British biologists called Elodea 'Canadian pondweed', suggesting
that they knew where it originally came from. It was observed in Scotland at
Berwick-upon-Tweed in August 1842, and this is the first recorded sighting in
Britain.
The arrival in England appears to been delayed for five years, when it
was found growing in reservoirs at Foxton locks near Market Harborough in
Leicestershire. It was probably brought in from Canada attached to timber
imported for building the railway at Rugby.
Elodea spread rapidly from Foxton locks: within twelve months it
was very common in Northamptonshire and Nottinghamshire. A year later it was
well established in Staffordshire, Warwickshire and Derbyshire.
Cuttings from the Foxton Lock plants were grown in a tub at the
Botanical Gardens in Cambridge in 1848. The curator, a Mr Murray, introduced
the plant into a waterway running alongside the Gardens. By 1851 it had choked
the rivers Cam and Ouse, and much of the low-lying fens of Cambridgeshire and
Lincolnshire. At this time it was a major problem to boats trying to sail
through the waterways.
Elodea continued its rapid advance through Europe: France and
Belgium by 1860, Germany by 1865. By 1870 it had reached east to Russia and
Hungary and north to Denmark and Sweden. Its advance was stopped at about
66° N by the low temperatures.
Elodea is still common in all of the areas that it colonised,
but it is no longer clogging up the rivers in the way that it once did.
Elodea grows vigorously for a few years after colonising a new area, and
takes over the waterways. After about five years its growth slows down, and it
stops being a troublesome weed. No one fully understands why, but it is thought
that the amount of iron at the bottom of the streams and lakes might be a
limiting factor for further growth.
If your school has a pond, try and find out how quickly the
Elodeais growing. It may have already started to slow down. Some people
think that it grows best when there is little oxygen in the mud at the bottom
of the pond.