Himalayan Balsam (or Impatiens glandulifera) is a not unattractive plant; it was introduced as a garden plant to the U.K sometime in the mid-nineteenth century. It is an annual plant and probably the tallest annual in the country - growing to heights of six foot. The flowers are somewhat variable in colour from a striking pink / purple to almost white. The shape has been likened to a policeman's helmet - hence it is sometimes known by that name. It flowers from June right through to October - which is quite late in the year; i.e. still producing seeds long after most other annuals. The black seeds form in capsules - which open explosively when mature, dispersing the seeds.
Unfortunately, the himalayan balsam did not stay in Victorian gardens. It spread. It is now found in a wide variety of habitats; waste land, roadside and railway lines, damp woodlands and particularly river banks, where it poses major problems. Its present distribution was probably helped by a number of people - see Professor Ian Rotherham's articles on invasives e.g. http://www.ukeconet.org ; quote "We have the stunningly beautiful Himalayan balsam spread along the River Sheaf and the River Derwent in the 1940s by Nobel Prize-winning scientist Sir Hans Krebs".
When the plant dies, river banks can be left devoid of significant vegetation and become muddy and liable to erosion. The mud or silt, when carried away by the river, can affect the spawning grounds of fish. Fish apart, there are other effects - the sheer size of the plants means that other plants are shaded out of existence, or the competition for minerals, water, or space is simply too much for them. There are 'knock on' effects in terms of the the invertebrate populations, insects, beetles, butterflies, spiders etc.
As an introduced species, the himalayan balsam arrived here (1839) without any of its natural 'enemies' - i.e. organisms that either feed on it (insects, caterpillars etc) or parasitise it (fungi, bacteria). Consequently, it has been able to grow and spread without check.
Traditional methods of control (weedkillers / manual removal) are often not very practical because of difficulty of accessing all sites along a river catchment or the area / site may have a high conservation status - so the use of herbicides is not appropriate. It has been estimated that a serious attempt at the removal of the Balsam (using such methods) across the U.K. would cost 300 million.
However, the CABI (formerly the Commonwealth Agricultural Bureau) is allowing the release of a rust fungus that attacks the himalayan balsam. A rust is an obligate, biotrophic fungus. That is, it is a parasite, which can only survive and reproduce in the living tissue of its host - in this case, the himalayan balsam (link opens a pdf). The life cycle of the rust is complex - and detailed information of the various stages can be found here : http://himalayanbalsam.cabi.org/the-proposed-solution/
Three locations are being used in the trial of this rust fungus - in Berkshire, Middlesex and Cornwall. It is hoped that this will provide a natural method of controlling the plant (sometimes referred to as biological control), whilst leaving native species unharmed. It may be that the fungus will reduce the vigour / size of the balsam plants - by interfering with the photosynthetic capacity of the plant. Then, over a period of 5 to 10 years, the fungus should spread and bring the Balsam populations under control (link opens pdf), so that they can then be managed or eliminated by physical (it has a shallow root) or chemical means.