Posted on September 11, 2021
Acorn knopper galls can be found on the young oak trees behind Greenwood House and also in the Charlton Down Nature Reserve. They are acorns which have developed in a distorted way because a particular species of small wasps have laid their eggs in the acorn flowers. The irritation as the egg hatches out causes a proliferation of tissue growth on which the larva can feed. Sometimes a whole acorn is transformed this way. Sometimes only part of it. And on some stems a perfectly normal acorn in its cup grows side by side with the gall acorn. The shapes are variable as you can see from the photographs.
If you would like a little more information about these galls, ten years ago I wrote in my other blog about some of the same type of galls that I noticed in Kew Gardens.