Species of the day: Horse Chestnut

James Duncan
, 01 May 2020
Species of the day: Horse Chestnut
Horse Chestnut © Derek Middleton

By James Duncan

Learning and Engagement Officer

The Horse Chestnut (Aesculus hippocastanum) is a ubiquitous tree species, best known for its substantial contribution to British culture in the form of 'conkers.' Now is the perfect time to see this beautiful tree in bloom, its magnificent 'candelabra' of snowy-white and pink flowers punctuating distinctively fingered (palmate) leaves. It has long been used as an ornamental avenue plant, and you may well spot its colourful canopy spreading along British country roads. For all its history and place in culture, you'd imagine its always been native but this isn't the case. Horse Chestnut was actually introduced from the Balkans in Tudor times during the sixteenth century - it's become a naturalised species that's been embraced by us ever since. There may be a couple of reasons for its common name. The first may relate to its origin in Turkey, where an observation noted that soldiers fed the conkers to their horses. The second is owing to the 'horseshoe-shaped' scar left on the stems after the leaves fall. It's a fast growing species and may reach 40 metres and a few hundred years old, though it has limited commercial value owing to its soft wood. 

Once the flowers have been pollinated, they develop within a viciously spiked green husk into the beautiful glossy-brown, super-tough seeds with which we're so familiar. Though the tree has been in Britain since the 1500's, it wasn't until 1848 that the game 'conkers' was first played using horse chestnut seeds. It had in fact previously utilised snail shells, and the name may partly have derived from the word 'conch.' Equally, it has strong associations with the word 'conquer', particularly from the nineteenth century onwards, and the game was alternatively named 'conquerers.' Interestingly, the seeds contain a glycoside called aesculin which is toxic to many animals, though they're still eaten by some mammals, including deer. They also contain aescin which has more beneficial effects for humans in the form of anti-inflammatories. 

Unfortunately for the Horse Chestnut, it's currently in the midst of difficult times. It's now got unwanted company in the form of a small moth species, the Horse Chestnut Leaf Miner (Cameraria ohridella). This moth was first recorded in the UK in 2002, though had been known in its native Southern Europe for nearly 20 years beforehand. Quite incredibly, it seems it may have been hitching an accidental free ride to Britain in the form of our cars, with pupae sometimes transported in leaf litter. The moth literally 'mines' its way within the leaf material leaving unsightly brown scars as the leaves eventually die in its wake. Though the tree itself won't perish, the attacks leave it weakened, causing premature leaf drop, and may affect its annual growth cycle. The moth larvae do, however, provide a fantastic food source for our birds, avid insectivores like Blue Tits reaping the benefits. Ultimately the leaf miner is here to stay, though perhaps its worst repercussion is leaving trees more vulnerable to other diseases such as 'bleeding canker', now estimated to affect nearly half of all British Horse Chestnuts. 

Horse Chestnut © Derek Middleton (1)

Horse Chestnut - 'conkers' © Derek Middleton

Horse Chestnut © Roger Wilmshurst

Horse Chestnut © Roger Wilmshurst

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