In the first place, they're so ugly. And sticky. And when they lie around on the drive outside your house, getting crushed by tyres, they make a disgusting goo. They are ridged like a sea shell, say a limpet. They are an excrescence which afflicts oak trees - the knopper gall. Correction: they afflict only the common or pedunculate oak, quercus robur, the great spreading tree, enveloping and killing the acorn. Our national tree, chosen not so long ago, quercus petraea, or the sessile oak, is exempt. Now how does the gall-fly that causes all the damage differentiate between the oaks? One of the mysteries no one has solved. And this is a comparatively new phenomenon. First discovered on these islands, it is said, in the 1970s. In Ireland it seemed to surface in the 1980s. The insect which causes the damage is andriscus quercuscalcis, but it cannot do its deadly work on the common or pedunculate oak unless there is another element in the area, and that is the presence of a Turkey oak. Charles Nelson writes in his majestic Trees of Ireland that this "humble insect demonstrates that it is a better taxonomist [i.e. scientific classifier] than many botanists". You might look long for a Turkey oak in the district and not find one. But some people believe that the range of the insect may be many miles. Nor is the Turkey outstandingly different to other strains when young. The indentations in the leaf are more deeply cut and the acorns are surrounded at the base by a fringe of whiskery green scales. But if the tree hasn't yet produced acorns?
One man planted one Turkey oak among some hundred other oaks about 15 years ago. He has had none of these awful knopper galls, and is tearing out his hair that he cannot distinguish the potential offender. If it hasn't produced acorns, he is stymied for the time being. He hopes that what he was sold as a Turkey oak has turned out not to be so. This time, he doesn't mind being diddled. Quercus cerris is the Latin. Be warned. At present it seems that only the east of the country is plagued. The wood of this oak, by the way, much prized in Turkey, is, in this part of the world, i.e. Europe, said to shrink and warp. That is the verdict of Herbert L. Edlin in his book The Tree Key, published by Frederic Warne. Meanwhile our friend will continue to inspect his trees for the sign of a whiskery sheaf around an acorn. The damage may already be done.