Changes in the status and distribution of Oenanthe pimpinelloides
Corky-fruited Water-dropwort, in Essex
KEN ADAMS
School of Health & Biosciences. University of East London E15 4LZ
There are seven species of the genus Oenanthe in Britain, all of them except O. pimpinelloides
being either aquatics or plants of marshy ground, and all to some degree poisonous to livestock,
particularly cattle. However O. pimpinelloides is characteristic of damp patches on neutral, lightly
grazed, often scrubby, pastures and hay meadows, often on quite well drained soils, and has a
strange distribution in Britain. Confined to southern England, it is found in virtually every 10km
square in N. Somerset, Dorset, and South Hants; extending into West Gloucestershire, the eastern
part of South Somerset, and formerly, well into South Devon. Elsewhere it extends sporadically
right across southern England from the tip of Cornwall to East Kent and just into Suffolk. It also
has a substantial presence along the Severn Valley in East Gloucestershire. This pattern of distribution
suggests that it is, or has been until recently, severely restricted by some climatic factor, which is
only met in local pockets further to the east. In Essex the biggest patches seem to be in shallow
hollows on south or south-west facing slopes, often associated with seepages. Its rapid spread in
Essex in recent years however, may mean that climatic change is promoting an infilling of its
distribution pattern. Its main strongholds seem to be enclosed by an area with a July mean temperature
in excess of 17.5°C (62°F) and a February minimum of 1.65°C (35°F), suggesting that temperature
is the controlling factor. To some extent its distribution has been fragmented by the ploughing up or
reseeding of suitable habitat, but its apparent preference for sheltered south facing slopes in the
south-east suggests that it may have been restricted in the past to thermophilic pockets, and is now
doing rather well in response to a spate of hot summers. Its highly toxic relative O. crocata, formerly
restricted in Vol 8 (South Essex) to the warm environs of the London area, has also begun to
colonise further east in Essex, being now known as far out as Southend.
O. pimpinelloides is not an easy plant to identify, particularly in coastal areas where it can be
mistaken or overlooked for O. lachenalii. When in leaf its characteristic three leaf morphologies
(parsley-like at the base with oval serrated leaflets, the upper stem leaves having capillary segments,
and the middle leaves having an intermediate shape) are shared with O. lachenalii. By the time it
flowers and is noticeable, from June to late August, the basal leaves have withered, and the partial-
umbels (each small cluster of fruits), are noticeably flat topped (in contrast to the rounded, irregular,
partial-umbels of O. lachenalii). The fruits, which are rather rectangular from the side and c.3.5mm,
as opposed to somewhat ovoid and c.2.5mm in O. lachenalii, are borne on rays that thicken up
after flowering (unlike O. lachenalii). Below ground it has potato-like root tubers arising some
distance out from the stem, in contrast to the Dahlia-like spindle-shaped tubers of O. lachenalii,
that taper out from the stem base, but this apparently is an unreliable character as they can sometimes
resemble each other.
Stanley Jermyn and the present author were unaware of its presence in Essex at the time of writing
the Flora of Essex (1974). Old sites recorded from Wigborough, Virley and Mersea, in the 1860s,
and Clacton in 1913, seemed likely to be long since gone. Then in 1984, while carrying out the
meadow survey forTL40, Marcus and Claire Grace discovered a large colony in the Bell Common
(Sheppard's) Meadows, near the centre of Epping, newly acquired by the Corporation of London.
Further investigation revealed its presence in three scrubby meadows. It has since been lost from
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Essex Naturalist (New Series) 21 (2004)