I always read Caroline Wheeler’s regular ‘Herbalist’s View’ articles with interest. 
The uses and
folklore superstitions associated with plants have always fascinated me, as
have the derivations of the common names of organisms. Her comments in White
Admiral 59 about the naming of the Horse Chestnut Aesculus hippocastanum
made me remember several things read in the past and so I dug out many
books lying unread for years upon my shelves to check on those memories.
Other ‘horse’ species include the horse leech Haemopsis sanguisuga and the horse mussel Modiolus modiolus, both impressively large compared with other similar species. Some books refer to the idea that ‘horse’ may be applied to any large species, the carthorse being the largest animal on the farm and the largest most people had ever seen. However, there are other explanations.
Certainly with Haemopsis the association may well be to do with the fact that such leeches spend a fair amount of their life cycle burrowing in damp soil and are thus rarely seen. H. sanguisuga inhabits the stream in my own garden. Until I re-profiled the stream a year ago I saw the leech on only a few nights per year when observing by torch. During re-profiling, dozens of the leeches were found in the wet clay that had been disturbed.
Those who frequent streams will have noticed that at sections used as horse and cattle drinking places, the banks are churned up by hooves. This churning up process also unearths the leeches and would make them more noticeable to agricultural workers. Once the association was established in the ploughboy’s mind it is likely the common name would evolve.
Returning to the horse chestnut, the size association may of course still apply. The leaves are certainly large, probably the largest of all the common compound leaves. Caroline’s explanation is that ‘horse’ in the plant’s name may be used to imply that it is not fit for human consumption. This is a suggestion I have seen mentioned a few times; it certainly reflects the English hatred of the idea of consuming horseflesh. However, given the importance of the horse to the rural economy, one wonders why such a ‘useless’ seed should be thus associated.
Vedel & Lange (1960, Denmark) seem to contradict inedibility as they suggest that
the seeds have sometimes been used as a remedy for vomiting. But there are no
details of how this was achieved, and as many other references are purely to do
with animal medicine, it may not have been a reference to use in humans. They also
refer to the use of conkers as a cure for coughs in horses, and as a substitute
for soap when crushed and ground. The same text cites the name ‘conker’ as a
corruption of conqueror linking to the game of the same name and
presumably to the
victor of the Battle of Hastings. Grigson (1973) supports
this and tells us that Ribwort Plantain Plantago lanceolata flowers were
used for a similarly named game in Somerset.
In the wonderful Forestry Commission booklet, Edlin (1968), (buy it if you see it second-hand!) we are told that the horse chestnut was brought to Britain from the Balkans in 1616 and that the name refers to the nuts being fed by the Turks to ailing horses. Strangely it also tells us that normally all animals refuse to touch the seeds. This is contradicted by several texts such as Proctor (1972) and my old Observers Book of Trees (1960) which cite cattle, sheep and deer as all being fond of this ‘bitter fruit’. Proctor also tells us that they were used in Persia as a medicine for ‘Broken winded’ horses. Vedel and Lange (1960) echo the idea that deer are fond of the fruits, stating that the trees are useful as a valuable source of food in deer parks.
As well as the references I found above, I remember reading somewhere that the name horse chestnut came from the use of conkers as an emetic in horses. That is if your horse had eaten the wrong fodder then conkers mixed into the feed would bring up the offending food before it passed through the system. I wonder if the offending plant could be ragwort, because Grigson (1973) reminds us that in Ireland and Scotland the local name for ragwort was ‘Fairy Horse’ as apparently in folklore the faeries and later witches were believed to magically ride upon its stalks.
We will never know for sure 
how our trees and plants derived their names but I for
one believe all of this adds extra interest to our natural history. I would be
very interested to hear from any farmers, vets or horse owners who may have
further information to share.
Finally, as a primary school teacher, the explanation I have used most often is the shape of the leaf scar on the winter twig of A. hippocastanum. Children can readily see this as more or less a horseshoe, complete with horseshoe nails. The latter are of course really the scars left by the main veins of the leaf when the abscission layer forms in the autumn. Explaining the ‘horse’ name thus has been a good way to get children to look at winter twigs - most have never noticed leaf scars at all. Many will quickly then go on to be able to distinguish the black buds of ash, the cones of alder etc. a real start at identification. Anything to stimulate children’s interest is worth pursuing and who is to say that horseshoe leaf scars did not help to give the tree its name.
Mabey, Richard (1996) Flora Britannica Sinclair – Stevenson. London.
Stokoe, W. J. (1960) The Observer’s Book of Trees Frederick Warne & Co. London.
Procter, Ray (1972) Trees of the World Hamlyn. London.
Vedel, H. & Lange, J. (1960) Trees & Bushes in wood & hedgerow Methuen & Co. London.
Grigson, Geoffrey (1958) The Englishman’s Flora Phoenix House. London.
Edlin, Herbert (1968) Know Your Broadleaves H.M.S.O. London.
Adrian Chalkley
© 2005 Suffolk Naturalists' Society
used by kind permision.