Starfruit, a tiny white flower found on the margins of old ponds, can now be found in only one location, in Surrey, where last week there were four surviving stems. This makes it rarer even than the lady's slipper orchid, the spectacular yellow-and-purple bloom often thought of as Britain's least common plant but which is well established at its sole location in the north of England.
Now the starfruit's last home pond, on West End Common, near Esher, is being invaded by New Zealand pygmyweed, a tough and resilient plant introduced into Britain by gardeners and, having escaped into the wild, is running riot. It is spreading rapidly throughout southern England and is impossible to control.
On a neighbouring pond the pygmyweed forms an impenetrable green mat which, when dragged up, is thicker than shagpile carpet. It is smothering other wild flowers such as water mint and marsh St John's wort.
So far it is less advanced on the starfruit pond, but its stems are creeping towards this year's starfruit plants, and are within inches of two of them.
Plantlife, the wild-flower conservation charity which is leading efforts to save the starfruit and many other rare plants, is hoping to work with Elmbridge Borough Council to hold back the pygmyweed.
But it is a daunting prospect. "I don't know of a single instance where people have successfully controlled New Zealand pygmyweed," said Ruth Davis, Plantlife's species co-ordinator. "If you've got it in a pond you've got big trouble. Once it's there, the most successful approach is to kill everything with weedkiller and start again.
"The problem is, it's incredibly efficient at reproducing itself. You only need the tiniest, tiniest bit of it left and off it goes; it will regrow. People have tried rolling up turves of it, and they've tried liquid nitrogen. They've even suggested flame-throwers."
Crassula helmsii, to give it its scientific name, is a native of south- eastern Australia and New Zealand and has been sold commercially in Britain since 1927 as a plant for garden ponds. It was first discovered as a naturalised plant at Greensted, Essex, in 1956 and since then has spread rapidly. Its ability to outstrip native plants is causing great concern and other rare aquatic wild flowers, such as brown galingale and three- lobed water crowfoot, are threatened by it. Dave Page, the Elmbridge Borough Council commons ranger, is responsible for trying to stop it killing off the starfruit. "It's impossible to get rid of it," he said. "You can't use chemicals, as they would kill everything, but we think you can control it by hand, on a cyclical basis ... as the starfruit is Britain's rarest flower, we're going to do all we can to save it."
Starfruit - Damasonium alisma - is not to be confused with the much larger star-shaped tropical fruit of the same name. It is a tiny flower of Old England that used to flourish at the edges of ponds cattle used for drinking, and was carried as seed by the cattle along the old drovers' roads. To germinate, it needs a pond edge that dries out in summer. It has suffered a big decline this century as old natural country ponds disappeared or have been replaced with duck and fish ponds where the water is kept permanently high. Its flower gives way to a star-shaped group of seed pods.
Lady's slipper: Cypripedium calceolus. Orchid to be found on a single site in the North of England
Snowdon lily: Lloydia serotina. Alpine plant, surviving on a handful of sites in Welsh mountains
Military orchid: Orchis militaris. Pink and white bloom thought to be extinct from 1902 to 1947
Adderstongue spearwort: Ranunculus ophioglossifolius. The rarest of the buttercup familyReuse content