The Alien that Conquered Britain

The conker season is here again – amply celebrated today in an article in the Times newspaper (see http://www.timesonline.co.uk/tol/news/uk/article6860922.ece). Horse chestnut Aesculus hippocastanum is our best-loved alien species which, during the 400 years since it was first introduced into Britain from the Balkans, has graced our parklands and become the focus of a traditional sport that has left generations of school-kids nursing aching knuckles after misjudged swings of an opponent’s conker.

The seeds of horse chestnut are truly exquisite objects. Their deep umber hue has given its name to a colour and their glossy texture contrasts wonderfully with the spiky husk that they are enclosed within. Sadly, it’s a beauty that doesn’t last – after a day or two those glossy seeds become leathery and lose their rippled patterns. In the USA Aesculus species are known as buckeyes, on account of the resemblance between those glossy brown orbs and the eyes of deer.

I may be wrong, but I suspect the horse chestnut seed is the largest amongst trees in Britain. They invest a lot of resources in giving their germinating seeds the best possible start in life, by providing them with a very large food store.... but at the expense of long-distance dispersal (take a look at http://beyondthehumaneye.blogspot.com/2009/10/travelling-light.html for a completely different plant seed dispersal strategy).

One of the perils of producing seeds with a large food store is that they simply become food for hungry animals (think of all the acorns that are guzzled by squirrels, jays and wood pigeons) - which may be why conkers contain high levels of toxic substances called saponins which are natural detergents and generally damaging to animal digestive systems. Ground-up conker flesh can provide a deterrent against slugs, which have a strong aversion for saponins and tend not to cross a barrier of conker meal. This might be the basis for a natural method of slug control, but for the fact that saponins are water soluble, so that any attempt to use them as slug deterrents would be washed away in the first shower of rain.