Today's entry on fairy ring fungus was kindly submitted and written by UBC Botanical Garden's director, Dr. Quentin Cronk – Daniel.
These fairy rings from central London are taken from Google Earth (latitude: 51 degrees 30' 28.13" North; longitude: 0 degrees 11' 22.41" West). They are in the extreme north west section of Kensington Gardens (a section not open to the public), just north of Kensington Palace. The fairy rings are marked by a dark green growth of grass, caused by the advancing hyphal front releasing nutrients and stimulating the growth of the grass. Inside this is usually a zone of poor grass growth caused by the available nutrients being taken up by the fungus. Further towards the centre of the ring the grass growth becomes better again as the old hyphae die and nutrients are released. There are several fungi that cause fairy rings, but these ones in Kensington gardens are most probably caused by Marasmius oreades, which is one of the commonest lawn fairy rings of this type in Britain. The largest of these fairy rings in Kensington Gardens appear to be 10-15 metres across and as fairy rings grow at about 10-20 cm per year, they may be over 100 years old. Certainly they are likely to date from after 1841 when the western part of the Kensington Palace estate was remodelled to develop the exclusive neighbourhood of Kensington Palace Gardens (just to the left of this picture).
Fairy ring growth is a curious process and has been the subject of many studies (including mathematical modelling). Usually when two rings meet they extinguish each other, as they cannot grow through each other's zone of depletion. However, in this photograph it can be seen that several of the rings have successfully crossed.
In folklore they were thought to be the work of fairies or elves and hence Prospero (in Shakespeare's Tempest) exclaims: “you demi-puppets that by moonshine do the green sour ringlets make”. Many gardeners try to get rid of the rings as they consider them unsightly. However, be warned! According to mycologist Gordon Rutter writing in the Fortean Times, fairy mischief may wreak havoc on the unwary who disturb the rings. It may be best to leave them well alone, as at Kensington Gardens.