On Sunday 2 March members of the Wren Group practical work team
donned waders and life-jackets to clear areas of Floating Pennywort
(Hydrocotyle ranunculoides) from the western end of Perch Pond. This
aquatic plant, which grows on the surface of the water, is actually
quite attractive, so why bother? Well, it is not a native species but
adapts very well to conditions in the UK. It is capable of growing up
to 15 metres from the side of a lake in a single growing season
(that’s an astonishing 20cm per day) and up to 50cm thick. Smothering
the surface of a water body, that inevitably means the ecology of the
lake, pond or ditch is dramatically altered. Other plants will be
crowded out, surface-feeding birds are denied a water surface to
dabble in, and invertebrates in the water beneath are denied light.
The plant is a native of North America but, being popular with the
aquatic nursery industry, has colonised many other parts of the
world, including South America and much of southern Europe. Although
warnings about the risk of colonisation were apparently voiced as
long ago as 1936, it first really began to spread in the UK after
being sold for garden ponds in the 1980s. Floating Pennywort is not
an easy plant to remove. Its fine roots grow from nodes and unless
these are picked from the water they will simply re-grow. This is an
example of good old-fashioned hand-removal being more effective,
certainly in small areas of infestation, than mechanical methods,
which can simply spread the plant to other areas.
Contractors brought in by the City of London Corporation had cleared
some large areas of the plant in late January, but we were able to
tackle some of the smaller clumps in the area near the tea hut. We
will now be on the lookout to see where it re-establishes itself,
since complete eradication is well nigh impossible. Having said that,
work carried out to remove it from Heronry Lake a few years ago does
appear to have been a complete success.
Tim Harris