with mud to make walls. In Celtic legend, hazelnuts are a metaphor for concentrated wisdom (sweet, compact and packed into a hard shell). He tells many stories (of Virgin’s Crowns, the use of Hazel in the rendering of rents, Hazel tally sticks and more). Perhaps most famously Hazel sticks were the preferred instruments of dowsers, those special individuals who earned a living by detecting underground water by means of a piece of hazel shaped like a Y. However, I tried it, and found it does not work except when the dowser already knows that water is there.
References
Birks HJB (1989) Isochrone maps and patterns of tree-spreading in the British Isles. Journal of Biogeography 16, 503-540.
Cabot D & Goodwillie R (2018) The Burren (New Naturalist Library). Collins, London.
Coppins S and Coppins BJ (2012) Atlantic Hazel: Scotland’s Special Woodlands. Atlantic Hazel Action Group, Kilmartin.
Hicks D (2022) Biological Flora of Britain and Ireland: Corylus avellana. Journal of Ecology p. 3053-3089. https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.14008
Mabey R (1996). Flora Britannica. Chatto and Windus, London.
Stace CA (2019) New Flora of the British Isles. C&A Floristics.
Websites consulted
https://www.woodlandtrust.org.uk/trees-woods-and-wildlife/british-trees/a-z-of-british-trees/hazel/
https://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2745.14008
https://www.irishtimes.com/news/burren-s-rare-plants-at-risk-from-scrub-study-1.1294097
https://www.plant-talk.org/scrub-threatens-burren-ireland.htm
https://kentishcobnutsassociation.org.uk/
http://speciesofuk.blogspot.com/2014/04/week-46-hazel-corylus-avellana.html
https://www.jeremybartlett.co.uk/2023/02/23/hazel-corylus-avellana/
©John Grace
Botany in Scotland 
