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A NEW WATERWEED IN ESSEX
We've had Canadian Pondweed, Azolla, Least Duckweed, Nuttal's Pondweed and
Crassula helmsii, the curse from Australia. Now we have another North American
invader, the Floating Pennywort, Hydrocotyle ranunculoides.
Floating Pennywort is usually a floating waterweed but it will also grow on wet mud and
only in this habitat does it flower and set fruit. In its floating form it has fragile creeping
stems with roots sprouting from the nodes. Any small fragments which break away can
form separate colonies which is especially prone to happen in running water. It seems to
be tolerant of frost and also shade, where its leaves grow even larger. It often occupies a
niche not filled by any native higher plant; forming a carpet beneath reeds or tall rushes.
It is capable of very rapid growth (about 20-30 cm per week) and it is already a serious
pest in Central and South America.
Now, as to its appearance, its relationship to our native Marsh Pennywort (Hydrocotyle
vulgaris) is obvious, it too has creeping stems and rounded leaves. Floating Pennywort is
usually much larger, the leaf stems tend to be longer and in water the leaves float on the
surface like miniature water lilies or more accurately, like a species of Water Crowfoot
(Ranunculus sp.). In fact the leaves which are lobed, closely resemble those of a Water
Crowfoot.
One doesn't have to look far to find the source of this alien invader. Look no further than
your local garden or aquatics centre. It is readily on sale, often wrongly labelled as Marsh
Pennywort. It was first noticed in the wild in the river Chelmer in Chelmsford by Martin
Heywood in September 1990. It remained unidentified until Martin recognised the plant
exhibited by Tim Pyner and myself at the Essex Field Club meeting in September 1991.
This plant Tim and I had found in a flooded gravel pit known as Friar's Park in
Shoeburyness. Later in 1991, Martin refound Floating Pennywort at its original site in
Chelmsford and also further down stream in the Chelmer-Blackwater canal. During
1992, Tim Pyner and I found that it had spread at least as far as Ulting, 12kms
downstream of Chelmsford along the canal.
Since 1992, several more records have surfaced and these are summarised below.
Benfleet Marsh, in a drainage ditch. S. Massey.
Shipwrights Wood, Benfleet, almost covering a small pond near playing fields. S.Massey.
Thorndon Park south, Brentwood, in the lake. J. F. Skinner.
Canvey Island, in Canvey lake.
Stanford-le-Hope, stream near housing estate.
Basildon, Langdon Hills Country Park, pond in Willow Park together with Crassula helmsii,
Azolla and the Brazilian pondweed, Myriophyllum aquaticum. R. G. Payne.
There is also a first record for Buckinghamshire at the edge of a balancing lake at
Tongwell, Milton Keynes. R. Maycock and A. Woods.
Unless Floating Pennywort is knocked back by a severe winter (it shows no signs of
suffering in cold spells) I can see this water plant being very common in a few years and
potentially a serious pest.