Lythrum hyssopifolia, Grass-poly
The site near Cholsey where Lythrum hyssopifolia is found, is subject to periodic flooding. Flooding removes the competition from rank species and allows the Lythrum to germinate and flower. Thus we have a pattern of appearance of the plant after flooding and non-appearance when there has been no flooding. The number of plants counted ranges from zero in a non-flooding year to tens of thousands.
The strip where Grass-poly grows by the Thames is carefully managed arable. Sometimes the owners are not able to cultivate the field due to wet weather.
The Cholsey site is now the only one known in Oxfordshire; a search at a historical site at Piddington produced no plants though that site contains a good population of Grass Vetchling Lathyrus nissolia.
Seed has been collected to send to the Kew Millennium Seed Bank at Wakehurst Place to be stored for future use.